Verticillium Wilt of Stone Fruits
Verticillium wilt is a disease caused by the fungus Verticillium dahliae, a common pathogen in crops such as potatoes and mint. This fungus builds up on these common hosts and remains in the soil as small resting structures for many years. Susceptible hosts of this disease organism include many plants, and tree fruits such as cherry, peach, nectarine, plum or any other close relative. Many relatively resistant plants may maintain the population of this organism by acting as low-grade hosts, so once the site is infested, it tends to remain so. When a susceptible host root grows near the Verticillium resting structure, the fungus breaks dormancy, penetrates the root, infests the young root tissue, then moves into the plant’s vascular system. If the vascular system is greatly damaged, the plant wilts, leading to the characteristic sudden yellowing and leaf drop. Individual limbs or sections of the tree may exhibit symptoms, while other parts of the tree appear quite healthy. If there are relatively few fungal resting structures in the soil and the host is relatively resistant, the stone fruit tree may be attacked for many years without showing symptoms. Each year, the vigorous young tree may grow a new layer of relatively healthy vascular tissue (new wood) that will support its growth. However, if tree growth slows, or the site has a relatively high number of Verticillium resting structures, the organism may overwhelm the tree, causing damage or death.
This disease is controlled temporarily in annual crops through crop rotation and soil fumigation. Despite this approach, the disease may still develop in the annual crop, but yields are only slightly affected. This is not a practical approach for tree fruits, as a temporary reduction of disease pressure will not prevent the disease for the expected life time of the orchard. Careful fumigation will lessen the degree that the tree is attacked by the fungus, and will have an effect if the disease potential is low. Stone fruit orchards are at great risk of developing this disease on sites where the disease pressure is high, despite long crop rotations and fumigation. There are no tests available to determine the precise disease potential on any specific site relative to tree fruits. There is no information available on the relative resistance of various stone fruit rootstocks. Growers must determine the cropping history of potential planting sites and avoid planting stone fruits on sites that produced potato, mint or other highly susceptible hosts.