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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Time to Clean Up Apple Storage Rooms

Monday, 28 July: The few remaining weeks before apple harvest begins provide an ideal time for cleaning and sanitizing apple storage rooms. All storage rooms should be swept clean and washed out with water to remove any dirt and debris on the floors. However, cleaning with water will not eliminate the fungal spores from the Penicillium species that cause blue mold decay in stored apples. Blue mold accounts for most of the losses to postharvest decays during apple storage, and most apple storage rooms contain at least  moderate populations of Penicillium spores even after rooms have been swept and washed. The spores survive washing by becoming airborne while the room is being washed, then settling back to the floor when there is no longer any air movement in the room.  They remain on the storage room floors until the cooling fans are turned on in the fall, at which time they become airborne again and are available to infect wounds in any freshly harvested apples that are moved into those rooms.

In an article that appeared in today’s issue of Scaffolds Fruit Journal, I describe some options for sanitizing storage room floors and the rationale for doing so.  As pointed out in that Scaffolds article, quaternary ammonium sanitizers (quats) have been considered the best option for sanitizing hard surfaces such as bins and storage rooms. Unlike chlorine solutions or peroxide-type sanitizers, the quats are more stable and provide some residual activity so that spores landing on treated surfaces after the treatment has dried may still be inactivated.

However, the European Union has recently enacted a maximum residue limit of 0.5 ppm for quats (see this for a list of EU countries and this for the EU document). No one has enough data to know if a quat sanitizer applied to a storage room floor could later be transferred on bin runners in sufficient quantity to generate excessive residues on apples as the bins are lowered into water flotation tanks on grader lines. Because of this uncertainty, storage operators who expect to export apples to the EU may wish to avoid using quats in their storage rooms or other fruit contact surfaces until more information becomes available.

The extremely low residue limit established by the EU should not be construed as an indication that residues from quats pose a health risk. Rather, it appears that the EU instituted a very conservative residue tolerance primarily because quats have been considered so safe that they have not undergone much testing. The EU uses the precautionary principle of establishing very low limits in situations where companies have not provided a full regimen of safety data.

Finally, it is worth noting that bacterial pathogens associated with food safety concerns are much more easily controlled with sanitizers than are spores of Penicillium species. Penicillium spores can survive for several years on dry surfaces, are easily dislodged and moved around by air currents, and require higher doses of sanitizers and/or longer exposure times for effective control. Thus, any treatment that eliminates Penicillium spores should be more than adequate for controlling any bacteria that might otherwise survive in storage rooms.

Penicillium spores do not in themselves pose any risks to human health, but high spore concentrations in storage rooms can result in higher incidences of fruit decay during storage. Fruit infected with Penicillium species often contain the mycotoxin known as patulin, and patulin levels in cider are regulated by law. Therefore fruit with blue mold decay should not be used in cider production.

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A Flood of Fungal Problems?

Friday, 4 July 2014:  Storms over the past three days have provided >4 inches of rain throughout much of the Hudson Valley, and those rains may trigger major disease-control problems in orchards where early-season disease control was less than 100% effective.  (At least the rains have erased for now the water-stress concerns that I wrote about just 4 days ago!)

Apple scab: Rains on 2 July probably removed most of the fungicide protection from apples, at least in the parts of the lower Hudson Valley that had more than 2 inches of rain on Wednesday.  Orchards where no scab was evident on leaves prior to these rains should not be at any risk of scab through the remainder of the season.  However, orchards that  had a little scab showing on leaves (I’ll call them at-risk orchards) might end up with some fruit scab as a result of the second deluge that began yesterday and continued this morning. Hot weather earlier this week, along with captan residues from the last fungicide application, will have reduced the numbers of viable scab conidia that were available at the start of the recent rain events, but apple scab can “revitalize” quickly in warm damp weather.

Where DMI fungicides are still effective , it might be wise to treat at-risk orchards with Inspire Super plus Captan as soon as possible and hopefully within 72-96 hours from the start of the rain on Thursday.  In the absence of DMI-resistance, Inspire Super should provide 96 hr of post-infection activity, and it will also provide excellent protection against summer diseases. If Inspire Super has already been applied 5 times this year (the maximum number of applications allowed per year), then Indar plus captan would be a viable alternative.

For at-risk orchards where DMIs are no longer effective, the best alternative may be Captan plus Merivon or Captan plus Luna Sensation (except in NY where the latter is not registered). Both Merivon and Luna Sensation should provide at least 48 hours of post-infection activity on leaves, although I’m not certain if anyone knows exactly how much post-infection activity they may have on fruit. Both Merivon and Luna Sensation will also provide excellent protection against summer diseases.

A third option for at-risk orchards would be Captan plus Syllit (with Syllit at 3 pt/A since the highest label rate is required for maximum post-infection activity). That option is only available if Syllit has not yet been used in three applications this season, and the disadvantage of Syllit is that it will not provide control of summer diseases. Furthermore, I am a bit concerned about the potential for fruit injury from Syllit-Captan combinations when they are applied in hot weather, although fruit at this point will be less susceptible to damage than they would have been in early June.

Finally, Flint plus Captan applied within 48 hours from the start of the rain may also prevent fruit scab. However, Flint and other strobilurins have generally been less effective than the combinations mentioned above for arresting scab after inoculum levels are high. Because the hot weather prior to our current rains may have inactivated most of the scab conidia, Flint may be sufficient to protect fruit in this situation, but there is a degree of uncertainty about efficacy of Flint in this situation.

Phytophthora root rot: If rains resulted in orchard flooding for even a few hours, then there is a possibility that Phythophthora could begin root infections in susceptible rootstocks. For apples, the risk of infection is greatest for trees on M.26 since the rest of the common rootstocks are fairly resistant. (MM.106 is quite susceptible, but most trees on this older root stock are now so large as to be at lower risk). Trees are M.26 are at greatest risk in their early years when the trunk diameter is still less than 5 or 6 inches.

Where Phythophthora is a concern, a phosphite fungicide should be included in the next spray.  Phosphites are very effective against Phythophthora and are quickly translocated from leaves to roots.  So far as I know, phosphites can be safely tank-mixed with most other pesticides except for copper-containing fungicides. If a phosphite was applied during June, then trees are probably still protected from that earlier application.

Phosphites can also be used on peaches and other stone fruits where there is a concern about Phythophthora.  Labels for the many available phosphites vary, so check to be certain that the product purchased is labeled for this disease on the crops to which it will be applied.

Cherry leaf spot: Sweet and tart cherries that already had a few leaves in the tops of trees turning yellow due to cherry leaf spot will be at considerable risk for secondary spread of this disease. Failure to control cherry leaf spot will result in premature defoliation that can lead to subsequent winter injury. Apply a fungicide as soon as possible after the rain stops. Most of the registered fungicides (other than captan) should provide at least a day or two of post-infection activity, but repeated use of DMIs for brown rot control may have contributed to DMI-resistant leaf spot in some orchards.  

Where cherry harvest has been completed, or where rains have ruined the crop, Syllit can be applied for leaf spot control. Syllit provides a different chemistry than products used for brown rot control and therefore should still be highly effective in most Hudson Valley orchards. However, Syllit will not control brown rot. Cracked fruit that are not protected with fungicides will mummify on the tree and provide inoculum for brown rot in the future. Note that Syllit has a 7-day PHI, so it cannot be used at this point in the season unless cherry harvest has been completed.

Fabraea leaf spot on pears (and quince): Reports I’ve heard suggest that this has been a tough year to control Fabraea on pears. The current rain patterns will further complicate matters for orchards where Fabraea was already evident. I don’t know if any of our fungicides have much post-infection activity against Fabraea, but Syllit is probably the best bet for the next spray if Syllit has not yet been applied three times this season. Flint and other strobilurin-containing fungicides provide some protection, but they have generally been less effective than Syllit.

Where Fabraea infections are already visible on leaves or fruit, one percent spray oil should be added to all fungicide applications through the remainder of the season. Research at the Hudson Valley Lab has shown that oil does not prevent infection, but it  significantly reduces the number of viable conidia that are produced in lesions. Including spray oil in every application after the first Fabraea lesions appear can slow the spread of the disease and help prevent premature defoliation. Repeated applications of oil may cause some lenticel enlargement on young shoots, along with other indications that the trees are not really happy to be repeatedly coated with oil. Because repeated oil applications cause some phytotoxicity and because oil does not prevent infection, oil is not recommended until the first lesions become apparent. Nevertheless, a bit of phytotoxicity from oil is far better than having trees with no leaves at the end of July. Remember that when using spray oil, the oil should be applied at 1% of whatever spray volume is being applied: oil should not be concentrated beyond 1% in the tank.

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Apple Summer Diseases, Herbicide Problems, and Irrigation

Monday, 30 June 2014:

In case you missed them, I recently published two articles in Scaffolds Fruit Journal that I will not duplicate here.  The article last week (23 June) was titled “Controlling Summer Diseases on Apples” and focused on spray timing and fungicide selection for controlling sooty blotch and flyspeck.  The article published today was titled “Controlling Summer Diseases on Apples — Part II: Bitter Rot” and describes my contention that maintaining adequate soil moisture during heat waves may be critical for controlling bitter rot on highly susceptible cultivars such as ‘Honeycrisp.’

Brad Majek, weed scientist in New Jersey, recently published a warning about potential phytotoxicity to tree fruits from glufosinate herbicide (Rely and generics). His short article appeared in the Rutgers Cooperative Extension Plant and Pest Advisory. One of the photos in Brad’s article looks very similar to the damage that I continue to see in many Hudson Valley apple orchards.

I’ve suggested in the past that glyphosate (Round-up and generics) may be causing trunk damage in some orchards. I continue to believe that glyphosate can be especially damaging to some cultivars of apples (e.g., Macoun) when those cultivars are grown on light or gravelly soils. My hypotheses were published about 10 years ago (Scaffolds, June 14 2004*), but we have never been able to actually prove that glyphosate caused the basal trunk cankers that were described in the 2004 article. Nevertheless, I am aware than many growers who observed similar trunk damage in their own orchards have switched away from glyphosate to alternative herbicides, and some have undoubtedly switched to glufosinate. The article and photos by Majek suggest that switching from glyphosate to glufosinate may be somewhat akin to jumping from the frying pan into the fire.

It is very clear, after many years of orchard use, that NEITHER glyphosate nor glufosinate cause trunk injury to apple trees every time that they are used or in every orchard in which they are applied. Nevertheless, observational evidence suggests that both of these products can damage trees under some conditions. The exact conditions that contribute to damage have not been defined, but I continue to suspect that the potential for damage is significantly higher if tree trunks are hit with either of these herbicides during or just prior to periods of drought stress. The additional desiccation from herbicide exposure combined with normal water stress during hot dry periods may predispose the trunks to invasion by Botryosphaeria dothidea, a canker pathogen that is incapable killing the cambium in healthy functioning trees, but which becomes very pathogenic in drought-stressed trees. In some cases, I suspect that gramoxone can also contribute to similar trunk damage, especially on young trees.

Ultimately, the best way to avoid herbicide damage on apple trunks is to avoid hitting the tree trunks with the herbicide sprays.  However, that is virtually impossible because avoiding the trunks would allow some weeds to persist around the trunks and in the line between adjacent trees.  A more feasible approach may be to avoid application of herbicides during periods when trees are already experiencing water stress or where water-stress can be expected in the near future (e.g., ahead of heat waves where temperatures are predicted to exceed 90 °F). If water stress is mitigated via trickle irrigation, then I suspect that trunk injury from herbicides will also be less likely than in orchards where trickle irrigation has not been  installed or where installed irrigation systems are not turned on soon enough.

Remember that in super-spindle orchards with trees planted 3 ft apart within the row and rows only 10-12 ft apart, the  root zone for each tree is limited to roughly a 3-ft by 3-ft plot. Completion between trees will limit root growth along the row and soil compaction from tractor traffic in the narrow rows will limit root growth into the row middles. If natural rainfall in any given region would not provide enough water to reliably maintain a productive apple tree in a 3-ft by 3-ft container, then it hardly seems logical to expect high density plantings to perform well in the field without supplemental irrigation. As noted at the beginning of this post, irrigation may be important for controlling bitter rot on some cultivars in some regions. The probability that high-density plantings without irrigation will encounter more problems from herbicide-related trunk cankers provides another reason to consider irrigation, especially for plantings located on shallow, light, or gravelly soils that have limited water holding capacity. Ultimately, most modern high-density apple orchards are going to need trickle irrigation to optimize productivity and avoid disease problems.

*Note: Links to photos in the 2004 Scaffolds article are no longer functional, so the photos associated with that article have been inserted below.

Fig 1A. Basal trunk canker on Macoun apple tree
Fig 1B. Evidence of killed tissue on the same tree
Fig 2A. Small trunk canker caused by B. dothidea
Fig 2B. Cankers from B. dothidea on a smaller limb

 

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More on Fire Blight

24 June 2014: As noted in my last post 10 days ago, fire blight is apparently pretty easy to find in the Hudson Valley this year.  Several people have told me that younger trees that were protected with streptomycin sprays are generally in pretty good shape, but many folks figured that older blocks of Empire, McIntosh, Stayman, Jonamac, etc. wouldn’t get  blight because they were considered less susceptible than younger trees and/or cultivars such as Honeycrisp and Gala that had blight problems in previous years (when all those Gala and Honeycrisp blocks were still young trees).

This year in the Hudson Valley, the number of degree-hours during bloom was more than double the minimum required to initiate blossom blight, and weather during bloom was very humid even on days when we had no rain. We have known for many years that when  fire blight models show heat unit accumulations at 2, 3, or 4 times the minimum required for infection, then blight tends to show up in more and more places, including in older trees and on cultivars that rarely show blight in years when degree-hour accumulations stay at less than 1.5 times the minimum threshold. Thus, I fully expected this to be a very bad fire blight year.  Nevertheless, I am surprised to hear that blight is showing up on mature trees on some relatively isolated farms that have not seen ANY fire blight for more than 20 years. I had suspected that fire blight inoculum was totally absent on many farms in the Hudson Valley because many farms never got blight even when they applied no streptomycin.  However, the wide spread occurrence of blight this year suggests that either I was wrong or else fire blight bacteria have an uncanny ability to migrate very quickly into areas where weather favors a fire blight party!

At this point, the only solution is to prune out the blight infections unless one wishes to try the combination of Double Nickel plus Cueva that I mentioned in my last post.  Incidentally, after my post of 14 June, several people told me that Double Nickel can work with copper because the active ingredients in Double Nickel are metabolites generated by bacterial growth in the fermentation facilities where this product is produced rather than the live bacteria in the formulated product. Apparently these bacterial metabolites are not degraded by copper.  In fact, the combination of Double Nickel and Cueva have apparently worked better together than they do independently, not only in the fire blight trial mentioned previously, but also in trials with several bacterial diseases of vegetable crops as well.

As I indicated in my last post, I don’t think that we will ever find a completely “safe” copper for apples, meaning that I suspect all copper formulations are capable of causing at least minimal damage to fruit of some cultivars if the copper is applied during summer under slow-drying conditions and/or with the wrong surfactants.  However, damaged fruit may be better than dead trees.  The latter is especially true if you have older trees with considerable blight immediately adjacent to a young block of a valuable cultivar (e.g., Honeycrisp, SweeTango, Snap Dragon, etc.) that was kept blight free via strep sprays during bloom.  Where old trees with active fire blight are next to younger trees that will continue growing into August, it may be worth risking fruit injury in the older trees by spraying them with Double Nickel plus Cueva so as to reduce the inoculum that will otherwise be blown into the adjacent young orchard. Young trees will remain susceptible to shoot blight infection so long as they are producing new terminal leaves. Spraying the young trees with copper may help, but the growing shoot tips will quickly outgrow the copper protection.  Applying copper to the adjacent older trees will help to reduce the inoculum that could otherwise result in heavy infection in the nearby younger trees.

How far will blight bacteria travel from infected trees?  That question cannot be answered because the distance is totally dependent on the kinds of storms that will move through the region over the next four to 6 weeks.  Gentle rains can generate a lot of airborne bacteria, but that bacterial aerosol probably will not move very far in the absence of high winds. However, a violent thunderstorm with gusty winds can easily carry inoculum a mile or more in sufficient quantity to destroy blocks of young trees that have the misfortune of being directly downwind. (I’ve seen this happen in the past!!) When blight is widespread in a region, it is almost impossible to totally prevent continued spread of fire blight that may occur during violent storms.  However, if weather over the next few weeks brings us only gentle rains, then using inoculum reduction (i.e., Double Nickel plus Cueva) in older trees immediately adjacent to younger blocks might prevent losses in the younger trees.  Incidentally, inoculum production will stop (or at least be drastically reduced) when those older trees set terminal buds. Even though young trees may remain susceptible to blight into mid-August because they will continue growing through summer, the risk of blight infection will drop off after adjacent older trees set terminal buds (assuming that those older trees are the primary source of inoculum).

Finally, a few more warnings about using copper during summer:

1.  NEVER combine copper with phosphite fungicides or acidifiers (e.g., LI-700) in the  spray tank. Phytotoxicity of copper is increased as pH is lowered.

2.  Yellow apple cultivars, especially Golden Delicious, seem to be more prone to injury from copper than most red-skinned cultivars.

3.  Phytotoxocity potential of copper sprays can be reduced by applying the copper only to dry leaves under fast-drying conditions and with low volumes of water (50-80 gal/A for medium sized trees) so that the spray droplets dry quickly and do not coalesce on fruit surfaces.

Posted in Fire blight info | Comments Off on More on Fire Blight

Dealing with Fire Blight

14 June 2014: Fire blight is causing problems across a broad swath of the mid-Atlantic and Northeast.  Dealing with fire blight at this time of year is always difficult.  Penn State Extension just published an excellent article by Dr. Kari Peter that is worth reading (click here). Of particular interest is her comment that Keith Yoder found that applications of Double Nickel plus Cueva (a copper product) helped to arrest spread of fire blight during summer in a test he conducted last year and reported in Plant Disease Management Reports.  Keith applied Double Nickel at 8 fl oz/100 gal of dilute spray plus Cueva at 2 qt/100 gal, and he made those applications at roughly monthly intervals.

I’m still a bit leery of applying any copper during summer to fresh market apples because of the risk of fruit injury, and frankly I don’t understand how one can mix copper (i.e., Cueva) with a biocontrol (Double Nickel) because one would assume that the copper (a biocide) would kill the bacteria in the biocontrol as soon as they are mixed in the spray tank.  Nevertheless, we have few options for slowing the spread of fire blight during summer, so this option may be worth considering if one is willing to risk some fruit injury (even though fruit injury was not a problem in the published test).

Remember that to minimize risks of fruit injury with copper sprays, applications should be made to dry leaves under rapid-drying conditions.

 

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Apple Diseases Appearing Now

Thursday, 29 May 2014: Despite my dire predictions in earlier blog posts, apple scab has been very slow to appear in the Hudson Valley this year. Severity of anticipated infection periods in mid-April was diminished when temperatures ended up being lower and/or wetting periods were shorter than weather forecasters had predicted.  However, the RIMpro model for apple scab that we began evaluating this year indicated that we had an “off-the-charts” infection period that started on 29 April (see graph below) followed by two more later in May.  The RIMpro model has been developed, extensively evaluated, and widely used in Europe for many years. For spore maturity predictions in the Hudson Valley, RIMpro accessed weather data from the Highland NEWA station to predict ascospore maturation and discharge as well as the potential for infection based weather during the infection period.

RIMpro scab data 29 May 14 copy

Once again nature has defied our expectations because neither I nor local consultants were able to find scab on unsprayed trees prior to last weekend (May 26-27). Even now the lesions appearing on unsprayed McIntosh trees at the Hudson Valley Lab are still rather faint (Fig. 1 below). However, starting earlier this week, consultants began finding scab in commercial orchards where spray coverage was inadequate during at least one of the critical infection periods in May. The position of infected leaves, both here at the lab and in commercial orchards, and the timing of lesion appearance indicates that the infections that appeared this week probably occurred May 9-11, not in late April, because the leaves showing infections had not yet formed in late April.  Thus, the predicted scab infection for late April apparently failed to result in detectable infections this year, perhaps because that infection period included 2.35 inches of rainfall and the heavy rain may have washed most spores out of the air and/or off of leaves before the infection process could be completed.

That was not the case with cedar apple rust, however, which is now visible on cluster leaves and fruit (Fig. 2) in plots at the Hudson Valley Lab where we have cedar trees planted between our test plots to ensure uniformly high levels of inoculum.  Cedar apple rust lesions are sometimes evident on fruitlet stems at this time of year where they are easily identified by the bright yellow color of the lesions (note the arrows in Fig. 2).  Quince rust, which I have not yet seen this year, can also infect fruit stems, but it causes distortion of the stems (Fig. 3) rather than bright yellow dots of color.  In some years we also find apple scab on fruitlet stems (Fig. 4).

When infections occur on flower pedicels and disease symptoms appear on fruitlet stems, the infections usually cause the fruitlets to die and abscise before harvest. In 1998 we were evaluating numerous cultivars for scab susceptibility and therefore left trees unsprayed during the early part of the season. That year the incidence of scab on fruitlet stems was so high that almost all of the fruit dropped from scab-susceptible cultivars before we could collect fruit for disease evaluations.

Although scab was slow to appear this year, we anticipate that it will rapidly become more apparent where fungicide coverage was inadequate. Where primary scab is present, fungicides will still be needed at weekly intervals for a few more weeks in order to prevent secondary infections of fruit and new leaves.

Posted in Apple diseases: general info, Apple scab info | Comments Off on Apple Diseases Appearing Now

Dealing with Fire Blight after Bloom

This gallery contains 9 photos.

Wednesday, 28 May 2014: The first fire blight symptoms are now appearing in Hudson Valley orchards. Dying blossom clusters or shoots noted while spraying orchards should be investigated carefully to determine if fire blight is causing the die-back. Early evidence … Continue reading →

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Assessing Hail-related Fire Blight Risks

7 pm, Thursday, May 22, 2014:  Unfortunately, some regions of the Hudson Valley received  hail within the last 12 hours. Where fire blight inoculum exists in apple and pear orchards, injuries to bark, leaves, and fruitets may provide entry points for the fire blight bacteria, but that is likely ONLY if trees were already carrying a lot of inoculum.  So, if you received hail today, what should you do?  This is a tough call, with no clear answers.

Following are five reasons to believe that hail-related risks of fire blight at this point are minimal and/or that there is little that anyone can do to modify those risks at this point:

1.  Risks that hail today will trigger fire blight are less than would be the case for hail occurring four weeks later in the season when blossom infections might be producing an abundance of new inoculum if strep sprays were less than 100% effective.

2. Orchards that were sprayed with strep during late bloom (i.e, around May 15) and have no flowers remaining should be OK because they had nothing to support bacterial growth over the past several days (i.e., no open flowers) and they therefore will not have had enough inoculum today to pose a problem.

3. Orchards that were treated with strep plus Apogee earlier during bloom are probably OK without any further action because Apogee will be helping to reduce risks by making the trees more resistant to infection.

4.  Trauma blight from hail at this time of year is unlikely if the epiphytic infection potential in the MaryBlyt model is below threshold (i.e., <100)  at the time of the hail event.  The EIP may actually need to approach 200 before trauma blight becomes really likely, but we just don’t have enough data to make a blanket statement. As shown in the printout from this morning, data for the Hudson Valley Lab (with predicted temperatures  entered for today and the next few days) indicate that we were just below the blossom blight threshold with an EIP of 99 today. The marginal EIP suggests low risks from hail.

5.  The only known option for reducing fire blight risks following hail is an application of streptomycin within 24 hours of the hail event.  However, we lack data indicating how effective strep actually is when applied AFTER hail events.  Furthermore, strep sprays should dry or at least remain on the trees for an hour (??) without rain so that the strep can be absorbed.  Spraying strep in the rain is not recommended. At the moment, the weather forecasts suggests that it will be difficult to find a window without rain or drizzle over the next 24 hours.

Finally, two warnings:

1.  If you know that you have (i) blight-susceptible cultivars (ii) that have a lot of recently opened flowers, (iii) that are less than six years old,  (iv) that were not treated with Apogee, (v) that have not received any strep since May 15 or 16, and (vi) that have an inoculum source nearby (i.e, orchard with blight last year), then you may want to attempt treating those blocks with strep if you get a decent treatment window in the next 24 hours. Do NOT include Regulaid at this time as it may impact activity of plant growth regulators applied within the next few days.

2. Note that the EIP may again reach infection thresholds on 25 May and for several days thereafter.  If the weather unfolds as predicted, any newly planted trees or cider apple cultivars that are in bloom will be at risk and should be protected.  Quince may also be at risk for fire blight if they are still in bloom by the weekend.

MaryBlyt predictions using data from the Highland-HVL NEWA station through 21 May and forecasts from Accuweather for dates after 21 May.
MaryBlyt predictions using data from the Highland-HVL NEWA station through 21 May and forecasts from Accuweather for dates after 21 May.

 

 

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Update on Fire Blight Risks

Tuesday afternoon, May 20, 2014:  Weather forecasters have adjusted their predictions, and the slightly cooler temperatures predicted have resulted in reduced risks for additional fire blight infections later this week. The potential infection periods that were predicted yesterday  disappeared when I inserted the new forecast into the MaryBlyt model using data from the Highland-HVL NEWA station and predicted temperatures from AccuWeather (see below).  Blossom blight infections may occur again next week (May 26-28) for newly planted trees and/or European cider apple varieties that are still blooming then, but it appears that major risks for infection of blossoms on bearing trees in the lower Hudson Valley this year occurred on May 11 and May 15. Any wetting on 12-14 May could also have triggered infections, but no wetting occurred on those dates in our Highland orchards.

14-05-20 Maryblyt

In the MaryBlyt output shown above, data below the blue horizontal line are from AccuWeather forecasts. The column labeled “EIP” (epiphytic infection potential) indicates when flowers have been exposed to enough heat (degree hours >65 F) to become infected.  The heat requirement is fulfilled when EIP ≥100.  In the column labeled BHWTR, the respective columns will show a “+” if the following conditions are met: B — open flowers; H— EIP>100; W— wetting from any rain, dew, or pesticide application on that day or >0.1 inch rain the previous day; T— mean temperature for the day is >60F. The final letter in that column will be an “I” indicating infection is likely if all of the other columns show a “+”, or the last column will be “H” for high risk if only three of the former criteria are met.

The column labeled “BBS” provides a prediction of when blossom blight symptoms might appear on trees. Symptoms from the 11 May infection event should appear on 25 May (i.e., when numbers in the BBS column exceed 100).  Symptoms from the second infection period should appear on 28 May when numbers in the BBS column that are followed by a “b” finally exceed 100.  In reality, the very early symptoms are often apparent only to very careful observers and it takes a few more days before the wilting-shoot symptoms become apparent to the casual observer.

Personally, I prefer to use the MaryBlyt model for timing blossom blight sprays rather than the Cougar Blight model that is available on the NEWA network.  The MaryBlyt model can be downloaded from http://www.caf.wvu.edu/kearneysville/Maryblyt/index.html, along with a manual that provides full documentation.  The disadvantage of using MaryBlyt is that the user must enter the maximum and minimum temperatures and rainfall for each day.  The advantage is that MaryBlyt model will accept multiple spray dates (in columns that I removed from the example above).  The model will adjust the output to indicate what risks will be after adjusting for any strep sprays that were applied. Using that function showed that a single strep spray could not adequately protect against infections on both 11 May and 15 May under our conditions at Highland. Thus, anyone with fire blight inoculum in their orchards should have applied at least two strep sprays this year sometime between 10 May and 16 May. We should know within two weeks just how effective those sprays actually were.

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Scary Weather for Apple Diseases

Friday, May 16, 2014: The current weather pattern is generating high risk for a number of apple diseases:

Fire blight risks have been very high through much of the last week.  I have not gone back through our historical records, but I suspect that it has been a very long time since the Hudson Valley has seen this duration of continuous fire blight infection conditions.  The long infection window means that there may well be gaps in protection provided by strep sprays. (Hopefully everyone has protected at-risk orchards with at least two strep sprays by now!)  No one knows how many orchards in the Hudson Valley actually contain fire blight inoculum and/or have been exposed to inoculum by wind-blown rain or pollinating insects, but this may be the year that we find out!

Given the high potential risks for fire blight, this is a year when Apogee (6 oz/100 gal of dilute spray) may prove valuable as a second line of defense against fire blight. Unfortunately, you can’t wait to see if you get fire blight before making a decision on Apogee.  To be effective for fire blight suppression, Apogee must be applied now (applications earlier this week were even better) because it takes about 10 days after application before an Apogee application will result in increased resistance to fire blight. If Apogee is applied during bloom or at petal fall, then the increased resistance to fire blight will kick in at about the same time that the first symptoms of fire blight appear on blossom clusters that were not adequately protected by strep sprays. Apogee-induced resistance to blight can be a critical component in reducing secondary spread of fire blight after bloom and will provide time to remove blossom blight infections before the disease becomes epidemic.

Remember that you should NOT apply Apogee to Empire or Staymen and that a water conditioner must be added to the spray tank before Apogee is added.  Read the product label! Also, applications of Apogee may interfere with thinning, but I suspect that most folks would rather have a thinning problem than a fire blight problem.

Quince rust is favored by long wetting periods with intermittent or light rains (like yesterday!). Heavy rainfall tends to wash the spores out of the air. I recall visiting a farm in Columbia County back in the 1980’s where a fungicide gap allowed about 40% of fruit to become infected with quince rust. Thus, this “minor disease” can cause major losses if trees are left unprotected. Captan does not control rust diseases, and 3 lb/A rate of mancozeb is too low to provide complete control if there is an abundance of inoculum (i.e., infected cedar trees) in the vicinity of the orchard.

The good news is that quince rust is fairly easy to control in most orchards so long as fungicides are applied ahead of infection periods. Fungicides that control quince rust include mancozeb, Polyram, Flint, DMIs, and SDHIs (Fontelis, Merivon, Luna Tranquility, and outside of NY, Luna Sensation). These fungicides protect against quince rust, but with the exception of the DMI fungicides, they lack any significant post-infection activity.

Rally has been shown to have really incredible post-infection activity against quince rust (perhaps as much as 7-days).  Other DMIs may also have post-infection activity, although I am uncertain about the post-infection capability of Indar and Inspire Super against quince rust.  The latter two fungicides are more surface-active than Rally. The quince rust pathogen grows toward the center of the fruit after infection and therefore may outgrow the reach of Indar and Inspire Super within a few days after infection. Thus, while Indar and Inspire Super will control quince rust when applied as protectants, the duration of their post-infection activity is unknown (at least to me).

Where there is any concern about whether fungicide protection was adequate during the past week, including Rally in the petal fall applications should eliminate quince rust.

Apple scab is obviously a significant threat at this point, but hopefully everyone knows how to control scab and has fungicide protection in place.

Posted in Fungicides for apples | Comments Off on Scary Weather for Apple Diseases

Bacterial Spot on Stone Fruits

Friday, May 16, 2014: Our warm, humid, misty, wet weather is providing excellent infection conditions for bacterial spot of stone fruits.  (Our current weather also favors brown rot infection, but you already knew that!) Bacterial spot can destroy peaches, nectarines, apricots, and plums by causing lesions on fruit.  The critical period for control is petal fall through shuck split, although control measures may need to be continued into the summer on susceptible cultivars. (See the Rutgers University assessments of peach-nectarine cultivar susceptibility here and susceptibility ratings plus control recommendations from Michigan here.)

The best control is achieved with applications of the antibiotic oxytetracycline (Mycoshield, FireLine), but these products are not labeled for plums and apricots. Low rates of copper can also be used to control the disease on peaches and nectarines, but I really don’t know what impact copper sprays might have on apricots and plums.  On peaches and nectarines, even low rates of copper will cause leaf spotting that can be almost as severe as the damage caused by bacterial spot itself, but copper applications can keep bacteria from infecting fruit. Because of the phytotoxicity caused by copper sprays (especially if applied too close together during periods with no rain), I suggest that stone fruit growers use Mycoshield or FireLine in at least the first few applications of the season.

For more details managing bacterial spot, see the excellent article by Dr. Norm Lalancette that just appeared in the Rutger’s Plant and Pest Advisory. Note that his comments on using Mycoshield/FireLine appear in the last paragraph of the article.  I am not certain if all of the copper products listed in the article are registered in New York, so check product labels carefully, as always.

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