Apple Scab Strategies
Alan Jones,MSU Dept. of Botany & Plant Pathology
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One thing we were painfully reminded of in the Grand Rapids area this past season is that is doesn’t take a whole lot of spores to create a severe infection. Very few spores were caught throughout the spring, but the scab lesions still managed to show up.
Often, the hardest parts of making decisions about apple scab control is when to start and when to stop. More often than not, the first rains that occur past the showing of green tissue are predicted as light showers. Should you spray or not? Growers who take the cautious approach will apply a protectant fungicide ahead of the infection periods. They may be wasting their time and a little money. Those who opt to wait until after the first infection period and then spray with an eradicant fungicide may be removing one or two fungicide applications from their spray bill. From this description, it seems that it makes good sense and would pay to wait until after the first infection period and use an eradicant.
However, there are some hidden risks in depending on eradicants for the first spray. First of all, the “back action” of eradicants does not last forever. You have to consider that if the infection period itself lasts one day, 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 or strobilurin fungicides.
Also, weather conditions after the infection period are often less-than-ideal for getting good coverage. Oftentimes, the frontal systems that bring clear weather also bring wind, so there’s a good chance that day two (and perhaps day three) after the infection period will be windy. If you’re willing to chance that day three or day four will provide ideal spraying conditions, don’t forget that there’s always the risk that the sprayer will break down (which can easily happen if this is the first spray of the season).
If sprays are applied in the wind, it’s safe to say that coverage will be less-than-perfect. However, when a protectant fungicide such as captan or mancozeb or metiram is applied under windy conditions ahead of a rain, one can take comfort in knowing that the same rains that discharge scab spores can also redistribute fungicide residues. Thus, protectant fungicides are a bit “forgiving” when it comes to coverage, whereby imperfect coverage does not necessarily result in poor scab control.
However, when fungicides are applied as eradicants, the scab spores have already been discharged to the leaves when the fungicide is applied. Therefore, complete spray coverage is essential for good scab control. Remember, wind and eradicants do not work well together.
In orchards that have high apple scab inoculum levels from last year, waiting to apply the first spray until after the first infection period is a high-risk strategy. There are lots of uncontrollable factors and the potential for many things that can go wrong. In orchards with low scab inoculum levels, using an eradicant as the first scab spray can save money. This is also a sensible approach in dry years. However, in high-inoculum orchards, it will be safer to get a protectant fungicide applied ahead of the first scab infection period. Perhaps it will allow you to sleep better and appreciate the rain for its irrigation qualities rather than its apple scab infection qualities.
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