Phomopsis cane and leaf spot of grape—Phomopsis viticola
Phomopsis viticola is a minor grape pathogen during most years. It becomes a significant problem when wet weather occurs after budbreak in the spring or if rain occurs late in the growing season before harvest.
Identification
Phomopsis cane and leaf spot can occur on infected grape berries, canes, leaves, rachides (rachis, stem of a grape cluster), and shoots.
Canes. During the dormant season, infected wood (e.g., spurs, the short arms on cordons from which leaves and fruit develop) appears severely bleached. Severely infected canes or spurs have irregular black to dark brown discoloration mixed with whitish areas of bleached wood. Black, pimplelike fungal fruiting bodies (pycnidia) develop on the infected wood during the dormant season. The diseased canes and spurs are weakened and may die during the dormant season.
Leaves. During the growing season the first symptoms of disease are tiny, black to dark brown spots with yellow margins on leaf blades and petioles. After budbreak these spots appear 3 to 4 weeks after a rain. If a large numbers of spots develop and merge, they kill the leaf or cause it to grow distorted and remain undersized. Viewed from a distance it may be apparent that a portion of the grape vine has numerous leaves that are distorted and undersized. When petioles are severely infected, leaves turn yellow and drop prematurely.
Rachides, clusters, and berries. When a rachis is infected, it develops dark lesions. Sometimes these cluster stems are so extensively infected that the berries wither into mummies. Cool, wet weather before harvest can cause individual berries to develop pale brown spots that enlarge and become dark brown lesions. Dark pycnidia may develop on the skin of infected berries and exude yellow masses of Phomopsis spores. Infected berries or bunches may rot.
Shoots. On shoots the first evidence of infection is small spots with black centers similar to those on infected leaves. Infections occur mainly on the basal portion of shoots. These lesions are oblong and if they grow to about 1/4 inch the outer layers of shoots (epidermis) typically crack at the point of infection. If the lesions are abundant they can merge and give infected shoots a scabby appearance. These infected shoots can break during windy conditions or from the weight of fruit as clusters mature.
Life cycle
The fungus overwinters in grape canes and spurs. In late winter small, black pycnidia develop on the infected wood. The pycnidia exude small, stringlike, whitish tendrils of spores (cirrhi). If cool rains are abundant after budbreak during March or April, the spores are splashed onto leaves and green stems. If green plant parts remain wet for at least 5 hours or more, the spores cause infections.
The fungal spores are spread by splashing rain and can infect and cause disease on any green tissue of grape. During years when rain is abundant during late winter and spring the pathogen can severely disease grape leaves and stems of certain cultivars. However, grapes cultivars vary greatly in their susceptibility to the pathogen.
Lesions on grape clusters, leaves, and shoots become inactive during the summer heat. Then if it rains just before harvest, this causes the fungus to become active and spread to cause lesions on fruit.
Damage
The impact of Phomopsis cane and leaf spot is minor during most years. Because prolonged moisture is required for infections the disease is most severe in northern regions of California such as the North Coast and Northern San Joaquin Valley where spring rains are more common after budbreak.
When cool, wet weather occurs early in the growing season, severe foliar infections can develop, resulting in damaged leaves that may drop prematurely. When numerous lesions grow together on a rachis, the rachis may be killed and break, reducing fruit yield. Severe cane infections also reduce fruit yield. If rain occurs late in the growing season before harvest, lesions can develop on berries and fruit may rot and shrivel.
Solutions
During the dormant season, examine canes and spurs. Look for irregular black to dark brown bark discoloration mixed with whitish areas of bleached wood. On this discolored bark and wood, look for pycnidia. During dry weather, prune off this infected wood and dispose of it way from where grapes are grown.
Where Phomopsis cane and leaf spot has been a problem during previous growing seasons, after pruning out infected canes and spurs, the dormant vines can be thoroughly sprayed with Bordeaux mixture before bud break to reduce the subsequent presence of disease. Just before or immediately after budbreak, but before rain and before shoots are longer than one-half inch, green tissue can be thoroughly sprayed with sulfur. If weather is rainy, vines can again be sprayed with sulfur when shoots are about 5 to 6 inches long. Do not apply sulfur within 3 weeks of an oil application or phytotoxicity may occur. After bud break mancozeb can be applied instead of sulfur.
Adapted from Grape Pest Management Third Edition from UC Agriculture and Natural Resources also available as an eBook and Pest Management Guidelines: Grape, University of California Statewide Integrated Pest Management Program (UC IPM). |
Tiny black spots with yellow margins that commonly follow leaf veins are a symptom of Phomopsis cane and leaf spot.
Dark, elongate lesions of Phomopsis cane and leaf spot can give green shoots a scabby appearance.
During the dormant season, canes and spurs with Phomopsis cane and leaf spot have a bleached appearance.
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