Thinning During a Difficult Period

D. Green, UMass Amherst, 17-May 2011

The current weather pattern move in when most orchards were at some stage of bloom between early bloom when king flowers were open to late bloom. The end result in most orchards was there were still bees in the orchard and consequently most were unable to apply the traditional petal fall spray containing carbaryl. The question we now face is what to do from a thinning perspective with the prospect of such unfavorable weather in the forecast this week.

During this very unfavorable period I am suggesting that only carbaryl be used. The next two days appear to be consistently rainy. However, if you see a period of time when carbaryl can be applied and have the spray dry, do apply it. Carbaryl is not concentration dependent and thus over thinning is very unlikely. If the spray is washed off you can reapply it with little fear of excessive thinning.

I am not recommending the use of other thinners during this very uncertain time because they are concentration dependent and the forecasted tempertures are not favorable. If you have an opportunity to apply one of these thinners in the next 2 or 3 days it will take at least 8 days to see the response. When the rainy weather stops and you have an opportunity to apply thinners under more favorable conditions, you do not want to have to ask the question of how much thinning did you get from an NAA or MaxCel application. There is no way you can answer this question in the short term.

Based upon the current weather forecast, Saturday may be the earliest opportunity for lower probability of precipitation and favorable temperature. While we dwell upon temperature as a primary factor, sun light is also a factor. Moderate temperatures coupled with cloudy conditions, low light) favor relatively strong thinning. However, until fruit show signs of growth carbohydrate deficits which favor strong thinning are not nearly as important as when growth starts accelerating (about 6 mm stage). Last year Wes Autio ran the carbohydrate model for the growing season. Carbohydrate deficits at petal fall were below -40 ( a strong deficit) but when thinners were applied then, only modest thinning was observed. Had that deficit occurred at the 10 mm stage and normal thinners were applied, I suspect that over thinning would have occurred.
In summary, apply carbaryl as soon as you have an opportunity. When the rainy weather subsides be prepared to apply thinners, probably this weekend. Do not be afraid to include carbaryl again.