Red leaves have become apparent in many red vinifera vineyards that might have leafroll or red blotch virus infections. In some cases, the leaves are rolling and in others they aren’t. Typically, these symptoms do not show up until veraison in August. However, vine development is advanced by about 2 weeks from normal, so virus symptom development is following along the same trajectory.
Symptoms of both leafroll and red blotch first show up on the mature leaves. The symptoms on white viniferas are yellowing and/or necrosis on the mature leaves. The young leaves at the top portion of the canopy might not show typical virus symptoms.
Both grapevine leafroll-associated virus-3 (GLRV) and grapevine red blotch virus (GRBV) have been identified in symptomatic samples that have been submitted.
Red or yellow areas on leaf margins that progress to scorched areas have also been observed. These symptoms are not caused by virus but rather from water stress-induced nutrient deficiency, likely potassium.
The only way to confirm that symptoms are caused by virus is to submit petiole samples to a qualified lab for molecular testing. Test, don’t guess!