Anticipating Outcomes from a Week of Rain

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

Risk factors and implications include the following:

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

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

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