Apple scab, plum curculio, thinning

May 28, 2015

by Terence Bradshaw

I was a bit absent earlier this week when some of this information may have been relevant, but here’s my take on the situation in Vermont orchards:

Apple scab:
Although there has been little moisture this season, infection periods have occurred, and I saw my first primary scab lesion on unsprayed McIntosh trees at the UVM Hort Farm this morning. Although the NEWA apple scab model shows near-complete ascospore maturity and release across the majority of the state, the model isn’t always accurate in these seasons of low rainfall. Researchers at UMASS have reported “plenty” of mature ascospores in their trapping, and the next soaking rainfall will likely lead so a major spore release and infection period. To date, the infection periods we have seen this season have generally been from combined, intermittent wetting events. These events require just as much protection as a full rain, and I recommend maintaining fungicide coverage for another couple of weeks until we are comfortable that the primary inoculum is expended.

Fire blight: With the warm weather we’ve been having, fire blight bacteria has had an excellent opportunity to continue developing in Vermont orchards. Growers, especially those who had fire blight anywhere on their farm last year, would be advised to have an application of streptomycin on-hand and be ready to apply immediately following a traumatic weather event (very high wind with rain, hail). Blossom blight symptoms from the May 9-11 infection event should be showing up any day now, so scout your orchards closely and be prepared to cut out diseased tissue using appropriate caution to prevent disease spread (cut 8-12 inches below visible symptoms, clean pruners with alcohol or 20% bleach solution between cuts, prune on a dry sunny day). DO NOT apply strep to visible fire blight infections- you will not get control of the disease, and you will be setting your orchard up for strep resistance in the local fire blight population.

Thinning: Bloom was heavy and pollination good across most of the state, and Vermont orchards will need to thin aggressively this year to break the alternate bearing pattern we have gotten into. Many orchards should have put a second thinner on at the beginning of this week when fruit were at 8-12 mm size and subsequent warm weather conducive to thinner activity. Good spray conditions should be coming this evening and tomorrow and relatively warm weather tomorrow and Saturday will provide a decent window of opportunity. By next week, many orchards will have fruit reaching 15-20 mm in size which are significantly more difficult to thin.

Insects: Plum curculio is still active, so keep covered for that one. Orchards that received a full-block spray of an effective curc material may get away with border row sprays for the next week or two. Codling moth are beginning egg laying in the Champlain Valley so materials that target early instar larvae may be applied any time. European apple sawfly are largely completed with their activities this season but if your orchard has problems with this pest, consider keeping covered with an effective material for another week or so.

Horticulture: Water any trees you can, especially new plantings. Now is the time to apply nitrogen fertilizers, and trees may benefit from potassium applications at this time as well. Foliar fertilizers may also be applied now, since nutrient demand is very high given the high fruit load and rapidly developing canopy.

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