Leafhoppers can transmit the red blotch virus in grapevines in a greenhouse, but may not be responsible for its spread outdoors.
While greenhouse tests have shown the insects can be infected with the virus and transmit the disease, it’s too early to jump to conclusions, a plant disease expert says.
Other insects, such as aphids, have previously been found to transmit viruses in greenhouse conditions without being the culprit in nature, said Bob Martin plant pathologist with USDA.
For that reason, Martin told winegrape growers at the recent 2014 Oregon Wine Symposium not to bother spraying for leafhoppers yet.
If they are the “vectors” for red blotch disease, then researchers may develop a targeted approach for killing them at the most opportune time, he said.
Red blotch disease was initially mistaken for leaf roll disease in California vineyards beginning around 2007, said Rhonda Smith, viticulture farm advisor with the University of California.
The leaf-discoloring disease was found to be caused by a different virus in 2011 and 2012 and is worrisome because it delays grape maturity, lowers sugar content and causes other unwanted effects, she said.
In Oregon’s Willamette Valley, the red blotch disease has been discovered in about 15 to 20 vineyards, said Martin.
In some cases, only a few plants are affected, while in others the symptoms are pervasive, he said.
Gauging the exact extent of the disease is tricky because tests are expensive so not every plant can be tested, he said.
At this point, many mysteries surround the virus.
“We know it’s a problem, we don’t know how severe it is, we don’t know how to control it,” said Martin.
Interestingly, researchers have found the disease not only in grapevines but also in three different vineyard weeds, he said.
In some vineyards, the virus is clearly spreading, Martin said. When symptomatic vines are removed, new plants exhibit signs of the disease the following year.
In other vineyards, however, the virus does not appear to be moving from plant to plant, he said.
Symptoms also don’t seem correlated with plant stress, said Smith.