Latin name: Didymella applanata
Reason for Concern:
1) Reported to reduce yields in several ways.
2) It can blight the fruit bearing spurs that are produced on the side branches, cause premature leaf drop, and kill buds on the canes that later develop into fruit bearing side branches.
3) Berries produced on diseased canes may be dry, small and seedy. It can also reduce winter hardiness.
4) Disease prevention is based partly on field history.
5) Wet weather during harvest increases the likelihood of continued infection on new canes.
Symptoms first appear on young primocanes in late spring or early summer. Purple to brown areas (lesions) appear just below the leaf or bud, usually on the lower portion of the stem. These lesions expand, sometimes covering all the area between 2 leaves. As a result, leaves fall off, especially on the lower parts of the stem. Infections on leaves are wedge-shaped with a brown central area surrounded by a yellow band. Petiole infections are not noticeable, but they grow into the new canes in mid-summer forming dark brown spots surrounding buds. The infection (brown areas) can spread up and down so that large sections of cane are totally infected. Buds surrounded by brown areas are not infected directly, but are greatly weakened by toxins produced by the fungus. These weakened buds may die or, if they do leaf out in the spring, may produce weak fruiting laterals with small, yellow leaves, which dry up early in the season. In late summer or early fall, bark in the affected area splits lengthwise and small black fungal specks appear in the lesions. They are followed shortly by many slightly larger, black, erupting spots. Infected cane areas remain dark brown until early winter when they become silvery-gray in color. When diseased canes become fruiting floricanes during the next season, side branches growing from diseased buds are often weak and withered.
The causal agent overwinters in lesions on diseased canes. The following spring and summer, during wet and rainy periods, spores release and spread by splashing rain and wind to nearby primocanes. They germinate in the presence of water and produce new infections, where the fungus will again overwinter. The fungus can infect leaves, petioles (leaf stems) and canes. New canes can be infected when they are 20 to 25 cm tall.
- At 4 locations in a 10 acre plot, sample at least 5 plants per location.Consider variety, age of planting, location and weather.
- From May to early July, be aware of wedge-shaped brown patches on leaves to obtain an indication of spur blight levels.
- After harvest through late winter, the fungus moves through the leaf and petiole and is most apparent as a purplish/brown lesion around the bud on the lower portion of primocanes.
- During the dormant season and early spring, check overwintering levels of the spur blight fungus by looking for cracked gray areas on the canes around buds.