Disease Management Update

Strobilurin ("Stroby" ) fungicides  -- A Clarification of  (1)  what is meant by "Excellent Redistribution" and (2) Minimal Rates

Strobilurin fungicides (i.e., Sovran, Flint)  are considered excellent protectants. However, unlike the standard protectants such as captan or the EBDCs where you could get redistribution of the material onto new leaf tissue during wet weather,  "redistribution" with the strobilurins is different.  That difference in the  "redistribution" characteristic of these new fungicides  is very important .   The following excerpt from an article written by Drs. Wilcox and Rosenberger entitled "Apple Disease Control with New Fungicides"  (Scaffolds, Vol. 9, March 27, 2000) should clear up any confusion:

"These materials are retained primarily within the waxy cuticle of leaves and fruit, which means that they are more rainfast than traditional protectants.  This also means that they don't redistribute very well from leaf to leaf in rainwater, although they do redistribute well within the waxy layers of a given leaf (or fruit).  Furthermore, a small portion of the total dose does diffuse from the surface of a sprayed leaf and, after a few days, enough accumulates on the other side so that it offers fungicidal protection on that unsprayed side (termed "translaminar" activity).  This general pattern of fungicide movement is unique to the strobilurins, and different manufacturers have made up their own trademarked names to describe it, e.g., "surface systemic" for Sovran and "mesosystemic" for Flint. "

In other words, redistribution is local -- redistribution is within the leaf or fruit.  Good initial coverage is essential. 

In regard to minimal rates,  Drs. Wilcox and Rosenberger in the same article cited above state  the following:

"... we suggest that apple growers in New York use Sovran at a minimum rate of 1.33 oz/100 gallons of dilute spray for tree-row-volume applications.  Technical support personnel from BASF recommend a minimum of 2.0 oz/acre even on the smallest trees, a recommendation that prudently recognizes the potential inefficiency of spray capture in small trees and the relatively high crop value in high-density plantings. We recognize that a rate of 1.0 oz/100 for Sovran may be adequate for purely protective sprays under modest pressure, but both the label and our personal experiences indicate the need for higher rates if postinfection activity is required. The minimum rate for Flint is 0.67 oz/100 gallons dilute basis.  The latter is based not only on the 2:1 formula for Sovran:Flint, but also is derived by dividing the lowest label rate (2 oz/A) by 3, using the increasingly standard assumption that per-acre rates must be divided by 3 to arrive at rates per 100 gallons for apples in New York State.  A common "fudge factor" (at least 150 gallons/A dilute basis, even on the smallest trees) yields a minimum per-acre rate of 1.0 oz for Flint.. "

Degree Day Model for Ascospore Maturity

Similar to what we did last year, we will be 'tracking' degree day accumulation at  sites around the state and then using the Degree Day  (DD) Model developed by Drs. Gadoury and MacHardy to estimate apple scab ascospore maturity.  The following is an estimate of  ascospore maturity  at 6 Vermont sites and a chart  that you can use to determine the ascospore maturity from your accumulated degree days at your orchard.

We are currently in the "lag phase" of ascospore maturity  that extends from 1 DD to approximately 300 DD, when about 20% of the season's ascospores mature.  This is the phase when ascospores mature slowly because of cool weather;  the daily average DD is only 15. Thus, matured ascospores do not accumulate rapidly in the pseudothecia between rain events. For example, during a 5-day dry period, only about 5% of the season's ascospores would mature. This phase usually ends about 20 days after bud break.