The other day a frantic grape grower contacted us convinced that PD had invaded the vineyard.  Of course the only sure way to diagnose PD is to do an expensive lab technique involving polymerase chain reaction for match up DNA with a known source (a control).  The problem was that the test came back negative and the final recommendation was that there was a nutrient deficiency.  The symptoms of PD are similar to symptoms of other disorders, including drought stress.  Before you go and have a sample tested (it costs $50 per sample), send us a photo or let us look at it up close first.  We can usually tell classic PD symptoms right away.  The following paragraph, written by Dr. Damon Smith, is directly from the “Handbook of Oklahoma Vineyard Establishment and Management”:

“The symptoms of PD are perennial and will appear late in the summer when weather conditions are predominately hot and dry, or when plants are under drought stress.  Plants will exhibit stress, with wilting of shoots and premature defoliation typically occurring.  Plants will also yield no fruit or have limited fruit production with poor quality.  Chlorosis and green fading colors will develop at the edges of leaves, which dry and turn brown.  Some vines will have a ‘matchstick’ symptom where the leaves have dropped from the plant, but petioles remain attached.  Marginal browning can take on an undulating appearance as it moves toward the veins of the leaves.  Leaf symptoms of PD can look very similar to drought stress symptoms; however, the yellow or red-brown band between green and scorched areas will be absent in vines suffering from drought stress.”

So there you have it.  I know PD is a concern for all vineyards, but crying wolf is not the answer.  Know what to look for, contact an expert to have a look if you suspect it, and then send in for a sample if visual symptoms are confirmed.