Symptoms and Signs
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. Some trees recover naturally from an initial infection, but others may die. This mostly depends on the strain of the fungus (defoliating or non-defoliating), the resistance of the cultivar, and the amount of inoculum in the soil.
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, cucurbits, eggplant, peppers, potato, or tomato. The Verticillium wilt pathogen is usually present in these soils. Soils containing over one microsclerotia per gram of soil should be avoided.
Inoculum levels can be reduced before planting by flooding the fields during summer, growing several seasons of grass cover crops (especially rye or sudangrass) or a combination of these treatments. It is unknown if these techniques decrease inoculum levels to result in significant disease reduction.
Verticillium microsclerotia (fungal survival structures) have been documented to survive for at least 30 years in the soil. 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; however some cultivars (Frantoio, Empeltre, and to a lesser extent Koroneiki) have exhibited tolerance to the pathogen (the pathogen can reproduce on the rootstock, but growth and fruit production are not affected by the disease). Use of a Verticillium-tolerant rootstock may not protect the scion, because the pathogen may grow through the graft union. Some tolerance has been reported in the table olive cultivars Ascolano and Kalamon (Kalamata).
After trees have been planted, there is no reliable method of control. Postplant soil solarization has 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. For more information, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377 (PDF).