Given what researchers know about Pierce’s disease (PD) they continue to chase clearer answers into cause and effect of the bacterial disease.
Matt Daugherty, Cooperative Extension specialist in the University of California, Riverside’s entomology department, says PD is a statewide problem, even though the main vector for the disease remains relegated primarily to Southern California and parts of the Central Valley.
Pierce’s disease can be found in all of the state’s grape-growing regions, though Daugherty admits that the fatal disease isn’t always a death sentence. That’s part of what researchers are trying to better understand.
This much they do know: common symptoms of the disease include progressive leaf scorch, defoliation, “matchstick” petioles, raisined grape clusters and vine dieback.
What researchers are still studying is how some vines have been able to recover from the disease.
Daugherty says there are indications that vines may be able to recover during colder winter months. It appears that winter temperatures below 40 degrees can have a positive impact on helping vines “lose their infection,” he says.
Pierce’s disease seems to be on the rise again in California, which he thinks may be due in part to warmer winters in the past several years, though that is not completely understood either, he says. Perhaps it’s due more with vector response to the warmer winters.
“Is there a novel new strain driving this current epidemic?,” he asked at a recent Cooperative Extension seminar for grape growers on the Central Coast? The jury is out on that question as well.
What is known is there has been a significant rise in the population of glassy-winged sharpshooters (GWSS), the principle vector for PD. An area of Kern County near Bakersfield has seen an approximate 10-fold increase in GWSS numbers over the past few years while the Temecula area of Southern California has experienced a two-to-three fold increase in GWSS numbers over the same period.
Daugherty says GWSS populations are the focus of much attention because of their sheer numbers. Even though they are not as efficient at spreading the disease as native sharpshooters, “it’s really a numbers game with them,” he says.
Native sharpshooters like the blue-green sharpshooter, are more efficient at vectoring the disease, he says, though their numbers tend to be lower. These are the predominant PD vectors in North and Central Coast vineyards. Their range extends along the West Coast from the Baja Peninsula to Canada.
Other native sharpshooters tend to have moderate abilities to spread disease and are also lower in numbers than the well-known glassy-winged sharpshooter.
Regardless of the species of sharpshooter, Daugherty says once they contract the bacterial disease the insects are infested for life.
Continuing studies seek answers ranging from the control of the pest to the possibility of finding or breeding plant materials that are resistant or tolerant to the disease. He is also looking at cold-weather impacts that could help vines lose their infections and thus repair themselves.
“We’re looking at some sites in Napa and Sonoma, but nothing stands out so far,” he said.
Vector control is not easy as there are few parasitoids and other biological controls. Chemical controls can help, but in the case of organic grapes, current products have shorter-lasting residual effects, leading to repeat applications and greater control costs.
Soil applications of Imidacloprid on the North Coast are not effective, leading growers to rely on foliar applications.
Daugherty says researchers are looking into another known vector of the bacterium Xylella fastidiosa, the cause of Pierce’s disease.
The spittlebug (Phiaenus spumarius) appears less efficient than the blue-green sharpshooter at vectoring the disease. He cautions that much of the biology of this pest remains unknown and must be studied to determine it possible impacts related to PD.
Grapevine leafroll-associated virus is said to be named for a group of viruses infecting grape vines.
According to Monica Cooper, viticulture advisor for the University of California Cooperative Extension in Napa County (North Coast), five different viruses cause this disease, which in the case of two of the viruses – leafroll 2 and leafroll 7 – researchers still don’t understand how they move throughout the vineyard.
She also addressed an estimated 200 at a recent viticulture seminar hosted by the University of California Cooperative Extension in San Luis Obispo, Calif.
The remaining leafroll diseases – leafroll’s 1, 3 and 4 – are vectored by insects, including mealybugs, soft scale and hard scale. It’s the three leafroll diseases vectored by these that researchers are most concerned with.
Leafroll associated-virus 3 is the most common and widespread of the viruses around the world, and as such is the most studied, she says.
This version leads to phloem degeneration, degraded chlorophyll and a decrease in photosynthesis, she says. The end result is decreased fruit quality and “severe economic impacts.”
Mealybugs and soft scale are the primary movement of this disease in vineyards.
According to Cooper, mealybugs feeding on infected vines can acquire the virus within an hour and are able to then inoculate healthy vines within a similar amount of time.
Unlike other vectors, which maintain disease profiles for their entire life cycle, mealybugs can lose their ability to transmit the virus after several days of feeding on healthy vines.
Cooper recommends growers employ consistent vine removal strategies to control disease in their vineyards. Because of the latency of the disease – the period between when vines are first infected and when symptoms appear – growers can sometimes think they’re not staying ahead of the disease.
She cited an example from the North Coast where a grower during the 2014-15 dormant season removed 936 vines, only to discover more disease in the 2015 growing season.
Thinking he got it all the first time, the grower was puzzled at the new symptoms, but was told that it takes about a year from infection for symptoms to appear. This led the grower to remove another 96 vines at the end of that growing season, and another 33 the following year.
It can take three-to-five years of removals before disease incidents are significantly reduced or eliminated, Cooper says.
Vine removal is just one of a two-pronged approach – mealybug control efforts remain an important method to manage disease in the vineyard, she says.