Definition - What does Leaf Spot mean?
Leaf spot is a common name used in viticulture that describes the symptoms of a set of diseases that cause the leaves of the vine to appear spotted. The diseases that cause leaf spot can be from fungi or bacteria, and it takes an experienced viticulturist to know the difference, as insects can complicated matters and create damage and symptoms in grapevines that appear to be the same as leaf spot.
WineFrog explains Leaf Spot
When grapevine leaves begin to show leaf spots, the vines are often already compromised. While leaf spot on wine vines can be from insects or bacteria, leaf spot is most often caused by the fungus Phomopsis viticola, which flourishes in wet cool weather and often takes hold in the vineyard during the spring. The leaf spot is characterized by small, black patches on the leaves with a yellow ring on the outside of the spot; while the leaves display a visual symptom, the vines overall will have less vigor and will not be able to produce wine grapes to their potential.
Phomopsis viticola and other leaf spot fungi, can live through the winter in the vineyard and release spores during the spring rains. The disease spreads as water splashes from the ground onto the vine and the vines sit with moisture on the leaves. The fungi prefer cooler temperatures and will stop developing in the hot months of summer, but will become active again in the fall, especially if there is rain.
In severe cases of leaf spot, the shoots, fruit bunches and canes can become infected. The shoots will develop small black spots, that if left untreated, will split the shoot. Infected fruit bunches will shrivel and canes will develop black spots along with white areas. Once a vine exhibits these symptoms, vineyards will experience crop loss both short term and long term as the health of the vine becomes compromised.
Vineyards can use fungicide in the early spring and frequently throughout the early season, if there is a history of leaf spot in the vineyard, vineyard managers can utilize effective canopy management techniques to ensure proper airflow through the vine to reduce moisture build-up.