Plum nursery mites and firing observed in tart cherry

Tart cherry growers may need to apply a miticide application to protect overwintering health of trees.

With this season’s hot and dry conditions, we have observed high plum nursery mite populations in tart cherry. In some cases, these mites coupled with the weather conditions have resulted in ‘firing’ of tart cherry branches. Firing looks similar to fireblight in apple where a branch or multiple branches on a tree appear dead and all, or the majority of leaves on that branch are completely brown and dried up. Branches will die as a result of this firing. Firing can occur without the presence of mites, but in most cases, the mite and weather combination cause this problem. In most years, plum nursery mites are present in tart cherry but with the hot weather of 2012, these mites have reproduced quickly and populations have exploded in orchards. We have observed higher than normal plum nursery mite populations in our miticide trial at the Northwest Michigan Horticultural Research Station.  

To determine if a miticide is warranted in trees this late in the season, growers should establish if mites are indeed present on the leaves of the tree. Plum nursery mite is a rust mite. These mites are extremely small and live on the upper and lower leaf surfaces and feed primarily on new growth. Growers should use a high powered hand lens (30x) and look for the tiny wormlike mites with two pairs of legs along the leaf midribs. If growers observe the mites, particularly in trees with firing, a miticide is likely needed to protect the overwintering health of the trees as it is at risk from mite injury. Now is a good time to apply this miticide application because plum nursery mites will continue to feed and reproduce into September. Vendex and Nexter are registered for plum nursery mites (rust mites) in cherry.

Miticide applications are not inexpensive, and a spray for mites is somewhat unusual this late in the season. Growers are understandably hesitant to spend the money on this application following a particularly difficult year. However, if mite populations are extremely high and this hot and dry weather continues, more firing and branch death can result in weakened trees and orchards as we head into winter.

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Drs. Rothwell and Wise's work is funded in part by MSU's AgBioResearch.

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