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What is Pierce’s Disease (PD)?
Pierce’s Disease (PD) is caused by the bacterial infection of grapevine xylem vessels (water-conducting pipes). Specifically, the vessels become occluded by the bacterium Xylella fastidiosa. The X. fastidiosa cells and the sticky polymers they secrete block the movement of water and nutrients from the roots to the leaves. Symptoms of PD in a grapevine include scorching of the leaf margin, matchstick petioles (where the leaf falls off the petiole before the petiole falls off the shoot) and green islands (areas on the shoot of non-uniform hardening off in late summer or early fall). Once a vine shows symptoms of PD it usually dies within in a few months to a year and must be torn out. Considering that vineyards are not profitable for the first several years after planting, replanting after a PD infection can be economically devastating to a vineyard owner.
The X. fastidiosa bacteria are transferred from vine to vine by insect vectors called sharpshooters. There are a number of sharpshooter species that may vector the disease, the largest being the Glassy Winged Sharpshooter (GWSS). Sharpshooters feed on the xylem sap of plants inoculating new plants with bacteria that they have acquired from previous feedings.
The Importance of Studying PD in Texas
In the 1990s a PD outbreak eliminated all of the grapevines in the Southern California Temecula area. The USDA, the state of California and the California grape growers combined resources to research the spread of the disease and slow the expansion of the major GWSS insect vector. The USDA has also been interested in the existence of PD in Texas where disease pressure is historically high (at least along the Gulf Coast) and where the current California GWSS population originated. As such, USDA-APHIS has funded the Texas PD Research and Education program to look at the ecology and epidemiology of the bacterium, vector and disease within Texas.
There is tremendous value in studying the biology and ecology of the PD vectors, as well as, plant and bacterium interactions within Texas. Numerous possible PD insect vectors reside within Texas. In fact, Texas has many more species of sharpshooters than California (over 20 species so far) and Texas is the likely home of the recent population of GWSS species in California. Reports by early Texas settlers (up to 200 years ago) stated they were not able to keep their European grapevines (Vitis vinifera) alive along the Gulf Coast of Texas. Members of the European species include varieties like Chardonnay, Cabernet Sauvignon, and countless others. One of the curiosities of this disease is that X. fastidiosa does not seem to cause any problem to the numerous wild grapevine species found in Texas or to kill most hybrids of European varieties and the American species. It makes sense evolutionary that the wild and hybrid vines would be resistant to PD if the bacterium has been in Texas for thousands of years, but the resistance mechanism within wild vines has yet to be identified.
With respect to the bacteria itself, the X. fastidiosa strain that causes PD was historically limited in Texas to the Texas Gulf region where PD pressure has always been high. The recent movement of PD into other areas in Texas has created questions about the spread of this disease. What can we learn about the evolution of the X. fastidiosa PD strain within Texas? Warm Gulf climates and large numbers of X. fastidiosa positive native Gulf plants suggest that the bacterium may have its evolutionary origins in the southeast US. Genetic comparison of X. fastidiosa isolates from California and Texas can help us answer evolutionary history questions such as this. What can we learn about the expansion of PD across Texas? Whether the disease is moving north within Texas because of warmer winter temperatures, insect movement or human transport is not yet known. This can be evaluated by comparing the DNA fingerprints of individual Texas isolates of X. fastidiosa taken from Texas vineyards
Studying PD at the University of Houston-Downtown
Dr. Morano is a member of the USDA-APHIS funded Texas Pierce’s Disease Research and Education Program. As such, she works collaboratively with other scientists at Texas A&M, Texas AgriLife Extension and UT Tyler. She is also working with collaborators on X. fastidiosa genetics at UC Riverside. All of the research conducted on PD at UH-Downtown is conducted by undergraduate research students in the UH-D PD Research Laboratory (see people).
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