||Spots with ashy gray centers on the fruit of a grape (Vitis 'Vidal Blanc') caused by anthracnose
Anthracnose of grapes, caused by the fungus Elsinoe ampelina, is a serious disease of home-grown grapes. It is also commonly called bird’s eye rot for the distinctive spots it causes on grape berries. The disease is most destructive in warm, wet seasons. It attacks all green parts of the vine – leaves, shoots, leaf and fruit stems, tendrils, and fruit. The most damaging effect is to the fruit. Warm, muggy weather in the spring and summer, along with unsprayed fruit of susceptible varieties, may cause significant fruit damage directly or leaf defoliation that will rob the fruit of sugar to ripen properly. Anthracnose is not difficult to control if good cultural practices are followed along with the use of protective fungicide sprays.
Along with black rot, anthracnose is one of the most serious fungal diseases of grapes in warm areas. Both can generally be controlled using the same disease management strategies and fungicides.
Symptoms and Diagnosis
Leaves and stems: On the leaves, small brown spots develop which fall out when the tissue dies creating a shot-hole effect. Young affected shoots become distorted and dieback. On stems, white-centered, bird’s eyes spots develop, which can grow and girdle the stem resulting in die back. Young, rapidly growing leaves and stems are most affected. Diseased leaves, stems and stem cankers are a source of infection later in the season as well as for infection the following year.
Fruit: Fruit develops round purple spots that later turn ashy gray in the centers with dark brown margins. The spots expand to infect the whole berry, which withers and drops from the cluster, but some mummified berries may persist. In black rot of grapes, diseased, withered berries turn coal black and do not drop off. The diseased fruit hangs on the cluster and are also referred to as mummies.
The anthracnose fungus overwinters in dense mycelial masses (called sclerotia) in infected woody tissue. Stem cankers can become a major infection source the following spring as well as throughout the year. During rain, microscopic spores (conidia) are released and are carried by rain-splash to young, expanding leaves and shoots as well as fruit. The conidia will germinate and infect when free water is present on the plants for 12 hours and temperatures are between 36-90 degrees F. The higher the temperature the faster infection will occur. When the weather is wet, spores can be released the entire spring and summer providing continuous infection. Cool weather slows growth of the fungus.
Integrated Pest Management Strategies
1. Planting. Space vines properly and choose a planting site where the vines will be exposed to full sun and good air circulation. Keep the vines off the ground and ensure they are properly tied, limiting the amount of time the vines remain wet, thus reducing infection.
2. Sanitation. Remove infected shoots, canes and fruit clusters as they develop and remove any wild grapes in the area as they can harbor the disease. Keep the fruit growing area and surrounding areas free of weeds and tall grass. This practice will promote lower relative humidity and rapid drying of vines, thereby limiting fungal infection.
3. Pruning. Prune the vines in early winter during dormancy. Select only a few strong, healthy canes from the previous year’s growth to produce the following season’s crop. Remove these prunings from the vineyard and burn or destroy. Remove and destroy any diseased shoots during the growing season, disinfecting tools between cuts.
4. Cultivation. Cultivate the vineyard before bud break to bury the diseased berries and leaves. Diseased berries covered with soil do not produce spores that will reach the developing vines. For homegrown grapes, use 2–3 inches of leaf mulch or fine bark to cover infected debris.
5. Fungicides. Use protective fungicide sprays. A delayed dormant spray of liquid lime sulfur or Bordeaux mixture just as the buds are breaking will help reduce anthracnose problems. Pesticides registered to protect the developing new growth and fruit are captan, chlorothalonil and mancozeb. Spray at 2-week intervals from bud break until the fruit begins to color.
6. Cultivars. Cultivars with large, juicy berries are the most susceptible. In general, grapes that ripen late in the season are affected the least. Most commercial cultivars are sufficiently resistant if adequately protected with a fungicide spray program.
Strategies 1, 2, 3 and 4 are strictly organic approaches. Of the fungicides listed in Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate organic lime-sulfur products. Some copper sprays, which can provide some protection, may also be allowed.