University of Vermont

By Terence Bradshaw

I know that some orchards are still in bloom, so the thinning and insect management portions of this message may not pertain. Hopefully everyone with any hint of risk for fire blight has treated sometime during this heat spell. Cooler weather this weekend and rapidly dropping blossoms will decrease risk, but until then, the pump is primed. Everyone also needs to keep an eye out for blossom blight symptoms, and for the shoot blight that will follow. Growers can apply Apogee (4.5-9 ounces per 100 gallons dilute**) any time now to reduce shoot blight incidence. Apogee 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.

All set on FB? Okay, we have some big fish to fry this week. Let’s cover them one by one.

1. This will all be said 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. I have an inkling that many of us will be up pretty late/ early tonight/tomorrow morning to get things covered while dodging the heat.
2. We are all headed for a pretty big apple scab infection period starting tomorrow Thursday 5/28 or Friday. Make sure you are covered going into it, I would not count on any material’s kick-back activity to handle this kind of scab load. As I’ve mentioned before, some materials (captan, sulfur) can cause leaf burning when applied ahead of hot weather. But, given what looks like an extended wetting period and an ample load of mature ascospores, I’d take the chance of a little leaf burn over a major scab outbreak. If you want to put on a lower rate now and follow up Sunday or so with a kick-back material (SDHI, strobilurin, or DMI, see here for more discussion, also mixed with a protectant like mancozeb or captan). 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.
3. 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.
4. 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 just about perfect, so I’d expect trees to need a decent thinning this year. 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. 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.

Where trade names or commercial products are used for identification,

no discrimination is intended and no endorsement is implied.

Always read the label before using any pesticide.

The label is the legal document for the product use.

Disregard any information in this message if it is in conflict with the

label.

The UVM Tree Fruit and Viticulture Program is supported by the

University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM

Grant, and USDA Risk Management Agency Funds.

By Terence Bradshaw

What an amazing weekend it’s been, weather-wise. I still need to check bud stages, my cider varieties here at 1500’ elevation in central Vermont are still at around half-inch green, and so are not an indicator of what things look like around the state in more typical orchards. However, I imagine many varieties are approaching petal fall, and growers may be itching to put on an insecticide of thinning spray.

The weather for the next three days (Tuesday-Thursday I mean) calls for pretty extreme heat, by May standards, anyway. Given that likely spray materials now include potentially phytotoxic fungicides (captan, sulfur) and insecticides (carbaryl), and that thinners will be very active if applied in this heat, I urge caution. I also don’t like to apply anything in temps of 85° or hotter.

I’ll look over bud stages in the next day or so and check the weather for later this week. Chances look decent for our first apple scab infection period in a while on Friday, and it could be a doozie. Combine that with need to thin and manage post-bloom insects, and we could be in for a complicated spray week.

If you spray anything now, I’d only consider streptomycin on highly susceptible varieties in orchards with recent fire blight infection. In that case, a coat of fungicide (not captan or sulfur because of phytotoxicity issues) may not be a bad idea. But in most cases, I’d lean towards no treatments and plan on an evening / night / crack-of-dawn-when-it’s-cooler treatment window later in the week.

Where trade names or commercial products are used for identification,

no discrimination is intended and no endorsement is implied.

Always read the label before using any pesticide.

The label is the legal document for the product use.

Disregard any information in this message if it is in conflict with the

label.

The UVM Tree Fruit and Viticulture Program is supported by the

University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM

Grant, and USDA Risk Management Agency Funds.

By Terence Bradshaw

I’ve received a few questions about the fire blight alert:

1. If you are in an inland/upland site and no bloom or fresh pruning wounds, this alert does not apply. However, I bet most apples in the state, aside from some late cider varieties, will be in bloom before this disease alert is done.
2. Strep can be applied within 24 hours before or after an infection / wetting event. So you could wait to see if there is dew, but need to know that there’s no dew if you decide to hold off. Dr. David Rosenberger from Cornell has also been discussing the possibility that high humidity may be enough to cause infections. Bottom line- I’d get out there sometime if you have any risk (susceptible blooming varieties, past history).
3. A treated flower is a treated flower. So if you’re at full bloom and you’re going to treat anyway, treat any time.
4. Rate: I was corrected on the rate of streptomycin that I’d recommended, which I’d passed on from another extension warning without reading the label. Harbour is the main (only?) brand of strep we use in the state, and it is labeled for 24-48 ounces per acre. The first application should include Regulaid or similar wetting agent, subsequent sprays, if applied, can and should omit the wetting agent to reduce phytotoxicity.
   The Harbour label is a bit confusing, as it makes some jumps from concentration to rate with an implied understanding of (and I’d say controversial application of) tree row volume. We’ll discuss TRV another time, not during an important disease event. Stick to the label rate, which is 24-48 ounces per acre.

Where trade names or commercial products are used for identification,

no discrimination is intended and no endorsement is implied.

Always read the label before using any pesticide.

The label is the legal document for the product use.

Disregard any information in this message if it is in conflict with the

label.

The UVM Tree Fruit and Viticulture Program is supported by the

University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM

Grant, and USDA Risk Management Agency Funds.

By Terence Bradshaw

I’ll keep this short and to the point. In spite of, or rather in keeping with, what I said the other day, fire blight risk is increasing in Vermont orchards. I’m seeing sustained warm, sunny, relatively windless, and dry conditions for the next 5-7 days, with increasing risk of rain as next week starts. The heat is driving the bacterial population to increase pretty rapidly. Warmth suitable for infection is present. Open blossoms are present. The question is whether ot not you a) have fire blight inoculum in or near your orchard and b) whether or not wou’ll get the necessary wetting to cause infection. For the first one, if you’ve ever had any fire blight in the past couple of years, assume that you have inoculum. The population is moving too rapidly to assume otherwise.

For the second, you may want to wait things out until just before a rain on Monday or Tuesday, but what if ypu miss a dew event? Sunday morning looks like there’s potential for some dew, and that’s enough.

Anyone in any stage of bloom, is to get a streptomycin (or pick your organic biocontrol of choice, but I can’t personally vouch for any at this time) spray on sometime between Friday and Sunday morning. Once a blossom is treated, it’s protected, so later is better to ensure you cover the maximum number of blossoms. But don’t wait so late that you miss a good spray window. Strep should be applied 16 oz per acre in sufficient water to get good wetting, amd I recommend Regulaid or another organosilicone wetting agent be included.

Where trade names or commercial products are used for identification,

no discrimination is intended and no endorsement is implied.

Always read the label before using any pesticide.

The label is the legal document for the product use.

Disregard any information in this message if it is in conflict with the

label.

The UVM Tree Fruit and Viticulture Program is supported by the

University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM

Grant, and USDA Risk Management Agency Funds.

By Terence Bradshaw

Apple bloom has started in much of the state, and the weather looks about as good as it gets for pollination and fertilization. It’s been dry since last Friday, and, in most sites, that rain wasn’t even as much as expected. So, water if you can, trees in bloom and setting fruit have high energy requirements, and water is a key part of mineral nutrient transfer and photosynthesis. While I’m at it, this is a good time to get your fertilizers on. Nitrogen fertilizers should be applied between bloom and June 15, while your potassium and other fertilizers can go down between now and midsummer. It’s also not too late to put on foliar fertilizers, such as the prebloom nutrient cocktail recommended by Cornell University and New England apple experts. However, given the bloom, the dry weather, and minimal disease pressure resulting from that, I’d put the sprayer away for a while- with the caveat I’ll leave below..

It looks like dry weather will be the rule for a while, so apple scab just isn’t an issue. That means that, when we do get some wetting, an pretty good infection period could result, so keep an eye on things and plan to cover ahead of any rains, which look to be at least a week out. When the weather is dry, ascospore maturation goes into hibernation so we can’t assume the spores will all mature and release with the next rain and scab season will be done with in one event. Of course, use NEWA to help keep an eye on your scab and other disease issues.

The disease we really think about during bloom, and especially during a warm/hot bloom, is fire blight. This bacterial disease acts differently from scab and other fungal diseases in two key ways- first, it is essentially internal in the tree, and therefore can act systemically and therefore tends to stick around for a while. Next, its infective pathogen can ramp up its population rapidly under warm conditions. The cool spring so far has kept fire blight danger (measured as epiphytic infection potential, or EIP) down to a minimum. However, that EIP is expected to ramp up fast in the next week:

The fire blight model does assume that you a) have inoculum present, and b) that you have water to move it into blossoms which allow entry into the tree’s vascular system. You can adjust the latter condition a little in NEWA, and the former is a judgement call as far as making an application is concerned. I would have at least one application of streptomycin on-hand and be prepared to treat blooming trees, particularly if you have a wetting event. This could include heavy dew, especially if you have a day afterward that has high relative humidity. High-value, fire blight-prone varieties may warrant a prophylactic treatment going into the weekend, but that’s a tough call.

For organic growers, Actinovate, BlightBan A506, Bloomtime Biological, Blossom Protect, Double Nickel, Regalia, and Serenade are labeled biological controls. I can’t vouch for any of their effectiveness, and I know that some growers apply copper or lime sulfur (warning- both are very phytotoxic) during bloom to manage the disease but again, no promises are offered from me there. If you can, I would suggest Serenade or one of the other biological materials if you have it on-hand and to watch carefully for symptoms which will require cutting out.

Once the EIP is sufficiently high (in NEWA, 100 is considered high enough to cause infection), you need three factors for infection to occur: open wounds (i.e. blossoms or fresh pruning cuts); mean temperature above 60°F; and wetting, even a spray or dew event can be enough to move bacteria into susceptible openings.

This risk is pretty much widespread across the state. The thing to remember is that protective measures, i.e. application of streptomycin in most orchards, or caustic materials/biological controls in organic orchards, must be applied to open wounds or blossoms within 24 hours before or after infection. So if we assume that an infection event occurs in a rain you’ll want strep or another material (really, if your orchard isn’t certified organic, strep is the only material to consider) on within a day. It will be hot Friday, so blossoms will be opening all day (or petals falling on cultivars that have finished bloom already) and potential for phytoxicity will be greater. The later you can go, the better to make sure you cover any blossoms that open, but don’t delay and miss it.

But, make sure to look at the forecast at least twice a day. We not only are not predicting rain, but also, at least for the next few days, predicting dew, either. The NEWA fire blight model output (see above) includes predictions for rain and dew.

Harbour is the strep material available to most growers. I would recommend applying at 1-2 pounds per acre based on tree canopy volume, if in doubt, err on the lower end. I also recommend including a wetting agent like Regulaid or LI-700. If possible, apply on its own, without other fungicides or insecticides (you are in bloom, so no insecticides anyway). You will get leaf yellowing from this application, expect it and the tree will soon grow out of it. One application Wednesday PM-Friday AM should cover you. Once any particular blossom is treated, it’s protected. Temperatures are expected to drop after Friday, which lowers risk, but if high risk continues through the weekend and you keep having blossoms open, then a second application Saturday-Monday (I have no good idea of what the weather will be five+ days from now) may be warranted.

Trees that are at full petal fall are not susceptible, but straggling late blooms can be infection sites. Ideally, the whole orchard would be treated. If you need to prioritize, go first for cultivars that had fire blight in the last two yearr or highly susceptible cultivars (Gala, Ginger Gold, Honeycrisp, Cortland, Paulared, etc), and of course blocks that are in bloom. Application well after any wetting event,will not give protection against it. Also, despite the wording on the Harbour label, continued treatment every 10-14 days after bloom is not recommended (at all- don’t do it), and doing so is a) expensive, b) a waste, and, most importantly, c) the best way to develop antibiotic resistance in E. amylovera populations.

As for insects, there are management actions you can be doing now. Of course, no insecticides during bloom. But you should be checking pheromone-baited codling moth traps at least every other day and noting the date of first capture in order to set your biofix for the NEWA models we’ll use later. Now is a good time to get mating disruption ties for dogwood borer or codling moth up in the orchard.

Finally, keep a good eye on the bloom and on pollinator activity in the orchard. I suspect that this will be a year that we’ll need to really work on crop thinning, even though we’re in an ‘even year’ which, in the past decade or so, has meant smaller total crop overall. We’ll talk about thinning next time.

Where trade names or commercial products are used for identification,

no discrimination is intended and no endorsement is implied.

Always read the label before using any pesticide.

The label is the legal document for the product use.

Disregard any information in this message if it is in conflict with the

label.

The UVM Tree Fruit and Viticulture Program is supported by the

University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM

Grant, and USDA Risk Management Agency Funds.