![]() When the disease is severe the entire cluster can be killed. Infected shoots, stems (pedicels), tendrils and young flower clusters are curled and thicker than non-infected tissue (image on the left photo courtesy- ). Masses of white, down-like spores will form on infected shoots, stems (image on the right, photo courtesy- ), and tendrils. Lesions with spores appear as a dense white mass (image on right, photo courtesy- agric.wa.gov.au) During periods of wet weather and high humidity, masses of spores form on the underside of the leaf lesion. Lesions are usually restricted in size by the leaf veins. As the leaf ages, the spots turn brown (necrotic) and become crusty (center image, photo courtesy- nicolewarduk.blogspot). Early downy mildew leaf symptoms are often confused with powdery mildew. As the disease develops the spots enlarge and turn yellow (image on left photo courtesy- ) and have an irregular shape to them. Initial infections appear as small, light yellow to green, oily spot (lesions) on the upper surface of the leaf. Young leaves are very susceptible to downy mildew. The pathogen overwinters as dormant spores in infected leaves on the ground. Severe infections can lead to complete defoliation. Most infection occurs during the period from early bloom through 3 to 4 weeks after bloom. Leaf symptoms vary depending on the age of the leaf and infections occur throughout the growing season. Susceptibility within North American species ( Vitis labrusca ) ranges from highly susceptible to resistant. All Vinifera vitis cultivars are susceptible to downy mildew. The disease is most severe during seasons that are excessively wet and warm.
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