Verticillium wilt is caused by two common soilborne fungi. These fungi have a wide host range and attack more than 300 woody and herbaceous plants. The disease can be widespread and extremely destructive. Black raspberries are more susceptible to the disease than red raspberries. Blackberries also are attacked by the pathogen but are not as prone to wilting.
Symptoms on plants become obvious by June or early July. Shoots are stunted and leaves, starting at the base of the infected plant, turn yellow, wilt, and drop. The entire shoot will wither and die shortly thereafter. Black raspberry canes might show a blue or purple streak from the soil line extending upward. This purple streak is not detectable on red raspberry canes. Fruiting canes, infected the previous year, either die in the spring or develop yellow and stunted leaves. If the canes die before reaching maturity, the fruit becomes mummified. Losses are heavier in black raspberries than in red raspberries.
The disease is caused by the fungi Verticillium dahliae and Verticillium albo-atrum. These fungi can exist in the soil prior to planting, may be brought in on planting stock, or may move in on wind-blown soil. The fungi can survive either in plant debris or free in the soil. The fungus enters the roots through breaks or wounds and moves into the vascular system, causing a systemic infection. After the plant or plant portions die, the fungus continues to survive in the soil for long periods of time. Factors that can increase disease are heavy soils and cold, wet spring weather.
Verticillium is favored by cool weather and is most severe in poorly drained soils following a cool, wet spring. There are no effective fungicides for management once the plants are in the ground. To minimize this disease, choose a planting site with no known history of this problem. Avoid land recently planted with tomatoes, potatoes, eggplants, peppers, strawberries, raspberries, or stone fruits; and land infested with horse nettle, ground cherry, red-root pigweed, and lamb's-quarter. Plant verticillium-free nursery stock. The number of years required to eliminate verticillium, especially the resting spores from the soil, is unknown. In spite of this, planting with verticillium-free black raspberry stock on uninfested soil usually ensures many years of avoidance of this disease.