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W i n e s & V i n e s s e p t e m b e r 2 0 1 4 77 WineEast To address those concerns, Dr. Misha Kwasniewski, assistant research professor and enology program leader at the Grape and Wine Institute at the University of Missouri, held a PD workshop titled "Evalu- ating and Mitigating Risk" on May 6. To provide growers with the latest information about what to look for and how to assess their vineyards, Kwasniewski invited Kamas from Texas and Dr. James Wolpert, viticul- ture extension specialist emeritus at the University of California, Davis. Kamas encouraged growers not to panic, noting that some vineyards in Texas have survived even with significant pressure from PD. He underscored the point that even if PD is a part of the landscape, it probably won't jump into a high-pressure situation and probably will be more chronic than acute. Part of the problem, according to Wol pert, is that the potential source for PD in Missouri is not clear. It may come from infected plant material, or there may be host plants in Missouri's woods. Research has shown that Xf cannot survive for long in sub-freezing temperatures, and during the 2013-14 winter in Missouri, temperatures dropped to -15° F and remained "stuck there" for several weeks While there is a strong relationship between cold cycling and the survival of PD-causing bacteria, there may be other factors involved. Wolpert stressed that it is most important for lines of communication to be open between growers and research- ers, since growers know their vineyards and can report possible symptoms if they see something. Their vigilance can help extension personnel know what is happen- Spotting the symptoms T he initial visual symptom that Pierce's disease (PD) is present often is irregular scorching of grapevine leaves. However, on different varieties the scorching may not look the same and may appear at different times of the growing cycle. According to Jim Kamas of Texas A&M, "On white varieties, the leaf margin starts to bleach and then becomes necrotic....On red varieties, a red 'facing' usually accompanies the scorch. As with whites, some varieties show scorching very soon after infection, and others may take years to show the first signs of infection." Another symptom is the abscission of the leaf blade while the leaf petiole is retained, and a third symptom is the uneven ripening of canes. The cane remains green at nodes, while the intermodal part of the stem turns brown. A trained eye can tell the difference in the field by looking at the vines. However, these visual symptoms should be confirmed by laboratory analysis. Leaf scorch- ing can be a sign of other problems including salt injury, pathogens such as Esca or black measles, nutritional deficiencies or phytotoxicity resulting from agricultural chemicals. The abscission symptom can be the result of other pathogens as well. Multiple insect vectors spread Pd While the glassy-winged sharpshooter is the main vector for PD in California, the bacteria Xylella fastidiosa can be transmitted by many insects including Cicadellidae sharpshooters (the family that includes the glassy-winged sharpshooter), Proconiini sharpshooters and three different species of spittlebug, for a total of more than 35 different species of competent vectors, according to Kamas. Kamas found two different kinds of sharpshooters in Missouri that were bigger than the glassy-winged sharpshooters and that could survive Missouri's colder winters. North Carolina has a similar, but more diverse, species. Texas has a high degree of parasites that help to reduce the number of sharpshooters. Kamas also noted that the more people look for PD, the more it is found. The ques- tion, then, is what is moving the pathogen around? He has found that traps placed at different heights in a vineyard will get different species of sharpshooters. L.J.M. grapevine leaves display evidence of irregular scorching as a result of Pd infection. this vine shows uneven periderm development. The potential source for Pierce's disease in Missouri may come from infected plant material, or there may be host plants in Missouri's woods.