VT Apple IPM: Petal fall edition

Video from the orchard this week: https://youtu.be/P1K7XWlg35M

Champlain Valley and other warm site orchards are approaching full petal fall, so there are a lot of decisions to make in the days ahead. Hopefully everyone with any hint of risk for fire blight treated sometime late last week or over the weekend, orchards in bloom may consider treating given this heat spell that’s about to pop up. Everyone also needs to keep an eye out for blossom blight symptoms, and for the shoot blight that will follow. Growers can apply prohexdione calcium (UMASS factsheet, Apogee label, or Kudos label, 6-12 ounces per 100 gallons dilute**) any time now to reduce shoot blight incidence. The plant growth regulator does not affect the bacteria that causes fire blight, it only reduces susceptibility of shoots to infection by thickening cell walls. Treatments will reduce shoot elongation and thicken cell walls for about 2-4 weeks post-application, so retreatment may be necessary every 1-4 weeks until terminal bud set.

In light of the heat that we’re experiencing in the next couple of days, if you can avoid spraying anything (streptomycin excepted) when the weather is >85°, that’s a good thing. Then again, don’t ignore the very real threat of the following pests that need to be protected against.

We are approaching the end of apple scab season, but don’t relax just yet. NEWA is predicting that all spores are mature in warm sites and will all be released with the next decent rain, but the model can lag behind biology. I recommend maintaining coverage for the next couple of weeks. For cooler inland and upland sites, there are likely still spores remaining on the overwintering bank, so stay covered and consider using a kickback material from FRAC class 3,7,9, or 11 if you have any question about residue heading into an infection period which we may see mid-week. As far as materials, everyone should plan on phasing out mancozeb soon, as it it toxic to beneficial predatory mites that do some great biological control or European red mite and two-spotted spider mite, and we’ll soon have to be thinking about its 77-day preharvest interval. Organic growers, I would apply sulfur before and after the rain event, and maybe consider lime sulfur (LS) for the second spray to provide some post-infection control. LS is caustic, nasty stuff, so use it wisely, wear all the appropriate gear, and wash everything down well as it is very corrosive to steel and other materials.

If you’re at total petal fall, then it’s time to start thinking about insect pests, especially plum curculio (PC). PC love this heat and will be ready to oviposit on fruit as they reach 7-10 mm diameter. Organic growers should plan on getting a coating or Surround on trees ASAP, and maintaining that coverage for about 400 degree days (base 50°F) after petal fall (NEWA has a good model for this). This is a longer window of coverage than for non-organic orchard management (308 dd base 50°F), because Surround does not kill the insects and so must be maintained longer until the biological urge to oviposit has completely subsided. For non-organic orchards, effective materials include Imidan, Actara, Avaunt, Voliam, and Agri-Flex. Carbaryl, if used for thinning (see below), will have some efficacy, but probably shouldn’t be your primary material of choice given the weather that is very conducive to PC activity. Thinning rates of carbaryl are about half the insecticide rate, and I would plan on using just that lower rate as a thinner and use a separate material for my insect management. Any of these materials will help to manage the other petal fall insects, including European apple sawfly and the various lepidopterans (obliquebanded leafroller, Oriental fruit moth, codling moth, etc.) that may be emerging at this time.

Thinning. Okay, this is always a tricky one. First, anything applied in the next 40 hours will be highly active because of the heat, so I’d err on lower rates and a lighter touch. A second application may be needed after this weather breaks. Now, I haven’t been in orchards all across the state, but where I have seen bloom from Connecticut valley, Addison county, and our own orchard in South Burlington, it was good to downright heavy. Pollination and fertilization conditions have been good, so I’d expect trees to need a decent thinning this year. The wild card is how much damage orchards may have seen from the April 25 freeze when many trees were at tight cluster bud stage. We are seeing some damage at the UVM orchard, but there are enough good fruit that are starting to set that I am ready to thin moderately aggressively this season. The New England Tree Fruit Management Guide has some good variety-by-variety recommendations, so I recommend starting there. A good, standard petal fall spray of 1 qt/acre of carbaryl and 8 oz/acre Fruitone N or L (I did the TRV adjustment for you) should do the trick for most orchards. As fruit approach 8-10 mm in size and the weather starts to warm up, the 6-BA thinners will be very active, and they are a great choice for small fruited cultivars (Macoun, Gala, Fuji, Empire). For organic orchards, it’s time to start hand thinning. A lime sulfur spray used for scab can help to knock some fruit off, but it’s not labeled specifically for that use.

I think that covers it for now.

**This reference to amount per 100 gallons dilute refers to Tree Row Volume (TRV), which is a somewhat out-of-vogue method for adjusting spray rates to compensate for canopy volume. I describe it some here, but in simple terms, it calls for measuring the tree canopy volume and estimating the number of gallons of water to saturate the canopy to wetness (dilute gallons per acre, DGA). No one sprays at full dilute, that wastes time, money, and water. For a good rule of thumb, large, standard trees 20 feet tall planted at 30 feet x 40 feet spacing had (notice the past tense) about 420 DGA. A more typical ‘large’ semidwarf orchard on M.7 or similar with 12 foot tall trees planted at 12 ft x 18 ft would have 200 DGA. DGA decreases down to around 100 and stays there pretty consistently for tall spindle and similar high density, narrow-canopy systems. BUT, we often do not recommend reducing TRV below 150, maybe 120 if you have excellent coverage and an easily sprayed canopy. And this TRV is only used to determine the rate of material used per acre, not how much water you put in the tank. So. Let’s just say use 200 DGA for semidwarf trees, 150 for trellised trees. Back to the Apogee example, let’s use 8 ounces per 100 DGA for simplicity’s sake, that would be 16 ounces per acre to the big trees, 12 ounces to the smaller high density trees. Then figure out how much to put in the tank based on the amount of water you spray per acre, which is likely 50 (or less?) to 150 gallons.

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