We are having a series of warm days in the last few weeks, and that made me wonder about the risk of PD for this year.  As you might have heard from our colleagues Dr. Doug Pfeiffer and Dr. Chris Bergh, you can predict the risk of outbreak or Pierce’s Disease (PD) by monitoring winter time temperature.  When your area receive more than 3 nights (or days) with temperature below 15F (-9.4C), the risk of PD outbreak will be low.  It is determined by the number of nights (or days) during the winter months, and it does not happen as three consecutive nights.  Here are records of 2011 winter from Virginia Cooperative Extension’s mesonet weather stations.

It seems that Accomack co., VA Beach, and Bedford co, had only one day with temperature below 15F, indicating that in these regions, the risk of PD outbreak is high.

Unfortunately, the mesonet stations are scattered around the states and you may not find the one nearby your place.  If you click on the link (a banner) to the weather underground on the right-hand side of this page, type in your zip code,  click “view” under “History and Almanac” section, and then click on monthly view, then you can find daily minimum temperatures of the weather station nearby your location in a calender format.

Here’s a picture of a PD infected vine.  As you can see, it shows a distinctive burning-like necrosis (or dessication) on the edges of the leaves, and often associated with”matchstick” appearance of petioles without leaf.

Once your vines are infected with PD,  the phytoplasma called Xylella fastidiosa (a bacteria-like pathogen of PD) will spread systemically in the infected vine.  X. fastidiosa is known to be vectored by several species of sharpshooters, including glassy-winged sharpshooters (Homalodisca coagulata); however, since these insects can fly and migrate, the insect control may not be the best option to prevent the spread of PD.  Thus, the only the practical remedy at this point is the removal of the infected vines in order to reduce the risk of secondary spread. 

If you want to learn more about PD, vectors, and the risk assessment, please visit Doug’s page at http://www.virginiafruit.ento.vt.edu/PDWinterRisk.html and http://www.virginiafruit.ento.vt.edu/PDsharpshooters.html