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The changeable weather we had last week left some of the Fruit Team in shorts and T-shirts at the Tuesday twilight meeting, and parkas at the Wednesday meeting. It also created some interesting situations regarding scab. For one thing, the hot weather pushed tree growth, scab maturity and grass growth.
Tree growth matters because when trees grow faster than usual, they can "outgrow" pesticide coverage. Going into rains last week the last application of fungicide may have been only 5 days earlier, but new leaf growth is unprotected unless the fungicide is redistributed by the rain. If it is, it is diluted, and the more new growth there is, the more diluted the fungicide becomes. There are not good measures for how much new growth effects fungicide coverage. However, if there is a question at this point, reapply the protection.
Scab maturity has blasted through 50% and is approaching 75% discharged or ready to discharge. So right now, we are still in the most dangerous part of primary scab season. However, we also went through about a week or more of maturation in 2 days, so we have shortened primary season by about 5 to 7 days. This could translate to saving a fungicide spray. We should reach the end of primary scab by the end of next week, in most areas.
What does grass have to do with scab? Well, the heavier the grass growth the better the interference with spore release. Where there's grass, the spores can't get out of the grass. On the other hand, where there is no grass, as there isn't in the herbicide strips, the spores will do just fine.
Keep a tight rein on scab control for the next 10 days. Check now for lesions in those spots where they are most likely - the fuzzy grey-green spots have started to show. If you don't see any, then you may be done with scab for the year by Memorial Day.
The traditional thinning window is upon us. Generally, favorable weather conditions for fruitlet retention are predicted, i.e. some sun and seasonable temperatures. NAA between 3 to 15 PPM (depending on cultivar) combined with Sevin XLR at one quart per 100 gallons (dilute) is often the ticket. If you applied a petal-fall thinning spray, you may want to use NAA or Sevin alone, or reduce the rates somewhat. Remember that thinning sprays are more effective when applied at warmer temperatures. Cloudy weather for a few days following application will also enhance their thinning effect. Accel plus Sevin combinations can worked well on McIntosh type cultivars (including Empire and Macoun), but it needs to be warm (80-85 degrees is ideal) after application. It's always a good idea to leave a tree or two unsprayed so you can evaluate the effectiveness of your thinning spray.
Pears can be troublesome to thin. NAD can be used at petal fall to thin Bartlett¹s, but NAD application to Bosc pears can result in complete defruiting. A safer bet for Bosc pears is to use NAA at 7.5 PPM in the 5 to 10 mm fruitlet size range. (One week to ten days after petal fall.) As with apples, warm weather during and following application increases the thinning, particularly if it is cloudy. If successful, good return bloom with Bosc pears in the 2-1/2 inch plus size category will be the result.
Chemical thinners do their job when absorbed by leaf tissue. Conditions that favor rapid absorption include warm temperatures and high humidity when the plant is actively growing and the chemical thinner has a longer time to be absorbed. Expect a good response when thinners are applied under these conditions. Sometimes we get asked what effect a wayward rain shower has on a recent thinning application? In general, as long as the spray is not immediately washed off (i.e. spraying in the rain!) you should get a response. Sevin in particular redistributes and remains effective for as long as a week after application. In addition, most absorption of chemical thinners occurs on the leaf underside, and most rain (presumably) hits the top of the leaf. Bottom line: no need to worry as long as the application had some time to dry and be absorbed before the precipitation.
The persistent hot weather of the first week of May certainly sparked a major immigration of plum curculio adults into most orchards. This set the stage for concentrated PC fruit-injuring activity at petal fall, and the opportunity to control the bulk of immigrant PCs with the first post-bloom treatment. In the past week, temperatures (particularly evening) have cooled markedly, and PCs present in orchards have exhibited very little egglaying activity.
In 12 commercial orchards spread across the state, cumulative fruit samples have yielded 11 PC oviposition scars on nearly 15,000 fruit. These data suggest that despite their presence in all monitored orchards, PC have not had the post-arrival weather conditions necessary to stimulate widespread egglaying activity. Growers who have already applied a petal fall treatment targeting PC should enjoy very good control of the sluggish population that has arrived; we highly recommend that those who have not yet made the first PC treatment regularly monitor perimeter trees for evidence of PC activity, and treat when the first signs of egglaying are detected. The next warm spell (a couple of days at or near 80 degrees) should clarify the weevil plan somewhat—whether continued immigration and long-term egglaying will be an issue this season.
In a few early-developing orchards, first-generation sap-feeding mines have begun to appear, likely from the earliest wave of egglaying adults. However, limited sampling suggests that it is still too early to apply a post-bloom LM treatment. It is likely that the cool temperatures of the past week have slowed LM egg development, and we recommend that growers time their LM treatments in accordance with appearance of threshold levels of first-generation mines.
Effective control of LM can become difficult with inconsistent weather patterns during egglaying and larval development, as growth stages can become strung out even in the first generation. Fortunately, as shown last season, newer materials (including Provado, Agri-Mek, and SpinTor) may pack enough residual whallop to compensate for disparate growth in the first generation.
If Provado, Agri-Mek, or SpinTor against first-generation mines is the management tactic of choice, we recommend that an application be made as soon as sap-feeding mines exceed the following thresholds:
Cumulative first-generation sap-feeding mines per 100 leaves : McIntosh 7 ; other varieties 14.
When monitoring for the abundance of sap-feeding mines, samples should be taken on the underside of the older fruit-cluster leaves, toward the interior of the tree. As mentioned in recent issues of Healthy Fruit, these thresholds can be used to determine if a spray is necessary when trap captures indicate a marginal population, or if the orchard’s LM species composition is mixed. In the event of high trap captures or excessive egg counts, these thresholds can also be used to trigger timing of the application.
Egg hatch of European red mites is complete in most areas, and a few sites harbor what may become troublesome populations. Notable ERM numbers are likely to appear earliest in blocks that did not receive full oil programs, as spring conditions did not favor 2 oil treatments in most orchards. As mentioned in last week’s issue of Healthy Fruit, growers who are not confident in the efficacy of oil treatments may benefit from a sample of fruit cluster leaves for presence of motile ERM. In the absence of a crystal ball, these samples offer the best estimate of the early-season ERM growth potential; research in New York suggests that if 30% of clusters harbor nymphs, a petal fall miticide treatment may be in order. More detailed information regarding the choices for early-season mite management can be found in last week’s Healthy Fruit or the 2000 March Message.
Healthy Fruit is written by Dan Cooley, Ron Prokopy, Jon Clements, Starker Wright, Arthur Tuttle, Wes Autio, and Duane Greene except where other contributors are noted. Publication is funded in part by the UMass Extension Agroecology Program, grower subscriptions, and the University of Massachusetts IPM Program. A text version can be e-mailed to you if you contact Doreen York. Please cite this source if reprinting information.
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