Tree Fruit Diseases: Observations and Archives

Can frost events trigger fire blight? Frost injury is considered a fire blight “trauma event”, but very little is known or published about exactly what conditions are required to trigger fire blight during a frost event.  Because frost was widespread throughout New York and New England on the morning of May 18, there is at least a slight risk that trauma blight may be appearing in some orchards next week. However, frost-induced fire blight is so rare that apparently no one thought about it on the days just before or after the frost when a strep spray might have been useful. However, as outlined below, even if I had thought of the potential for frost-induced trauma blight, I probably would not have recommended applying strep because the the late timing and dry weather associated with this year’s frost event may have negated most of the risk.

Frost can create an infection event for Erwinia amylovora because frost injury provides entry points and because melting ice crystals in fruitlets and shoot tips create a slight “suction” as they melt, thereby increasing the likelihood that bacteria will be drawn into the injured tissue. Limiting factors for frost-induce fire blight presumably involve presence/absence of abundant inoculum on the surface of host tissues at the time of the frost as well as other poorly understood effects of the environmental conditions before and after the frost event.

Over the course of my career, I have seen at least one, and perhaps two, devastating fire blight outbreaks in apples related to frost trauma. The first occurred in the early 1980’s south of Gardiner, NY, at the time when Dr. Paul Steiner was developing his MaryBlyt model.  I was sending Paul data to use in building his model, and the outbreak near Gardiner did not fit any scenarios for blossom blight infection. However, Paul noted that a frost event had occurred, and he used the timing of symptom appearance to trace the infection back to that frost event (which occurred during bloom, as I recall). I saw the second probable frost-related blight outbreak in Quebec about a decade later.  In both cases, fire blight was so extensive that it could not be pruned out and in some cases, orchards were removed due to severity of the fire blight later that season.

After spending several hours looking at the weather data and fire blight predictions for numerous locations on the NEWA website, I can summarize as follows:

1. Most locations in the Hudson Valley and southern New England were registering EIP’s of 100 to 250 on May 16, but the EIP levels dropped by about a third on May 17 due to cooler weather. An EIP >100 is required to trigger blossom blight when moisture is present, but no one knows whether enough inoculum may be present even at lower EIP levels to allow infection via frost injury. The NEWA EIP model showed only high risk, not infection, for fire blight on May 15-16 because weather was dry. Risk levels for Cougar Blight were more elevated.
2. Note that the program used to calculate EIP zeros out the EIP number after a frost. However, that does not mean that the risk of frost-induced trauma blight was also zero at the time of the frost. Bacteria not drawn into frost-damage tissue apparently do not survive frosts well enough to be of concern after the event, but the potential for frost-induced trauma blight should be judged based on the EIP the day before the frost.  As noted above, we don’t know if bacteria can trigger frost-induced infections even when EIP values are below 100 at the time of the frost. However, I suspect that risks of frost-induced trauma blight are rather low when the EIP is below 100 at the time of the frost (in this case, on May 17), although presence of oozing cankers may override the importance of EIP during frost events.
3. In the lower Hudson Valley and southern New England, most trees were well past petal fall by May 18. The fire blight bacterium multiplies primarily on the stigma of flowers, and after bloom that site is no longer available.  Thus, inoculum levels at the time of frost on May 18 should have been quite low EXCEPT THAT ….
4. Orchards with active cankers from previous years might have had high inoculum levels on shoot tips and fruitlets due to dissemination of bacteria from oozing cankers by various insects. Also, newly planted orchards and cider apple cultivars may still have had open flowers on May 18, and those could also have contributed inoculum for frost-induce trauma blight.
5. Orchards with actively oozing cankers AND open flowers were almost certainly at high risk on May 18. In the Gardener, NY, frost event noted earlier, we found large cankers in several old trees that may have contributed inoculum for that devastating frost-trauma event. Also, Decker and Daemen (1999) noted that presence of over-wintering cankers appeared to be critical for frost-induced blight infections on pear trees in Europe.

In conclusion, I suspect that risks of frost-induced blight are relatively low this year because the frost occurred after bloom (in most orchards), weather was dry during the preceding days, and inoculum levels therefore should have been too low to allow frost-induced infections. However, growers should be alert for symptoms of fire blight next week (May 29-June 3). That’s the good news.  The bad news is that frost-induced fire blight, when it appears, may be so severe that it will be impossible to remove infected shoots via pruning. If that occurs, I see only two options. On young trees (<8 years old), it may be possible to restart badly infected trees by cutting off the entire top of the trees below the last branch if such pruning is done before the bacteria can be translocated down into the lower trunk or to the rootstock (if it is a blight susceptible rootstock). Detail pruning to salvage tree structure will likely be a waste of time on young trees severely affected by frost-induced trauma blight. On older trees that have settled down, it may be best to walk away and allow blight to take its course if there are scores of infections per tree, then salvage what remains during winter pruning next year. In both cases, if severe infection is limited to just a small number of trees, then removing those trees is the best option.

More suggestions on using the NEWA fire blight model:  For those who wish to check their local conditions around the time of the frost event, the following may be helpful.

1. On NEWA, go to the “Crop and IPM tools”, then scroll down to “Fire blight”.
2. Enter a NEWA station location.
3. When the fire blight page opens, if you wish to see the EIP levels prior to the frost, enter a blossom date. The actual date is not important at this point, so just enter April 30.
4. For “Date of interest”, enter May 17 and the table will show EIP levels for May 15-22.
5. If you wish to see effects of strep sprays applied prior to the frost, scroll down past the infection and wetness tables and enter a spray date for strep, then scroll back up to the EIP table to see how strep applied on that date would have affected EIP levels. For locations that I tested, it appears that strep applied any time after May 12 should have kept the EIP below 100 right up through the frost event on May 18. (Not sure how relevant that is?)
6. If you wish to see WHEN frost-induced trauma blight might appear in your orchard, clear the date for first blossom opened, click on May 18 as the date of interest (while leaving first blossom date blank), then scroll down to “Scouting for shoot blight associated with a trauma event” and enter May 18 (the date of the frost). That will trigger a predicted date for appearance of trauma blight symptoms. For some reason, the option for entering a trauma blight date disappears if you enter a date for first bloom.

One final comment: Frost events can also trigger shoot blight caused by Pseudomonas syringae, a bacterial pathogen that likes cool damp weather. On pears, this causes pear blast disease, which looks very much like fire blight. A similar blast disease caused by P. syringae has very occasionally been seen on apples following frost injury. I suspect that the dry weather prior to the May 18 frost and the lack of flowers on most trees at the time of the frost did not favor development of Pseudomonas infections this year.

Nevertheless, it is important to distinguish pear blast from fire blight. Pear blast will NOT spread beyond the current season growth and will not produce ooze droplets on infected tissue like fire blight does. Removal by pruning is not important for managing pear blast, whereas removing fire blight strikes via pruning is advised unless the number of infections is so large as to make it unfeasible.

Literature cited:

Deckers, T. and Daemen, E. 1999. Determining factors reducing the host susceptibility of fruit trees for fire blight infections. In: Proc. 8^th International Workshop on Fire Blight, Momol and Saygili (eds.) Acta Horticulturae 489:483-489

Note: This article was first published in Scaffolds Fruit
Journal 29:1 (23 March 2020) but some of the hyperlinks
in that article did not function correctly.

Attention to a few disease-control details in early spring can make life easier through the rest of the summer. Following is my list of reminders:

For apple scab and Marssonina: In blocks or around trees where these diseases were a problem last year, disease control this year will be much easier if over-wintering leaves can be removed, shredded with a mower, or sprayed with urea prior to bud break. The latter two approaches will speed decay of the leaves, thereby destroying the overwintering phases of the fungi before the full complement of spores can be released. Details for these processes have been described elsewhere (Rosenberger, 2005; Acimovic 2020). Where neither of these diseases were a problem last year, there is no reason to spend extra time and money on managing leaf litter.

For fire blight: Where fire blight was a problem last year, all diseased twigs, branches, and cankers should have been removed during winter pruning. The pathogen in winter prunings will lose viability as the prunings dry out. So far as we know, there is no risk of spreading the pathogen by shredding the prunings with a flail mower. After winter pruning to remove cankers, the next line of defense is provided by a copper spray applied at silver tip or green tip. The preponderance of evidence suggests that new low-rate coppers provide relatively short residual activity on trees (Rosenberger et al., 2013).  For the silver tip spray, we generally suggest copper products that can be applied at rates that will result in application of at least 2 lb of elemental copper per acre, thereby providing more extended protection. In years with a late spring when trees are expected to progress rapidly from green tip to tight cluster, using lower rates of copper may help to reduce the risk of fruit russetting that can occur if copper residues persist too long and injure tissue at the base of flower buds that later develops into fruit. However, weather so far this year suggests that we may have a more drawn-out spring were higher rates of copper will prove beneficial for controlling fire blight in problem blocks. In blocks where no fire blight has been found in either of the past two years, there is little if any reason to apply copper. Copper is used to suppress bacteria emerging from obscure infections that remain in the orchard after pruning. If there has been no evidence of fire blight for two years, then presumably there are no active blight infections and therefore no targets for the copper sprays in apples or pears.

For bitter rot: In blocks were bitter rot was a problem last year, remove all fallen fruit, fruit mummies, and pruned twigs from beneath trees and either dispose of them away from the orchard or flail-chop them in row middles to break them down for more rapid decay. Rotted fruit left on the orchard floor over winter have been recognized as inoculum sources since 1903 (Schrenk and Spaulding, 1903; also see commentary at https://blogs.cornell.edu/plantpathhvl/apple-diseases/summer-diseases/bitter-rot/ ). Twigs pruned from trees last summer or this spring can be colonized by the bitter rot pathogens (Fig. 1). Those colonized twigs may produce inoculum for fruit decay in summer.

For crab apple pollenizor trees: Where crab apples are used to enhance cross pollination, any cankers in the crab apple trees should be removed during winter pruning. In Washington State, cankers on crab apple pollenizer trees have been identified as the inoculum sources for several important postharvest pathogens of apples and pears (Xiao et al., 2014). The diseases identified on the west coast are not very common as postharvest pathogens in eastern apple orchards, perhaps because those diseases are suppressed by the extensive summer fungicide programs that we must employ to protect fruit from flyspeck and other summer fruit rots. Nevertheless, it is unwise to allow fungal cankers to persist within orchards even if they are found only on pollenizer trees.

Black knot: For those growing plums and tart cherries, any black knots (Fig. 2) found in trees should be pruned out before trees reach the white bud stage by cutting at least 8 inches below the existing knot. The knots that are removed should be either burned immediately, buried, or bagged for disposal with other trash that is sent to a landfill.  Do NOT dispose of the pruned-out knots in a hedgerow beside your orchard: Spores from the prunings will blow back into the orchard and cause new infections. (Even though I should have known this, I learned it the hard way in a research block many years ago!) Removing black knots from infected trees is the primary method for controlling this disease. Fungicides applied during bloom will NOT protect trees when there is an abundance of inoculum from knots that were not removed during winter.

Peach leaf curl: Peach leaf curl (Fig. 3) is relatively easy to control using copper sprays or one of the labeled fungicides that are listed in Cornell’s Pest Management Guidelines for Commercial Tree Fruit Production. Applications can be made during leaf drop in autumn or at bud swell in spring. If the peach crop was frozen out last year and no brown rot fungicides were applied to protect fruit last summer, then trees may be more prone to leaf curl this year. It seems that fungicides applied during summer to control brown rot reduce survival of leaf curl during summer and thereby reduce disease pressure for the following year. Summer fungicides alone are not enough: they must still be supplemented with either the leaf fall or the swollen bud application recommended for controlling peach leaf curl. Nevertheless, leaf curl disasters in the Hudson Valley have often been associated with orchards where the leaf curl spray was omitted or mis-timed in a year following a frozen-out crop.

Leaf curl sprays in spring should not be applied before buds are swollen because the leaf curl fungus overwinters in the buds and fungicides applied to fully dormant buds can be washed off before the buds swell enough to allow the fungicide to contact the overwintering fungus. Leaf curl sprays applied after buds have opened may be less than 100% effective, but applying a leaf curl spray a bit late is better than not applying any control at all.

For all of the above: “A stitch in time saves nine.”  Stopping or slowing diseases before they get started is much easier and more cost-effective than trying to stop them after they begin to spread.  For an apt illustration, check your local news feed for commentary on the coronavirus.

Literature cited:

Aćimović, S.G. 2020. Reduce Inoculum of Overwintering Fungi in Apple Leaf Litter: Marssonina coronaria & Venturia inaequalis.

Rosenberger, D.A., Rugh, A.L., Feldman, P.M., and Truncali, D.N. 2013. Comparison of copper products applied at green hip to control fire blight on apples. Pages 7-10 in  blogs.cornell.edu/dist/d/3767/files/2013/11/2013-HVL-Field-trial-data-compr-203ay9s.pdf

Rosenberger, D.A. 2005.  Jump starting apple scab control programs in high-inoculum orchards. Scaffolds Fruit Journal 14(1): 3-5. http://www.scaffolds.entomology.cornell.edu/2005/050321.html#disease

Schrenk, H.V. and Spaulding, P. 1903. The bitter rot of apples. USDA Bureau of Plant Industries Bull. 41. 63 p. https:// blogs.cornell.edu/dist/d/3767/files/2017/02/Shrenk-and-Spaulding-1903b-Bitter-rot-of-apple-18k8awc.pdf

Xiao, C. L., Kim, Y. K., and Boal, R. J. 2014. Sources and availability of inoculum and seasonal survival of Sphaeropsis pyriputrescens in apple orchards. Plant Dis. 98:1043-1049.  (Note: color photos in the middle of the article are useful: https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-12-13-1218-RE )

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 1^st  and that are predicted to continue through Saturday, May 7^th 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 29^th 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.

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.

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 28^th, 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.

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.

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.