The series of long wetting periods since May 3 are providing ideal conditions for quince rust. Trees are most susceptible to quince rust, which infects fruit but not leaves, when long wetting periods occur either from tight cluster to pink or from petal fall to first cover. (The open petals may prevent spores from landing on susceptible tissue during bloom.) Long wetting periods allow time for spores that are released from the galls on cedar trees to travel greater distances than would be possible during shorter wetting periods. Thus, given the weather of the past week in the Hudson Valley, the spore cloud is likely to envelop all orchards, even those with no adjacent cedar trees. The only factor that might reduce the severity of quince rust is that fact that heavy rains, such as those occurring today (Friday) may knock many of the spores out of the air before they can find susceptible host tissue.
Mancozeb is a good protectant against rust diseases, but we have had two periods of rain, May 3-5 and May 7-10, with enough rainfall to completely remove mancozeb protection before the end of the rainy periods. The only way to ensure that fruit will not develop quince rust is to apply DMI fungicides (FRAC group 3) that provide extended post-infection activity against rust diseases. I don’t think that anyone knows exactly how much post-infection activity the DMI fungicides can provide against quince rust, but the duration of post-infection activity is much greater than for apple scab, perhaps as much as 7 days or even longer. Thus, I suggest that apple growers in southeastern NY and New England include a DMI fungicide in the next spray. So far as I know, none of the other available chemistries will provide post-infection activity equal to that provided by DMI fungicides.
Remember that DMI fungicides may not provide their normal range of activity if they are applied in the rain or if the sprays cannot dry on the leaves before rain resumes. Also, the developing fruitlets will be protected from quince rust only if they are covered with the fungicide: one cannot count on redistribution for post-infection activity. Thus, good spray coverage is important.
The potential threat from quince rust should not be ignored. In the early 1980’s I visited an orchard near Hudson, NY, where a grower had failed to adjust his spray program for quince rust and about 50% of Red Delicious in that otherwise well-managed orchard were affected by quince rust. That orchard had no adjacent cedar trees. In research plots at the Hudson Valley Lab where orchards were adjacent to cedar trees, quince rust in one year showed up on so many fruitlet stems that most of the fruit dropped before the end of June. Quince rust can affect virtually all apple cultivars, unlike cedar apple rust where many cultivars are resistant. Thus, protection against quince rust must include all apple cultivars. Pears and quince are also susceptible.
For photos of quince rust infections, see https://blogs-dev.cornell.edu/plantpathhvl/apple-diseases/rust-diseases/
