Orchard Considerations: Scab (yes…), Fire Blight, Insect Management

June 17, 2020

It’s dry, I know. If you have the ability to irrigate, you should be. At a minimum, newly planted trees should be watered regularly. This dry weather has been great for management of most diseases- apple scab, plus rusts and some of the other ‘minor’ diseases should be pretty low in abundance in managed orchards. It’s not hard to find scab lesions in unsprayed orchards, even at the especially-dry UVM Horticulture Research & Education Center (HREC), so the potential is there. This dry weather is slowing the end of scab season, though. Ascospore development follows a fairly predictable curve in years with normal precipitation patterns. However, in dry conditions, the fungus enters a dormant state, so development slows. We’re not entirely sure how extended the dry window needs to be for the inoculum to die out rather than just wait it out until rains come and re-energize it. Most orchard’s NEWA models indicate that the primary scab season is over, but others who are using the RIMpro model indicate another infection period is waiting in the wings. I would feel better with one fungicide protection going into next week when rains are expected. I don’t know if any of the models adequately plan for weather conditions like this.

The disease of greater concern to me is fire blight. Weather during bloom was hot this year with pretty high potential for infection. However, wetness events were rare, and inoculum isn’t guaranteed in all orchards. We have some active fire blight research going on at the HREC, and I can assure you that trees that were inoculated have plenty of the disease. We also have other sections of orchard with little FB history where I have found a few strikes. Everyone should keep an eye out and do a thorough scouting of your orchards for signs of infection and plan to cut it out ASAP. This dry weather we’re having is conducive to cutting while minimizing disease spread. Dan Cooley and others wrote a nice fact sheet on fire blight management that can be found at: https://ag.umass.edu/sites/ag.umass.edu/files/fact-sheets/pdf/f-133.pdf. I have also recorded a video on removing strikes that I’m posting to my YouTube channel (https://www.youtube.com/user/UVMOrchard). My rural DSL upload capability is taking quite a while to get it there, so check back later today.
Insect management is key in orchards now. Where codling moth has been a problem, application of a CM-specific material is warranted most any time now, depending on your biofix (the date of first catch in pheromones traps in your orchard). For upland and inland sites, you may be a week away. Plum curculio are likely still active in cooler sites, but I have not seen fresh damage in the warmer sites (Champlain / Connecticut Valleys) so if a suitable material was applied in the last week or so, you’re probably covered there. Hot, dry weather is also conducive to mite flare-ups. A weekly or, if the numbers indicate, bi-weekly scouting will help to indicate if there are high enough mite numbers to consider treatment. Information on monitoring: https://netreefruit.org/apples/insects/mites. Mites should be treated based on the following thresholds: in June, 1-2 mites per leaf; July, 5 mites per leaf; in August, trees are more tolerant of feeding so treatment should only be applied if there are over 7.5 mites per leaf.

Think about applying calcium in all of your sprays now, especially on Honeycrisp, Cortland, and other large-fruited cultivars, and especially especially on those cultivars if they have a relatively small crop this year.

Many growers, myself included, have observed poor fruit set or over thinning this year. That warm spell around June 4-6 and again 10-12 seems to really have activated thinners, and there may have also been some frost damage going into bloom. I hope your crop has turned out okay, please let me know if there are issues. I’m not sure if weather events contributed to overthinning is an insurable loss under most crop insurance policies, but it’s worth looking into.

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