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Ask Your PCA: How can farmers combat rust and shot hole in orchards?

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Issue Date: October 14, 2020
Justin Nay, PCA, Integral Ag Services, Durham
Justin Nay

Both rust and shot hole are pathogens of stone fruit—almonds, peaches, prunes. Under the right conditions, if they build up to high levels, they cause leaf drop. If that happens over a large-enough area, it can cause problems with harvesting or prevent fruit from maturing, as well as limit the crop the following season. These diseases can overwinter on leaf tissue as well as other parts of the tree, but primarily on the leaves.

During the growing season, when the leaves should be functioning and healthy, shot hole and rust cause premature leaf drop. When this happens, the tree has to use its carbohydrate reserves to re-leaf instead of for nut production or for energy that would be banked for next year.

When the trees re-leaf at the end of summer, those leaves don't come off the tree as easily in the fall, and they may not come off even with foliar zinc treatments, because they're young and tender.

In recent years, with warmer winters and growers pushing for higher production, it's common to see leaves on almond trees—and on stone fruit trees in general—all the way into January/February of the following year.

When the leaves remain on the tree, there's no break between the inoculum carryover from one year to the next. Often, there will be orchards simultaneously with new leaves and the previous year's leaves.

A November application of foliar zinc plays a major role in limiting the severity of these diseases, especially rust. Shot hole isn't as prevalent as it has been in the past, and that may be a result of spring fungicide treatments. Problems with shot hole have been replaced with Botrytis during petal fall.

If the trees don't defoliate, there will be rust actively building up in the wintertime, and as long as there are leaves and moisture, it's cycling and building up.

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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