Green tip in Vermont orchards and disease management

By Terence Bradshaw

April 15, 2016

By later today most orchards in the Champlain and Connecticut valleys and Bennington area should be into green tip and disease management activities will need to commence soon for the 2016 season. The weather forecast for rain on Monday has been somewhat inconsistent with chances decreasing since last night, but growers should be ready for when any rains do come. That said, aside from the 30-40% chance called for on Monday, next week looks relatively dry. Depending on when we assume mature scab ascospores to be present in the orchard, we may have 2-6% ascospore maturity by Tuesday in most orchards. That may not seem like much, but many orchards had significant scab last year and 2% of a lot is still quite a spore load.

The threat of freezing weather is diminishing with forecast lows above freezing in the major fruit growing regions of the state after tonight. This means that both copper and oil should be okay to apply starting tomorrow through next week. I have seen and heard of increasing problems with San Jose scale in Vermont orchards, and a thorough oil application is the first line of defense against those insects as well as overwintering mites. Horticultural oil should be applied at 2% volume and in as much water as practical, certainly no less than 100 gallons per acre and more if you can on larger trees. This means slower travel and more refilling of the tank, so plan accordingly. Copper is the first line of defense against fire blight and while that disease was not present at high levels in Vermont orchards last year (in general…), it is out there and has become an increasing problem in the state in the past decade. A copper spray will also provide about seven days’ protection against apple scab and can be combined with oil. Fixed copper materials are best at this time, and specific materials and rates can be found in the 2015 New England Tree Fruit Management Guide. Care however must be taken with copper application, as the danger of phytotoxicity increases after green tip and especially by half-inch green tip. My suggestion is to get out there as soon as possible starting tomorrow and cover the whole orchard with a copper-oil mix before buds advance too far. When using oil, it is important to not use Captan for 7-10 days to avoid phytoxicity.

For orchards that have green tissue and were not covered with a fungicide, if an infection period occurs on Monday-Tuesday April 18-19, a kick-back fungicide may be called for to prevent infection because copper has no post-infection activity. The anilinopyrimidine (AP) fungicides Vangard and Scala have decent (48-72 hour) post-infection activity, work well in cooler weather, and are poor on fruit scab and are therefore good choices to use early in the season. Like all post-infection materials, there is potential for resistance development to the fungicide in local scab populations so a tank mix with a contact fungicide is recommended. I do not know of issues with copper and AP compatibility although there are no warnings on the label. If not using copper at this time, then mancozebs are another good material that will provide some protection against the disease (but have no kick-back activity of their own).

After this current round of treatment is completed, growers should begin their early season fungicide schedule going into the following week. Generally the mancozebs are the best candidates for maintaining protective coverage, although total use restriction must be followed (i.e. no more than four sprays through bloom at the six pound per acre rate of Manzate or equivalent or seven sprays up to the 77-day preharvest interval at the three pound per acre rate). After the seven-day ‘embargo’ following an oil application, Captan may also be used either alone or in combination with mancozeb (using a half-rate of each). If using Captan, recognize that there is a 40 pound per acre seasonal limit, so early season uses may limit its use later in the season for secondary scan and summer disease control.

Organic growers of course will be limited primarily to elemental fungicides including copper (apply that now, by the way) and sulfur materials. After the seven-day window that you buy with a copper spray, elemental sulfur applied at ten pounds per acre on a weekly basis or more often if more than one inch of rain falls is the primary line of defense against apple scab on susceptible cultivars. Growers do have the option of using liquid lime sulfur which has some post-infection activity but is very caustic, phytotoxic, and frankly nasty to apply, so good preventative coverage with elemental sulfur is important to manage the disease.

As always, apple scab and other pest management decisions should be made using best information on the conditions in your orchard. The apple scab primary ascospore infection season lasts roughly from green tip (judged by 50% of McIntosh buds having reached that stage) through 900 degree days base 32°F. Growers located near a NEWA station may use the models for the station closest to them, with the caveat that they may not be exact for their specific site. When using the scab model for a particular site, NEWA asks for a green tip date to be entered, and the default date is quite a bit ahead of what we are seeing in orchards. That said, entering an earlier date will allow you to be more conservative regarding ascospore development early in the season. For example, the default green tip date for Putney is March 30, which estimates 6% ascospore maturity by April 20, whereas entering a date of April 15 gives an estimated ascospore maturity of 2%. The only way to truly know the ascospore maturity level in orchards is to perform spore trapping or squash mounts using a microscope and very exacting and time-consuming technique, which is rarely done. The models are just that, computed estimates of biological activity in the orchard and only output what the model and entered conditions tell it to. That said, we know we are entering the scab season for this season, so growers should plan to apply coverage through the end of primary ascospore release. However, as we know that leaf wetness is required to initiate scab infection, extended dry periods will reduce the need for preventative coverage, but growers should be careful to not get caught entering a wet period without any coverage on trees.

New NEWA stations in Vermont! This spring we have brought one more NEWA station on-line in Bennington and have moved the Calais station from my house to a relatively new orchard in East Montpelier, which brings the total number of NEWA stations to ten, in addition to the airport sites located around the state. Remember that airport sites do not have leaf wetness sensors so apple scab and other disease models requiring that information cannot be run, but insect models will function fine.

Best wishes and good luck on the start of the season.