How to Manage Pests
UC Pest Management Guidelines
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.
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.
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 soil solarization, flooding the fields during summer, growing several seasons of grass cover crops (especially rye or sudangrass) or a combination of these treatments. Whether any of these practices are sufficient to make much of a difference is unknown. Verticillium microsclerotia (resistant spores) 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, although some tolerance has been reported in the cultivar Ascolano.
After trees have been planted, there is no reliable method of control. 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.
UC IPM Pest Management Guidelines: Olive