Verticillium Wilt

Leaves on plants infected with Verticillium wilt turn yellow, wither, and fall, beginning at the base of canes and progressing upward. Fruiting canes may take on a bluish black cast and die during summer as fruit are maturing. Symptoms sometimes appear first on only one side of the plant while the opposite side remains healthy before also becoming infected. Vascular discoloration, which is typical of Verticillium wilt for many crops, can be slight or absent for raspberries. Small groups of plants throughout the field may be affected. Primocanes (current season's growth) are usually free of the disease; however, a severe infection in newly planted fields may kill the plants the first year.

In California, Verticillium is only rarely found on raspberries and sometimes on blackberries.

The fungus persists in the soil in an actively growing state when susceptible crops or weeds are present or otherwise as dormant resting structures (microsclerotia). It often occurs in soil that was formerly planted to tomatoes, potatoes, strawberries, cotton, eggplants, peppers, cucurbits, and many other plants. Infection occurs when roots come into contact with the microsclerotia. The disease is favored by cool weather and can infect through either healthy or wounded roots and root hairs. After the initial infection, the fungus grows into the water-conducting tissues of the root (xylem) and spreads upward into the cane xylem. Eventually, the xylem tissues become plugged by the growth of the fungus or by the plant's internal defense mechanisms, such as the deposition of gums or the development of tyloses (overgrowths of parenchyma cells that are adjacent to xylem vessels), and the canes wilt and die. The fungus is then returned to the soil as the dead roots decompose and microsclerotia become available to infect new plants.

Verticillium wilt can be a difficult disease to manage. Avoid planting in fields that have a recent crop history of highly susceptible plants such as vegetable crops; such fields possibly contain high levels of the Verticillium microsclerotia. Fields that have been infested with weeds such as pigweed, nightshade, and lambsquarters can also contain high levels of the fungus. Select fields that are not infested and use clean planting stock.

In Central Valley locations, soil solarization can be used to reduce the level of inoculum in the soil before the canes are planted. (See Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds—PDF, UC ANR Publication 21377.)

The use of clean planting stock, crop rotation, and soil solarization are acceptable management methods in an organically certified crop, but soil solarization has not worked well in the coastal valleys.

Preplant fumigation reduces the levels of inoculum in the soil.