Zinc deficiency can cause spring symptoms of small, misshapen leaves with interveinal yellowing (chlorosis). Plants should respond favorably to zinc fertilization if zinc deficiency is the cause of these symptoms.
Salt burn or chemical injuries from other sprays or fertilizer can cause marginal browning of leaves. Marginal scorching caused by high salt concentrations in soil or irrigation water usually uniformly affects large areas of vines, not just individual vines. In addition, scorching due to salts usually is not preceded by yellow or discolored tissue.
Individual leaves on entirely healthy and vigorous vines sometimes have water shortage symptoms because the leaf is detached from the stem by the rapid expansion of the stem's diameter during rapid growth. In such cases, the leaf petiole can be pulled from the stem to reveal a brown cork layer formed between the petiole base and the stem. This condition differs from Pierce's disease in that leaf scorch caused by growth-caused detachment of leaves affects only the oldest leaves on otherwise vigorous stems.
Measles is a disease suspected to
be caused by fungi that has symptoms similar to those on leaves of
vines with PD. Differences from Pierce's disease symptoms are mainly
that the grapes on measles vines have small black spots on the fruit
skins, small areas of interveinal yellowing and discoloration that
become necrotic tend to be in the center of the leaf rather than the margin alone, and symptoms appear about a month or more earlier than Pierce's disease.
For more details on measles go to:
http://axp.ipm.ucdavis.edu/PMG/r302100511.html
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