Sep 22, 2010

Virus of tart cherry confirmed in northwest Michigan

Growers in northwest Michigan have noticed yellowing leaves in their tart cherry orchards. Most assumed that the yellow is caused by cherry leaf spot (CLS), which indeed has been showing up in the region’s orchards. However, on closer inspection, there are no CLS lesions on the leaves.

George Sundin and Nikki Rothwell of Michigan State University (MSU) issued a Crop Advisory Team (CAT) alert because they suspected that they were seeing green ring mottle virus (GRMV) in some orchards, while in others they believed the yellowing was caused by cherry yellows virus. The GRMV diagnosis was confirmed by laboratory testing conducted at Washington State University, Sundin said.

Symptoms of GRMV infection are usually expressed in late June to mid-July. The main symptom of GRMV infection, according to the CAT alert, is bright yellow leaves with circular green blotches, MSU said. These infected leaves can be removed from trees during shaking at harvest, or will defoliate naturally. Severe cases of GRMV can result in a loss of 40 to 50 percent of the leaves on a tree.

In northwest Michigan, GRMV symptoms have been appearing sporadically (one to a few infected trees per block), but significant levels of defoliation are often associated with the disease. Sometimes fruit is affected by GRMV and will look indented with streaks of dead tissue to the pit, according to the CAT alert. Natural spread of GRMV is slow and appears to occur tree-to-tree via root grafts. Grafting at the nursery stage can also spread the virus, MSU said.

“Indeed, with the virus-indexing capabilities nurseries employ, we should not be seeing this disease in virus-free nursery stock,” Sundin said.

There is no known control for GRMV infection. MSU currently has little knowledge why symptoms of GRMV have appeared this season and not in previous years. Growers that identify GRMV in their orchards should flag infected trees and have MSU specialists observe if the GRMV symptoms reappear, and if the virus spreads between trees, Sundin said. Similar to GRMV, cherry yellows causes yellowing of leaves and defoliation, but this virus is typically more widespread than GRMV, according to the CAT alert.

In most cases, both viruses are more apparent in older orchards, Sundin said. Cherry yellows is caused by the prune dwarf virus (PDV), and the symptoms of leaf yellowing commonly occur three to four weeks after petal fall. Unlike GRMV, PDV is seed-borne, pollen-borne and transmissible through grafting.