Red blotch is a plant disease that poses a threat to agricultural industries, particularly impacting grapevines globally. It can diminish crop quality and yield, affecting vineyards across North America and beyond. Understanding its characteristics and how it spreads is important for managing its presence.
Understanding Red Blotch
Red blotch is a plant disease primarily identified by distinct visual symptoms on affected grapevines. In red grape varieties, irregular red blotches appear on the leaf blades, often originating from the leaf margin or within the leaf blade itself, with primary and secondary veins frequently turning red. For white grape varieties, symptoms manifest as pale green to pale yellow patches on the leaves, which can sometimes progress to necrosis late in the season.
These blotches typically begin on the basal (older) leaves of the shoots in mid-to-late summer and then spread upwards through the canopy. While similar to grapevine leafroll disease, red blotch does not cause the downward rolling of leaf margins. The severity and onset of symptoms can vary depending on the grape cultivar, vineyard location, and growing season conditions.
Origins of Red Blotch
Red blotch is caused by the Grapevine Red Blotch Virus (GRBV), a single-stranded circular DNA virus. This virus was first identified in symptomatic grapevines in California in 2011. GRBV has since been confirmed in major grape-producing regions across the United States, Canada, and many other countries globally.
The virus primarily spreads through infected plant material, particularly via grafting and propagation. An insect vector, the three-cornered alfalfa hopper (Spissistilus festinus), also transmits GRBV from infected to healthy grapevines. This occurs when the insect feeds on the phloem of an infected plant, acquiring the virus, which it then transmits to a new plant through feeding.
Effects on Plant Health and Harvest
Red blotch disease impacts the physiological processes of grapevines, affecting crop quality and yield. The virus inhibits the translocation of sugars from leaves to fruits, disrupting normal ripening. This results in a reduction in total soluble solids (Brix), a measure of sugar content, in the grape juice, often decreasing by 2% to 20% compared to healthy grapes.
Beyond sugar levels, the disease can alter fruit composition, leading to increased malic acid and changes in pH and titratable acidity. There is also a reduction in anthocyanin levels, which are responsible for color in red grape varieties, affecting the visual appeal and quality of wine. These changes contribute to delayed and uneven berry ripening, smaller berry size, and altered sensory attributes in wine, such as reduced fruit aromas and poor color development, which can reduce the market value of the crop.
Controlling Red Blotch
Controlling red blotch disease requires a multi-faceted approach, focusing on prevention and the removal of infected plants. A key strategy involves using certified virus-free planting material, as GRBV is transmitted through grafting and propagation. Growers should request plants certified free of GRBV from nurseries to ensure healthy vine stock.
Monitoring vineyards for symptoms and testing symptomatic vines for GRBV presence is important for early detection. If disease incidence is below 30%, removing individual diseased vines and replacing them with clean stock is recommended to minimize losses and limit secondary spread. If the incidence exceeds 30%, a full vineyard replacement may be necessary to manage the disease. While chemical control for the insect vector, the three-cornered alfalfa hopper, is not currently a standard recommendation, managing cover crops less attractive to these insects may help reduce their presence in vineyards. Maintaining overall vineyard health through proper nutrition, irrigation, and managing crop load can also help mitigate some of the negative effects of the disease.