Verticillium wilt is a soil-borne fungus disease caused by the organism known as Verticillium dahliae.  Verticillium wilt can kill trees and is difficult or impossible to control and is considered a serious disease in olive trees.

SYMPTOMS

Symptoms of Verticillium wilt appear when leaves on one or more branches of the tree suddenly wilt early in the growing season; this process intensifies as the season progresses. Death of mature trees infected with Verticillium is possible. Darkening of xylem tissue, a key symptom for distinguishing Verticillium wilt in many crops, is frequently not apparent in olives.

COMMENTS ON THE DISEASE

The fungus survives from season to season in the soil and probably in the roots of infected trees. In early summer the fungus can be readily isolated from diseased tissue in infected trees. Verticillium wilt tends to be most troublesome in the southern San Joaquin Valley.

MANAGEMENT

The most effective management strategies to protect trees from Verticillium wilt are those taken before planting. When considering a new site for an olive grove, avoid land that has been planted for a number of years to crops that are highly susceptible to Verticillium wilt, such as cotton, eggplant, peppers, potato, or tomato. The Verticillium wilt pathogen is usually present in these soils.

Inoculum levels can be reduced before planting by fumigating the soil, soil solarization, flooding the fields during summer, growing several seasons of grass cover crops (especially rye or sudangrass) or a combination of these treatments. When replanting in an area where susceptible perennials were previously grown, remove as many roots of the trees or vines as possible. A resistant rootstock is not available, although some tolerance has been reported in the cultivar Ascolano.

After trees have been planted, no reliable method of control have been developed. Soil fumigation and soil solarization have provided inconsistent control in established plantings.

Soil Solarization. Beginning in late spring cover the surface of an entire block with transparent plastic that has a UV-inhibitor additive. Leave the plastic on throughout the summer and as long as practical. Inferior plastic will break down and render the treatment ineffective. Solarization gives inconsistent results when used in replant spots.

Soil Fumigation. Dry soil by withholding water during summer and using cover crops not susceptible to Verticillium wilt, such as sudangrass. The drier the soil, the better for deep penetration of methyl bromide. Deep-till the area after drying. If the soil is dusty, wait for an early rain before ripping and fumigating. Ripping a dry soil that is silty can result in large clods on the surface. Inject methyl bromide 18 to 30 inches deep with chisels and cover with gas-proof cover. Increasing the dose tends to increase the depth of penetration, but it cannot be relied upon to penetrate wet soils, especially if soils are high in clay. Do not remove the cover for at least 2 weeks and aerate 1 month before planting. Treatments may be made from late summer to early fall. In tree crops, methyl bromide often gives inconsistent control.