New Considerations for Controlling Bitter Rot on Apples

In previous commentary, fungicides recommended for control of bitter rot in the Northeast included captan, ziram, and QoI fungicides, but continued dependence on the latter is probably neither sustainable nor advisable. Read my recent article published in Scaffolds Fruit Journal for the rationale in suggesting that either Aprovia or Fontelis might be advisable in summer sprays during July in blocks where bitter rot has been a problem. Click here to access the complete article. New York readers should note that Aprovia is not registered for use in New York State.

For links to other articles that discuss recent research on the bitter rot pathogen, see earlier information posted here.  Several photos have been posted under “Bitter rot” on my “Apple Diseases” page.

Posted in Fungicides for apples, Seasonal notes | Comments Off on New Considerations for Controlling Bitter Rot on Apples

A New Option for Learning to Use RIMpro

RIMpro is a proprietary interactive decision support system that can help fruit growers manage apple scab and fire blight in spring. I discussed RIMpro briefly in a blog post last year. I have evaluated RIMpro for three years and have found it very useful for assessing infection risks for both apple scab and for the blossom blight phase of fire blight. Information provided by RIMpro can assist with precision timing of the fungicide and bactericide sprays required to control these diseases.

This year Dr. Srdjan Acimovic, plant pathologist at the Hudson Valley Lab (HVL), is launching an apple grower/researcher partnership to further explore the usefulness of RIMpro for apple growers in New York and other parts of eastern United States. Growers participating in this partnership will be provided with assistance in accessing and understanding the biological significance of the apple scab and fire blight infection predictions that are provided by RIMpro. I will be assisting in the launch of this project and with interpretation of the data generated by RIMpro for locations in eastern NY.

Details of this new program are outlined in a 4-page summary and a powerpoint, both of which were used for a recent informational meeting held at the Hudson Valley Lab on 2 Feb 2016.

Following is a short summary from those documents:

  1. Anyone who has a NEWA weather station on or near their farm can use weather data from that station to drive the RIMpro software if they subscribe to RIMpro in 2017.
  2. Those wishing to use RIMpro will need to pay $50 to NEWA for a RIMpro-compatible data stream and 200 Euros to RIMpro as outlined on the RIMpro website at http://www.rimpro.eu. Click the button for “Create New RIMpro Account” on the right side of that page for pricing information.
  3. Growers in eastern NY (Long Island to Champlain Valley) who wish to participate in the partnership will pay $80 to the Hudson Valley Lab to cover both the NEWA data stream ($50) and some technical support ($30) for the e-mailed tutorials and RIMpro interpretations that HVL staff will be providing Mon-Wed-Fri throughout the primary scab season.
  4. Because RIMpro is a proprietary program, only folks who are RIMpro subscribers  will be eligible to receive the thrice-weekly e-mails that assist with RIMpro set-up and interpretation.
  5. Growers outside of eastern NY who subscribe to RIMpro are welcome to join our RIMpro user group, and they can sign up to receive the thrice-weekly e-mails describing scab and blight conditions in eastern New York. Growers outside the area are not required to pay HVL the $30 fee for technical support because, while the tutorial info that we put into e-mails should still be useful, the comments on RIMpro outputs that we will be highlighting in our e-mails are unlikely to be timely or relevant for other regions.

Anyone who wishes to part of our RIMpro user group network for 2017 should contact either Dave Rosenberger (dar22@cornell.edu) or Dr. Srdjan Acimovic (sa979@cornell.edu) as soon as possible but no later than March 10.

Posted 3 Feb 2017

Posted in Apple scab info, Fire blight info | Comments Off on A New Option for Learning to Use RIMpro

Trunk-Related Problems in Apples: SAD

Apple growers and extension specialists in northeastern North America are seeing an alarming increase in the numbers of orchards, mostly young orchards, where trees are declining or dying due to dysfunctional trunks. The phenomenon has been named sudden apple decline (SAD) or rapid apple decline (RAD) because affected trees seem to decline very quickly. The visibly affected areas of the trunks exhibit cankers, cracks, or dead phloem and cambium, but causes and contributing factors are often difficult to determine. In some cases, multiple factors may be contributing to the tree losses.

The SAD phenomenon was discussed in a recent presentation that I made at the Empire State Expo in Syracuse, NY, that was held 17-19 January 2017.  An updated version of the proceedings from that talk  and of the Power Point used for the presentation ( 25.8 MB) are accessible by the indicated links.

For another perspective on this problem, see the excellent article by Dr. Kari Peter, Plant Pathologist at Penn State’s Biglerville station, that appeared in December 2016 and that can be accessed here.

On another note, I have updated my previous blog post on bitter rot with a link to the Power Point on bitter rot that I used at the Cumberland-Shenandoah Fruit Workers conference last December.

Posted 23 Jan 2017

Posted in Apple diseases: general info, Seasonal notes | Comments Off on Trunk-Related Problems in Apples: SAD

Recent Changes in Our Understanding of Bitter Rot

Bitter rot is an increasingly important disease of apples and pears in many production regions around the world. This post provides a link to a review of the scientific literature on bitter rot that I recently completed and that formed the basis for a presentation at the Cumberland-Shenandoah Fruit Workers Conference on 1-2 Dec 2016. The powerpoint I used at that conference includes photos and additional comments on research that is needed before we can formulate more effect control strategies for this disease.

The review explains how our understanding of bitter rot has changed in recent years as scientists have used molecular techniques to determine that bitter rot, which in 2006 was believed to be caused by three different fungi, is now known to be caused by at least 18 different Colletotrichum species. Different species predominate in different geographic regions and vary in their susceptibility to fungicides. A table on page 10 of the review provides a useful list of the species causing bitter rot and where and by whom they have been reported.

This literature review for bitter rot will probably be of more interest to scientists, extension specialists, and private consultants than to apple growers who are searching for information on control strategies. A shorter summary covering some of the same information, along with suggested control strategies for bitter rot in northeastern United States, was published 29 August 2016 in Scaffolds Fruit Journal

In addition to the control strategies listed at the end of the Scaffolds article, fruit growers should recognize that the fungi that cause bitter rot can sporulate abundantly in dead twigs that are left in or beneath trees after either winter or summer pruning. Thus, removal of all fallen fruit and pruning debris from herbicide strips beneath trees can help to reduce inoculum and is especially important in blocks with a history of bitter rot problems. Moving prunings to the row middles where they can be shredded with a flail mower is probably sufficient, but more research is needed to determine exactly where and how the fungi causing bitter rot overwinter in our orchards.

Posted 20 Jan 2017; Updated 7 Feb 2017

Posted in Review of relevant publications | Comments Off on Recent Changes in Our Understanding of Bitter Rot

Anticipating Outcomes from a Week of Rain

Tuesday evening, May 3rd: Hudson Valley fruit growers should be thinking very carefully about their disease control strategies for the next few days. The good news is that the weather is too cool for fire blight to be a concern at this point.  However, many other diseases will benefit from the cool wet weather.

Risk factors and implications include the following:

  1. In Highland, most apple cultivars are between full bloom and petal fall, the growth stages when trees are especially susceptible to a multitude of diseases.
  2. We are in the midst of what will almost certainly be the most important apple scab infection period of the season for our area. The RIMpro program is indicating that 20 to 25% of the season’s ascospores will be causing infections in unprotected trees during this week. (See Dr. Dan Cooley’s explanation of current RIMpro graphs on You-Tube). The risk of scab infection is also increased at this time because trees in the Hudson Valley are now pushing terminal shoots, so new scab-susceptible leaves are unfolding each day.
  3. The intermittent rains and wetting periods that began on Sunday, May 1st  and that are predicted to continue through Saturday, May 7th could result in an epidemic of quince rust on fruit. For more information on rust diseases, see my recent article in Scaffolds  and additional photos and info on rust diseases that I recently posted elsewhere on this website.
  4. This long wetting period during late bloom and petal fall also favors development of blossom-end rot diseases, moldy core, and Botrytis infections of dying petals. The latter have the potential to colonize the flower sepals (which later form the apple fruit calyx) where they remain quiescent during summer, only to activate and invade fruit during long-term storage where they cause losses to postharvest gray mold unless such infections are prevent via postharvest fungicide treatments.
  5. Plum trees are at peak susceptibility for black knot infections.
  6. For stone fruits where the crop was not eliminated by spring frosts, the wet weather will favor infection of flowers and/or fruitlets by both brown rot and Botrytis. Botrytis is generally not recognized as a major pathogen of stone fruits, but infections during late bloom and petal fall can cause either pollination failures or abscission of pollinated fruitlets.
  7. Since Friday, we have had nearly 2 inches of rain in Highland, with another 1.1 inches forecasted for the rest of this week. Most fungicide protection will be removed by 2 inches of rain, so that means that orchards last sprayed before Friday, April 29th are now running out of protection. Orchards sprayed last Saturday have been exposed to about ½ inch less rain, but they will also run out of protection before the end of the week if current forecasts are accurate.
  8. This extended period of cloudy, wet weather will be leaving foliage and fruit especially susceptible to injury from spray materials applied during the next week. Captan, although it almost never causes injury when applied alone, is of special concern at times like this if it is applied in complex tank mixtures. (For more info, see my 2014 article in New York Fruit Quarterly and a 2015 research report by Villani, Breth, and Cox that addresses concerns about phytotoxicity from captan.)

Suggestions for action (see items 7 and 8 for stone fruit concerns):

  1. Given the massive apple scab spore releases that have been and are occurring, apple orchards should be recovered ASAP so as to prevent any additional infections during the remainder of this wet week. Ideally, I would prefer to apply a tank mix containing both a protectant (in this case, mancozeb or Polyram) plus a post-infection material, the latter being applied as an insurance policy for any coverage lapses in the last spray or any excessive wash-off that occurred since then. However, there is no point in applying expensive fungicides that must dry to be absorbed into leaves if one cannot find a spray window that will allow drying. (Almost all of the fungicides with post-infection activity need at least 30 minutes of drying time to be fully effective, and some need several hours of drying time.)
  2. At this point it is probably more important to get something applied to apples rather than waiting for ideal conditions to apply a post-infection material. If there are no windows where sprays will dry before rains resume, then one should apply either mancozeb or Polyram alone, or apply either of them in combination with Captan for more potent scab activity. (Applying captan alone is not advised because it will not control rust diseases!) However, do not apply captan mixtures with any foliar nutrients or adjuvants that might enhance uptake into the fruitlets. The contact fungicides (mancozeb, Polyram, captan) will provide excellent scab and rust control even if they are applied in light rains, and for organic growers, the same is true of sulfur. Thus, with these products there is no need to wait for the “dry window” that one needs if fungicides with post-infection activity are being applied.
  3. If apples can be sprayed during a “dry window” in the next day or two, then I would suggest using Inspire Super plus mancozeb. Inspire Super, like other DMI fungicide such as Rally and Rubigan, will provide top-notch protection against rust diseases. The difenoconazole component of Inspire Super will provide the best scab control within the DMI fungicide group, especially in blocks where scab is showing reduced sensitivity to DMIs. The Vangard component in Inspire Super will provide up to 72 hr of post-infection activity against apple scab, thereby ensuring further activity against DMI-resistant strains of apple scab, and Vangard should also protect against Botrytis (except where populations are already resistant to the AP fungicides). Finally, there are some reports in the literature suggesting the difenoconazole applied as a protectant can prevent moldy core, although I would take that attribute of Inspire Super with a grain of salt, especially if it was not applied before this lengthy rain started last week.
  4. If only contact fungicides can be applied to apples this week, then blocks should be recovered with a post-infection fungicide (in a mixture with a contact) on Sunday or Monday after the rains end but while there will still be some benefit from the 48 to 72 hr post-infection activity.
  5. As we approach petal fall, tank mixes on apples by necessity become more complex. I would not use captan in any complex tank mixes until after we have had  several days of dry windy weather to promote cuticle production on new leaves and young fruitlets.
  6. Whereas Syllit has been a good post-infection apple scab material in the past, the label has recently been changed to state that Syllit may not be applied after pink bud. Thus, Syllit is no longer an option for reach-back activity at this time (unless one still has Syllit with the older label that allows application after pink). That’s not all bad because Syllit could, under some conditions, be rather harsh on fruit finish.
  7. Plum orchards, whether they have a crop or not, should be recovered with chlorothalonil (Bravo or generics) as soon as possible if the last application of chlorothalonil was more than 9 days ago. (The labels for at least some of these products indicate that the minimum retreatment interval is 10 days.) If Bravo was applied less than 10 days ago, Indar may be the next best fungicide for suppressing black knot.
  8. For stone fruits that still have cropping potential for this year, Indar will also be effective against brown rot (and perhaps Botrytis)  whereas chlorothalonil alone is not very strong against American brown rot and will not control the European brown rot that can attack European tart cherry cultivars during cool bloom-time rains. There should be no significant risk of brown rot on trees that had no flowers.
Posted in Seasonal notes | Comments Off on Anticipating Outcomes from a Week of Rain

Credibility of Fire Blight Forecasts

Given that apples and pears are blooming again in the Hudson Valley, it’s time to think about fire blight. The primary risk from fire blight occurs during bloom, as everyone should know by now. The fire blight bacterium, Erwinia amylovora, is spread by insects and/or rain splash to the stigmas of open flowers where the bacteria multiply very rapidly at temperatures above 65 F. If rain occurs after populations reach infective levels, then the bacteria are washed from the stigmas to the base of the flowers where infection occurs through the flower nectaries.

Two fire blight risk models, MaryBlyt and Cougar Blight, have been widely used in North America to predict when infections are likely to occur and thus, when sprays are needed to protect blossoms from fire blight. Last year (2015) both of these models indicated that temperatures were favorable for bacterial multiplication on stigmas throughout most of the bloom period. Given those conditions, infections were expected to have occurred on days when there was enough rain to move the bacteria down to the nectaries. The very small amount of rain needed to make pavement look wet is considered sufficient for moving bacteria into the nectaries. In the lower Hudson Valley, we had three light rains during bloom. Based on the model outputs, I expected severe fire blight outbreaks would occur where streptomycin was not applied at least twice during the season. In fact, I sprayed the plant pathology blocks at our research station three times to cover the three potential infection periods that occurred during bloom in 2015.

What happened? Very little fire blight occurred in the Hudson Valley last year, and it appeared that any blocks that were sprayed at least one time during the bloom period were completely protected from blight regardless of when that spray was applied whereas blocks that had received no strep sprays sometimes had a few strikes. At the end of the season, I was left asking how the risk models (and my own predictions based on those models) could have been so wrong.

We still do not have a definite answer to that question, but I suspect that blight failed to develop last year because the relative humidity throughout bloom was abnormally low. In a research paper published in 2000 (Phytopathology 90:1352-1357), Dr. Larry Pusey reported that very little blossom infection occurred in growth chamber trials with detached flowers when the relative humidity was below 80 to 85%. In most years, wetting periods in eastern United States are associated with at least short periods of high relative humidity, but that was not the case during any of the bloom-time wetting periods at the Hudson Valley Lab in 2015. Unfortunately, no one knows the minimum duration of high RH required for fire blight infections, nor do we know how the sequence of high RH periods (i.e., before, during, or after rains) might impact infection potential under the varying conditions that we find in the field.

So how should we interpret fire blight risk models in 2016? The conservative approach will be to continue following risk models just as has been done for the past decade or more and ignore RH as a factor until researchers can sort out the details. Extreme caution is required if one chooses to ignore strep sprays when our current models indicate fire blight risk is high because (1) we still do not have data indicating how to incorporate RH into the existing models, and (2) missing a strep spray when it is really needed will be far, far more costly than applying an extra strep spray when perhaps it was not needed. Furthermore, the amount of inoculum that is present will be very important in determining the severity of blossom blight, and right now we have no way to evaluate the presence of inoculum other than basing our estimates on orchard history.  That means that a very conservative approach is especially important in blocks that had a lot of blight last year.

Nevertheless, for those who wish to consider RH as a factor in fire blight forecasting, I suggest the following process as a starting point:

  1. Follow MaryBlyt or Cougar Blight models as one normally would during bloom.
  2. When the models predict either high risk with rain expected (Cougar Blight) or infection in MaryBlyt (which only occurs when wetting is added to other factors), then check relative humidity levels.
  3. Finally (and this is the part where we are really flying by the seat of the pants), I would suggest that if RH does not exceed 85% for at least four hours either during the wetting event or continuous with it (i.e., immediately before or after the wetting period), then the risk of infection may be rather low even if the models suggest otherwise.

One way that I have habitually crosschecked conclusions about blossom blight risks over the past 30 years is by applying what I have called the “personal discomfort index” or PDI. The PDI works as follows: “In early spring, if outdoor work that requires a modest exertion causes a person to feel uncomfortably hot and sticky, then one should check to see if apples and pears are in bloom. If flowers are present and strep has not been applied in the past two or three days, then begin strep sprays immediately.” If a PDI event is triggered, it is almost always because warm temperatures have combined with high RH to create that feeling of personal discomfort. Thus, in retrospect, my PDI for identifying blight infection periods may have been ahead of it’s time. (OK, I can hear all of my colleagues laughing! And I admit that what works for me in the Hudson Valley may not work for other folks in other places.  This is certainly true for those who never get out of the office or for those whose body temperature sensors fluctuate wildly 🙂

Last year when our risk models were predicting severe blossom blight conditions, I was a bit concerned because the conditions never triggered a PDI-event. I decided the models had to be more scientific than my subjective PDI analysis of blight risk and therefore warned about the potential for severe blossom blight if strep sprays were not applied. In retrospect, I wish I had trusted my PDI analysis a bit more. Since we now know that the models did not work well in the Hudson Valley in 2016, I’ve opted to give you both a quasi-scientific “seat of the pants” approach for adjusting model output this year with RH considerations, and I’ve also risked explaining my PDI approach for evaluating blight risk. As you can imagine, I will refuse to take responsibility for control failures that might be associated with using either of these unproven approaches!

What are the models showing now for the lower Hudson Valley? When I input into MaryBlyt a first bloom date of April 20 (Wednesday) along with the Accuweather forecasted temperatures for today (Thursday) and next week, MaryBlyt suggested that we could have an infection event with rains tomorrow (Friday) and again next Tuesday. I think that the risk of infection on Friday of this week is virtually nil, both because many orchards did not have many flowers open yesterday and because the predicted highs of 78 F for both today and tomorrow may never be reached. Furthermore, both Cougar Blight and RIMpro discount infection predictions during the first few days of bloom. The predicted infection for next week, which shows up in both MaryBlyt and the RIMpro fire blight models, is more “iffy” and bears watching. However, the current forecast for next week suggests only scattered showers are likely, and those often occur without generating extended periods of high RH. Thus, in my opinion, fire blight risk is relatively low in the lower Hudson Valley for at least the next few days and quite possibly for all of next week.

Posted in Seasonal notes | Comments Off on Credibility of Fire Blight Forecasts

Adding Insult to Injury: Scab Warning

In addition to cold injury from our current cold snap, a scab infection period may arrive in the Hudson Valley with the warm rains predicted for Thursday. (No the snow did NOT kill the scab spores, and yes, the spores are certainly mature enough to cause havoc in unsprayed trees at this point if we get a wetting period long enough for infections to occur!) Orchards sprayed last week should still have enough residues to squeak through this potential infection period, but orchards that have not yet been sprayed and those that were sprayed prior to the rains last Monday, March 28th, could be at risk. The situation is further complicated by weather predictions for windy weather through much of the remainder of the week and by concerns that pesticides applied after freezes may exacerbate leaf injury.

Apples in most locations in the Hudson Valley suffered some cold injury last night, with more cold predicted for tonight. It will take some time (probably several weeks) to fully assess the degree of damage. Earlier budding cultivars such as Empire have probably sustained more damage than later-budding cultivars such as Golden Delicious. Additional cold temperatures in the mid-20’s are predicted for Saturday night, although that forecast may still change before Saturday arrives.

If the current forecast for warm rains on Thursday remains unchanged, my recommendation would be to use available spray windows to apply fungicide to the most scab-susceptible cultivars, and perhaps to the entire farm if time allows, either before the Thursday rains or as soon as possible thereafter. However, because cold damage to leaves will increase the potential for phytotoxicity, use the following precautions:

  1. Do not apply anything to leaves that are still frozen. Ideally we would like a day or two for leaves to recover after severe freezes before any pesticides are applied, but that may not be possible in this situation if the scab infection period develops as predicted on Thursday.
  2. If you opt to spray before the Thursday rains, use only mancozeb or Polyram. DO NOT APPLY CAPTAN or SYLLIT or OIL or COPPER to cold injured leaves for at least several days after the last injury event because uptake of these products into the cold injured tissue could make the injury worse. Also, it may be best to apply mancozeb or Polyram without any adjuvants that might enhance penetration.
  3. If you decide to wait until after the rains (which may be the safest bet, given the cold damage and predicted windy conditions), then I suspect that the combination of Inspire Super plus mancozeb will provide the best disease control, especially if it can be applied within 72 hr from the time that rains start on Thursday. It may still provide some benefits if applied within 96 hr because cold weather will prevail again (at least during nights) after the Thursday rain, thereby slowing the fungal infection process. Waiting until after the rain also allows one to see if the predicted rains really do turn into an infection period. If we are lucky, the forecasters will be wrong, the predicted infection period will fizzle, and no fungicide will be required until after leaves have had a chance to heal.

Dr. Warren Stiles was a strong proponent of using a “spring tonic” spray of zinc, boron, and urea to help trees recover from cold injury. His recommendation was to apply 1 lb or 1 qt of boron, 1 qt of zinc chelate, plus 3 lb of urea at tight cluster to pink, with all of those rates being rates per 100 gal of dilute spray. Application of this foliar nutrient mixture was believed to assist with repair of damaged cells and flower parts that might otherwise die or only partially recover from the cold injury.

I really don’t know how beneficial the spring tonic spray might be in the current situation because I’m not sure that anyone has published evidence from controlled trials to validate the benefits. (It definitely will NOT resuscitate flowers that have been totally killed by the cold!) I probably would not apply the spring tonic, and certainly not the urea, until after the potential freeze event predicted for Saturday night because urea could make tissue even more susceptible to freeze injury. I don’t know if boron and zinc might adversely affect leaves just coming out of a severe freeze, so it may not be advisable to add the foliar nutrients to a fungicide spray if that spray will be applied before next week. Nevertheless, this appears to be a season where any surviving buds will need all the help that they can get, and therefore Dr. Stiles spring tonic spray might be worth considered for next week if we still have live buds after all of these cold nights.

A final caution: Very low temperatures are again forecast for early tomorrow morning, but winds may drop after mid-night.  I have heard that some folks are hoping to use wind machines if there is any temperature inversion, and that may make sense if orchard heaters will also be used in the vicinity of the wind machine.  Be aware, however, that running wind machines on very cold nights with low humidity can cause freeze-drying of the foliage. I don’t know the exact conditions that contribute to this phenomenon, but I recall a situation many years ago when I was call to a farm to examine browned leaves that the grower assumed were caused by phytotoxicity from something he had sprayed. We were all scratching our heads until we realized that the damaged was only evident in a ring-pattern around his wind machine!  Having browned foliage is a small price to pay if one can keep the flower buds alive, so wind machines may still be useful tonight. However, deciding how and when to use them may involve choosing the lesser of several evils when degrees of evil cannot be clearly assessed at the time decisions must be made.

Posted in Seasonal notes | Comments Off on Adding Insult to Injury: Scab Warning

Scab Forecasts Converge as Rains Approach

The predicted scab infection risk for April 1-3 has diminished somewhat, but caution is still advised.  Below are the RIMpro graphs for the two systems that I described in a post yesterday morning that was titled “Weather or Not.” The RIM value for the Highland-MB station has now dropped to about 300, which is the risk level that we are currently using as the threshold for economically significant infection periods in clean orchards. The RIM-value for the Highland-N system has remained rather steady over the past 48 hours and indicates an even lower risk of infection.

NOTE: In my two previous posts, I erroneously used the right-hand axis to reference RIM-values indicated by the red line. Thus, yesterday I indicated that the RIM-value for the Highland-MB graph was near 600 whereas it really was over 700 when read using the left-hand RIM-value axis. As forecasts have been modified over the past 48 hours, the RIM-value for the predicted wetting periods on April 1-3 has dropped from over 700 to slightly over 300 on the Highland-MB graphs.

16-03-30pm RIMpro comparison

Those who have not yet applied any fungicide in their orchards will want to closely monitor both the bud stage (how much green tissue is really out there) and wetting periods as they develop over the next few days.

If the forecasted rains for April 1 to 3 ultimately bring us more lengthy wetting periods than currently forecasted, then it may be wise to apply post-infection fungicides ASAP after the infection events (perhaps Saturday morning between showers).  The two fungicides in the AP class, Vangard and Scala, will both provide about 72 hr of post-infection activity counting from the start of the wetting period. Inspire Super, which contains Vangard plus the DMI fungicide difenoconazole, should be even more effective so long as the scab population in the orchard is not highly resistant to DMI fungicides. However, all of these fungicide need to dry on the leaves to be fully effective. All of them should also be applied with mancozeb or captan, but these two contact fungicides will only provide 12-18 hours of post-infection activity (counting from the start of rains) under the warm conditions that are predicted.

Although the scab risk from the predicted rains seems to be diminishing, it is important to note that if scab gets started this early in the season, it can potentially cause problems throughout the rest of the season and risks of getting fruit scab due to secondary spread will be significantly increased. Factors that suggest scab risk over the next few days will be minimal include the forecasts as noted above plus the fact that most orchards had very little scab last year, so inoculum levels are low.

On the flip side, one factor that concerns me is the fact that this will be a warm rain and warm rains trigger far more release of mature spores than cold rains. The RIMpro model includes temperatures during rains in assessing risks, but this is the kind of scenario where I would be more comfortable if we had more experience with RIMpro and if we had a more definitive biofix for starting the RIMpro program this year.

Given the uncertainties, as I suggested yesterday, decisions about whether or not to spray ahead of these predicted rains are very much dependent on individual approaches to risk management.

Posted in Apple scab info, Seasonal notes | Comments Off on Scab Forecasts Converge as Rains Approach

Revisiting Scab Risks for This Week

The scab infection risk for April 1-3 in the lower Hudson Valley may be waning, although caution is still advised. As suggested in my earlier post this morning, weather predictions that vary widely for events that are three or four days into the future often converge as the event of interest approaches.  The RIMpro output using the MeteoBlue weather input now shows a shorter predicted wetting period for April 1-2 than it did this morning, and that change has resulted in a lowering of the forecasted RIM value from over 600 to a bit under 400 as shown below. The changes will be apparent if this graph is compared to the one that I posted in a previous message earlier today.

16-03-29pm RIMpro-MB graph

 

A RIM value of nearly 400 is still a concern, but given that the MeteoBlue forecast is shifting toward the forecasts for shorter wetting periods that came from other weather sources (see my previous post), my bet is that further reductions in wetting periods will be forthcoming from MeteoBlue and that the predicted scab infection period for April 1-3 will mostly disappear. Most cultivars in our orchards at the Hudson Valley Lab have not progressed beyond quarter-inch green. Putting it all together, I am reversing my earlier opinion about scab risk by suggesting that, given current information, the forecasted rains on April 1-2 now seem unlikely to produce significant scab infections in orchards that were scab-free last year. However, that’s still a judgment call and the decision on whether or not to apply a fungicide tomorrow will largely depend on one’s personal approach for risk management.

Those who have not yet applied any sprays (neither copper nor a fungicide) may wish to hedge their bets by applying a spray on Wednesday to at least their higher-risk blocks (i.e., scab-susceptible cultivars, sites with advanced bud stages, orchards with slight amounts of carry-over inoculum). However, one point that I had overlooked when I made the earlier post this morning is that temperatures tonight are predicted to drop below freezing and may not rise above freezing until sometime near sunrise tomorrow. Applying sprays to frozen foliage is never a good idea, so the cold weather tonight may shorten the spray window for tomorrow.

In most blocks, it is still not too late to apply copper (in terms of the bud stage), but applying copper to green tissue shortly after a freeze may result in damage to early cluster leaves, and that damage will be even greater if oil is included with the copper. Where there is a significant freeze tonight, it may be best to stick with mancozeb as as a fungicide for any applications made tomorrow morning.

Posted in Seasonal notes | Comments Off on Revisiting Scab Risks for This Week

Apple Scab –– Weather or Not

The first scab infection period of the year may occur later this week in the Hudson Valley, but  the probabilities of that happening depend on which weather forecast you prefer to believe.  The weather variable is the critical factor regardless of what model or predictive system you might be using, but I will illustrate the problem with the output from the RIMpro program.

This year I am evaluating RIMpro using two different data input systems, both of which are providing outputs for the orchard at the Hudson Valley Lab. One data input system uses our NEWA weather station at the Hudson Valley Lab (Highland) to log weather data as it measured by the on-the-ground equipment. This system uses the Norwegian “yr.no” weather forecast system for weather predictions.  I call this model system “Highland-N”

The other input system uses only the MeteoBlue data for both past weather and predicted weather; I call this Highland-MB.  This is a virtual station that never accesses any real data from our orchard. MeteoBlue is a Swiss weather forecasting system that provides data for all locations around the world.

In addition, I subscribe to Accuweather for weather forecasts and occasionally check the US Weather Service, which I can access through the NEWA Network. Thus, I can access four different forecasting systems when I want to know what to expect over the next few days.

Last night (Monday, 28 March) the output for Highland-MB indicated that a major scab infection period with a RIM value over 600 will occur April 1-2-3 whereas the Highland-N output indicated that the wetting periods April 1-2-3 will be too short to allow any significant infection, generating a RIM value of only about 40. (On the RIMpro graphs, rains are shown in the bottom bar-graph in dark blue with associated wetting periods in light blue, and anytime the RIM value shown by the red line exceeds 300, one can anticipate a significant scab infection period.)  Accuweather agrees with the yr.no forecast whereas the US Weather Service forecast agrees with MeteoBlue.

Below is a screen shot that compares the two NEWA forecasts alligned so that the dates correspond vertically, with the Highland-MB output uppermost in the photo. Note that MeteoBlue provides only a 7-day forecast whereas yr.no provides a 10-day forecast. As a result, the RIMpro output must be alligned by date, not by the edges of the model output graphs.

16-03-29pm RIMpro comparison

Here is the interesting part of the story: The spray window, based on wind conditions as predicted by US Weather Service will be Wednesday, starting about 1 AM and continuing until rains begin on Thursday morning.  Accuweather was providing the same wind forecast.

So, should Hudson Valley growers spray on Wednesday (as suggested by the RIMpro with MeteoBlue forecast), or can we expect to ride through the rains this week-end without getting a scab infection period?  The predicted infection periods are still too far away to get an accurate read (i.e., weather forecasts have a lot of error when they are for more than 48 hr in advance), but we should have a better idea by Tuesday evening.  Often the various weather models begin to converge as we get closer to the predicted event.

I pass along this info primarily to help everyone understand how I find value in RIMpro, but also to illustrate that the RIMpro output is no better than the weather forecast that it is using. If you are using RIMpro, it can give a valuable heads-up about the potential for major infection periods as they approach. However, because weather forecasts are often inaccurate, expecially beyond 48 hours, RIMpro will predict false-positives at times (as may be the case right now with the MeteoBlue forecast), or it may give false negatives as may be the case right now with the yr.no forecast.

For critical spray decisions such as many will be facing this week, it would always be wise to look at the weather data going into RIMpro  and then compare that with at least one other source of weather forecasts.  If you can wait to start spraying until 48 to 24 hours before the predicted infection event, then you will have more accurate predictions because the accuracy of weather forecasts is better when the prediction period is short.  However, I know that waiting until 24 to 48 hours before an event to make decisions often is not possible, either because of wind conditions or because of the time required to cover all acreage.

Right now, if I had my orchard at risk (i.e., without recent fungicide coverage), I’d be planning to spray on Wednesday given the risk levels as I see them, especially considering that the forecast includes warm temperatures (high 60’s Thursday and Friday) that will allow rapid bud growth, and given that the four weather forecasts that I have checked are evenly split on whether or not we will be getting enough rain and wetting for a major scab infection period.

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Follow-up on Scab Development

Rains today (March 10th) pose no threat for apple scab in the Hudson Valley because we still do not have any green tissue on apples buds.  According to one degree-day model*, McIntosh can be expected to reach the green tip bud stage after degree day accumulations starting from January 1 reach 97 to 147 degree days, base 43 (using the Baskerville-Emmons degree-day calculation model). I used the degree day accumulations shown on the NEWA website for the Hudson Valley Lab this morning and then used a spread sheet to add degree day accumulations expected for the next 10 days based on the Accuweather forecast.  Those calculations suggest that we should have green-tip sometime between Saturday, March 12th, and Thursday, March 17th.

I checked another batch of overwintering leaves from another abandoned orchard for spore discharge in the shooting tower on Tuesday and found no spores discharged from that sample. However, there’s a hitch that negates the value of that observation. Although the leaves were from an abandoned orchard just north of the lab, last summer had such light scab pressure that those leaves apparently had very little scab on them at the end of the season. As a result, I was unable to find any scab pseudothecia in these leaves when scanning them under the dissecting scope. Now I don’t claim to be an expert at spotting scab pseudothecia because throughout my career that job was handled by my very capable technician, Mr. Frederick Meyer. However, I do think that even with my limited skill-set, I would have been able to find at least one pseudothecium in those leaves if any had been present, considering that I spent an hour searching over the surfaces of nearly 30 leaves. The bottom line is that I have not duplicated my finding of mature spores as indicated in my post on March 7th, but I still believe that my observation of mature spores on March 7th is adequate to serve as a biofix for RIMpro.

Rains are predicted for early next week, March 14-15, and according to RIMpro as I have run it using virtual data from the MeteoBlue weather service, that wetting period could trigger a light scab infection if trees actually do reach green tip on Saturday. However, any such infection period (if the rain develops and trees have green tissue by then) would only be a concern in orchards with a lot of overwintering scab. Given our dry year last year, I suspect that most orchards have a very low inoculum load, and my observations of leaves from abandoned orchards suggest that the influx of spores from abandoned trees will also be low this year in the Hudson Valley. Furthermore, I suspect that green tip in most blocks in the Hudson Valley will be delayed until the middle of next week, thereby negating any scab risk from potential rains on Monday-Tuesday of next week.

The bottom line: It appears that apple growers can enjoy one more week-end before we need to worry about apple scab.

*The degree-day model referenced above is from the last page of Scaffolds Fruit Journal (13 April 2015) vol. 22, issue 3, where Art Agnello has posted the green tip date observed at Geneva over the past 26 years calculated as noted above and expressed as a range that is derived from the mean +/- one standard deviation.  Thus, as of 2015 the  range for green tip was 97-145 base 43F (i.e., the mean value is 121, with a STD DEV of 24).

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Scab season alert

We found the first mature apple scab ascospores this afternoon, 7 March 2016.  It appears that we may need to be ready for a really fast start to the scab season in 2016.

Dan Donahue, extension educator for the Eastern NY Commercial Horticulture Program, picked up leaves yesterday from several abandoned orchards north of Red Hook, NY, which is about 30 miles north of the Hudson Valley Lab. Because last year was very dry during late summer, we had some difficulty finding good scab lesions and scab pseudothecia in his samples.  However, I finally located a fat scab pseudothecium, the black globular structure embedded in leaf tissue that contains the asci and apple scab ascospores.

When I checked a squash mount of this pseudothecium under the microscope, mature ascospores were immediately evident within some of the asci (Fig. 1). Each ascus contains eight ascospores.  More importantly, as I watched, some of the asci began discharging their spores, a process that involves the sudden rupture of the end of the ascal tube with spores rapidly streaming out through that opening. For microscopic observations, the squash mounts are prepared in water and the pseudothecium is “squashed” between the slide and a glass cover slip.  Thus, the asci are in water and the ascospores are therefore ejected into the thin film of water  between the slide and cover slip.  When the tip of the ascus ruptures, there is a sudden and dramatic movement of the spores as they rapidly stream out of the ascus.

Earlier today, we had run a “tower shoot” in which wet leaves were placed into a spore tower and a vacuum was applied to the base of the tower so that any ascospores released would be drawn down through the tower and captured on greased slides. We did not find any released ascospores in our tower shoot, but that sample of leaves came from a Golden Delicious orchard and the leaves seemed to have very little over-wintering scab. The leaves in which I found active ascospores were from an unknown cultivar in a different block.  We will be testing leaves from yet another block just north of the lab tomorrow morning in an attempt to confirm the presence of mature ascospores.  However, conditions that prevailed for many of the past 60 days (daytime temperatures well above freezing with enough moisture to keep leaf litter wet) would favor early maturation of ascospores, so I am not too surprised that we found mature spores today.

What does the detection of discharging spores mean from a grower perspective? First of all, so far as I know there is no green tissue yet on any apple trees in the Hudson Valley, and scab is not a threat until we have both green tissue and a Mill’s infection period.  However, the Accuweather forecast for Highland, NY, for the next three days (Tues, Wed, Thurs) is calling for daytime highs of 65, 78, and 70 F before things cool down with predicted rains after Thursday of this week. Night time lows for those same days are predicted as 41, 52, and 48 F. Thus, both apple buds and spore maturity will move rapidly during the next three days, although I am uncertain whether trees will reach green-tip before the predicted rains for Thursday.

The real value of today’s report on mature ascospores is that this detection of mature spores is the “biofix” that we need to initiate the RIMpro scab program. Since the leaves we checked today were picked up yesterday and left in a cool truck over night, I will be entering March 6th (the collection date) as my biofix date for mature spores. Hereafter, assuming we can confirm mature spores from another orchard tomorrow, the RIMpro program will use temperature and rainfall records to predict scab risk.  I’ll post more about those predictions tomorrow, but remember that there is no scab risk in the orchard until we can see green tissue on the trees.

Apple scab ascospores 6 Mar 2016
Figure 1. Apple scab ascospores observed 7 March 2016 from a leaf sample collected on 6 March.

 

 

 

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RIMpro, A Useful Apple Scab Model for 2016

RIMpro contains a proprietary apple scab model that is useful for (i) identifying the start of the scab season, (ii)  quantifying risks associated with key infection periods between green tip and first cover, and (iii)  determining the end of the primary scab season. RIMpro will provide more accurate information on apple scab infection risk than the traditional pseudothecial squash mounts that have been used for many years to assess apple scab ascospore maturity in spring.  I’ve been working with RIMpro on a trial basis for the past two seasons, and I’m very impressed with the apple scab model. I suspect that over the next few years it will become as important for apple scab management as MaryBlyt has become from fire blight management.

More information about RIMpro is available in this PDF file.

posted 21 Jan 2016.

 

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Trunk Disease Problems on ‘Crimson Topaz’

Crimson Topaz is a scab-resistant cultivar from Europe that has been gaining in popularity because of its attractive appearance and good eating quality. However, we are learning that this cultivar is rather susceptible to several diseases that cause cankers on the trunks. In Europe, trees in some locations have proven susceptible to European apple tree canker, a fungal disease caused by Nectria galligena. However, European apple tree canker is relatively uncommon in eastern United States because it prefers maritime climates that allow infections to occur at leaf scars during long wet periods in late autumn and early winter. Note that Nectria galligena is a different species than Nectria cinnabarina, a fungus that is common in northeastern United States and that causes twig die-backs and cankers at pruning cuts.

Of more concern than Nectria are indications that Crimson Topaz may be quite susceptible to trunk cankers caused by Phytophthora species. Phytophthora usually causes root rot or crown rot on apple trees, but these diseases have become less common as the apple industry adopted rootstocks that are relatively resistant to Phytophthora. However, Phytophthora can also invade tree trunks directly if cultivars are highly susceptible and if spores from the soil are splashed onto the trunks. Historically, this was a severe problem with the Grimes Golden cultivar (see this 1939 report by R.C. Baines). Now, similar problems are being reported in a few plantings of Crimson Topaz.

The first report of Phytophthora problems on Crimson Topaz was relayed to me by Dr. Mike Ellis from Ohio State, but similar problems have since been reported for other locations across the Northeast. Phytophthora is probably endemic in most orchard soils, so we don’t know why most plantings of Crimson Topaz remain healthy while others experience catastrophic tree losses (up to 50% or more?) within four to six years after planting.

Several strategies can be suggested for minimizing the risks of Phytophthora infection, but I don’t think that any of these strategies have been vetted in actual field trials with Crimson Topaz. Nevertheless, reports suggest that affected blocks can decline very quickly, so it may be wise to apply preventive measures to existing plantings rather than opting to delay control measures until the first trees show symptoms. Four strategies for dealing with this disease threat are listed below:

1. The easiest and presumably the most effective strategy is to protect Crimson Topaz trees from infection by treating them with phosphite fungicides twice per year. Phosphite fungicides are very effective against many Phytophthora species. Following foliar applications, these fungicides are rapidly translocated throughout trees. Residues within trees dissipate very slowly and therefore provide extended periods of control from a single spray. Optimal timing has not been determined, but I would suggest treating Crimson Topaz with a phosphite fungicide sometime between bloom and first cover (after trees have enough leaves to ensure good uptake) and again in late summer.

2. One consultant in Europe reports that growers there are resolving the problem by top-working Crimson Topaz on trunks of some other cultivar so as to put more distance between the soil and the susceptible Crimson Topaz portion of the tree. This solution is only useful for those who are already thinking about using top-working to change cultivars. Growers who top-work trees should always check with nurseries to determine the procedures for working with patent-protected cultivars.

3. Organic growers cannot use phosphite fungicides, but they may be able minimize the risks of Phytophthora infection by surrounding trees with a wood chip mulch so as to prevent soil (and the Phytophthora spores contained in the soil) from splashing onto the tree trunks.

4. Planting trees on slight berms will not eliminate the risk, but it could help to ensure that water will not puddle around tree trunks even during heavy rains. Water pooling beneath trees will almost certainly increase the risks that Phytophthora spores will be splashed upward onto the trunks.

Finally, the fact that Crimson Topaz is apparently susceptible to Phytophthora trunk cankers does not mean that this cultivar should be avoided. Rather, this disease alert is intended to ensure that growers who like this cultivar can continue to grow it without risking tree loss from a heretofore uncommon trunk canker pathogen.

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