Scaffolds 99 index

APPLE SCAB UPDATE

(Dave Rosenberger dar22@cornell.edu & Fritz Meyer fwm4@cornell.edu, Plant Pathology, Highland)

An apple scab squash mount assessment done on April 7 showed 13% mature spores, 0 % empty asci, and 2 spores per scan in the shooting-tower discharge test. Economically significant ascospore release usually begins when our squash mount counts show 15% mature spores. We had several warm days toward the end of last week, so spore maturity in the Hudson Valley is now sufficiently advanced to ensure that significant ascospore discharges will occur with the next rains.

Wetting periods occurred April 9 (9.6 hrs, 43°F.) and April 11-12 (16 hrs, 38°F.) Neither of these provided adequate temperature/wetness duration for scab infection to occur.

MORE ON CONTROLLING APPLE SCAB

Weather conditions during the early scab season have created the usual "unique" dilemmas concerning when and what to spray for apple scab. In the Hudson Valley, weather forecasters have predicted "scattered showers" on three different occasions since Green Tip. Any one of these events could have turned into a scab infection period. As it turned out, none of them did.

With conditions like these, Hudson Valley growers who took the cautious approach and applied protectant fungicides ahead of the infection periods probably "wasted" their money. Those who opted to wait until after the first infection period and then spray with an eradicant fungicide may have shaved one or two fungicide applications from their spray bill. On paper, it appears that delaying the first scab spray until after the first infection period makes good sense. Those opting for eradicant fungicides have the option of using either Vangard (with 48-hour kickback activity) or one of the SI-protectant combinations (96-hr kickback).

However, there are some hidden risks in depending on eradicants for the first spray. First, weather conditions after the infection period are often less-than-ideal for getting good coverage. Figure that the infection period itself lasts one day, so that leaves only one more day of spray time to take advantage of the eradicant activity of Vangard or three days of eradicant activity for SI fungicides. Then, remember that the frontal systems that bring clear weather also bring wind, so there's a good chance that day 2 (and perhaps day 3) after the infection period will be windy. Maybe you're willing to gamble that day 3 or day 4 will provide ideal spraying conditions, but then there's always the risk that the sprayer will break down, especially if this is the first spray of the season.

If sprays are applied in the wind, one must assume that coverage will be less-than-perfect. When a protectant fungicide (mancozeb, metiram, or captan) is applied under windy conditions ahead of a rain, one can take comfort in knowing that the same rains that discharge scab spores will also redistribute fungicide residues. Thus, protectant fungicides are a bit "forgiving" when it comes to coverage, and imperfect coverage does not necessarily result in poor scab control. When fungicides are applied as eradicants, however, the scab spores are already in place when the fungicide is applied and complete coverage is essential for good control.

In my opinion, waiting to apply the first spray until after the first infection period is a high-risk strategy for orchards where inoculum levels are high. There are just too many uncontrolled variables and too many things that can go wrong. Using an eradicant as the first scab spray can save money in dry years and is a sensible approach for low-inoculum orchards. In high-inoculum orchards, it will be safer to get a protectant fungicide applied ahead of the first scab infection period. It will allow you to sleep better and appreciate the rain!