grapevine red blotch

Beginner

Scientific Name
Grapevine red blotch, Geminiviridae

Identification
Leaves:

No visible difference between diseased and healthy vines in the spring
Red spots or blotches start to develop in mid- to late-summer coalesce as the season progresses, with most of the leaf blade becoming red. Shades of red varied from crimson to purple on red varieties. On white varieties foliar symptoms are less conspicuous and generally involve irregular chlorotic areas that can become necrotic later in the autumn
Older leaves at base of the canopy first affected
Late in the season, heavily symptomatic leaves often drop off prematurely.

Fruit:

Clusters smaller than normal but with normal-sized berries; up to 30-50% loss in yield
Often matures late and irregularly
Low in sugar (25-50% reduction), soluable solids and has poor pigmentation

Often Confused With
Potassium deficiency – reddish-purple interveinal discolouration starting at leaf margin but without the green coloration bordering the veins; symptoms first develop on leaves at mid-shoot

Magnesium deficiency – in red varieties, reddish interveinal discolouration starting at leaf margin but without the green coloration bordering the veins; symptoms first develop on basal leaves

Grape leafroll disease – downward rolling of leaf margins; interveinal reddening while veins stay green in red varieties.

Period of Activity
Most visible symptoms appear in mid to late summer.

Scouting Notes
Basal leaves will exhibit symptoms in mid- to late summer.

Threshold
None.

Advanced

Scientific Name
Grapevine red blotch, Geminiviridae

Identification
Leaves:

No visible difference between diseased and healthy vines in the spring
Red spots or blotches start to develop in mid- to late-summer coalesce as the season progresses, with most of the leaf blade becoming red. Shades of red varied from crimson to purple on red varieties. On white varieties foliar symptoms are less conspicuous and generally involve irregular chlorotic areas that can become necrotic later in the autumn
Older leaves at base of the canopy first affected
Late in the season, heavily symptomatic leaves often drop off prematurely.

Fruit:

50% loss in yield
Often matures late and irregularly
Low in sugar (25-50% reduction), soluable solids and has poor pigmentation

Often Confused With

Potassium deficiency – reddish-purple interveinal discolouration starting at leaf margin but without the green coloration bordering the veins; symptoms first develop on leaves at mid-shoot

Magnesium deficiency – in red varieties, reddish interveinal discolouration starting at leaf margin but without the green coloration bordering the veins; symptoms first develop on basal leaves

Grape leafroll disease – downward rolling of leaf margins; interveinal reddening while veins stay green in red varieties.

Biology
Red blotch has been identified in hybrids, rootstocks and juice and table grapes as well as vinifera varieties and wild grapevines. The incidence and severity of symptoms vary depending on the grape cultivar, environmental conditions, and cultural practices.

Red blotch is transmitted by grafting and persists in propagative material (budwood, rootstocks, and grafted vines) which are largely responsible for its dissemination over medium and long distances. Infected rootstocks are generally symptomless aside from reduced vigour so the risk of disseminating the disease is increased if untested rootstocks are used.

GRBV also affects berry physiology, causing uneven ripening, higher titratable acidity, and lower sugar and anthocyanin content.

Two species of treehopper, the 3-cornered alfalfa treehopper and the buffalo treehopper, have been identified as vectors of GRBV. In the Eastern US and BC, red blotch infections do not appear to have spread within infected vineyards so it is possible the vectors are not present in adequate numbers to pose a problem.