Grapevine disease symptoms in the Fall

Pierce’s disease

The first sign of Pierces disease infection is drying or “scorching” of leaves.  This usually develops by mid or late summer. Initially, scorching of leaves may be observed as yellow or red color along the leaf margins.  Leaves continue to dry from the edges in concentric zones.  Different varieties show slight variation in the development of the leaf symptoms. Infected leaves fall off leaving the petiole attached to the cane.  Fruit clusters may also wilt and dry up. The bark on affected canes often matures unevenly. From summer through harvest, look for the above symptoms in the vineyard and mark vines showing these symptoms. Laboratory testing will confirm the diagnosis. 

Red leaves – leafroll disease or something else?

Red leaf symptoms are alarming and may serve as a warning sign of something being wrong with the vine since normal fall color of red skin varieties is yellow, not red.  Red leaf symptoms can be caused by several different reasons- Leafroll disease is one candidate but nutrient deficiencies such as potassium, magnesium or phosphorus may also cause reddening of leaves which, similar to leafroll, starts on older leaves. Some root diseases or gopher damage may also lead to appearance of red leaves. Feeding of mites may also turn leaves red as well as physical girdling of trunk or cordon.

Laboratory testing helps out to identify the cause of red leaf symptoms. Proper sampling is necessary to ensure that the virus was distributed into the sample being tested. Pick from symptomatic leaves and from multiple sites.

Grapevine Red Blotch Disease

A new DNA virus, Grapevine Red Blotch associated Virus, or GRBaV, was discovered in grapes in 2012. The virus is named after the symptoms it was found to be associated with: irregular blotches of red color appearing in leaves and leaf blades of red varieties in late August. Characteristic of the disease is that leaf veins also turn red. Brix values on the grapes infected with the Red Blotch virus appear to be 4-5 units below normal.

Although red blotch disease symptoms may sometimes resemble those of the leafroll diseases, distinguishing characteristic of this disease is that leaf blades do not roll down as they do in leafroll-infected vines. Also, in GRBaV-infected vines secondary and tertiary veins are clearly red, while in leafroll-infected vines, veins always stay green.

A laboratory assay for qPCR detection of GRBaV is available at AL&L Crop Solutions.