Vascular Wilt Disease
Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
Vascular Wilt Diseases in Potatoes
The vascular wilt diseases are common and destructive to potatoes in Utah, both in commercial plantings and in home gardens. Wilts are usually associated with species of the fungus Verticillium but may be caused by species of Fusarium. Many other vegetable and ornamental plants are affected by these fungi as well.
In potatoes, a vascular wilt disease appears first as premature yellowing or other discoloration of the leaves, while the stems and leaf petioles remain green. Infected plants may wilt during the day and revive at night, but eventually the vine wilts permanently and dies prematurely. The wilt is due to the plugging of the water-conducting tissues by the fungus. These symptoms often occur only on one side of the plant, with other portions of the plant remaining healthy.
When infected by Fusarium, plants may show bunching of upper leaves and shortening of stem internodes, creating a rosetting symptom. Both fungi can cause discoloration in the vascular system in the lower stems of the potato plant, most easily seen when the stem is sliced through at a shallow angle. This vascular discoloration in both diseases can extend into the tuber but is more common and pronounced when Fusarium causes the disease. In addition, some species of Fusarium cause rot of the underground stem, tuber blemishes, and decay, especially at the stem end.
In potatoes, vascular wilt diseases are caused by Verticillium albo-atrum and V. dahliae, and by Fusarium eumartii, F. oxysporum, F. avenaceum, and F. solani. Verticillium dahliae is most common in Utah.
All of these fungi are inhabitants of the soil. Fusarium species are capable of living saprophytically, in the absence of a host plant, and usually persist in the soil for many years. In contrast, Verticillium species are poor soil competitors, and do not survive for long periods in the absence of a host; but they do survive from one season to the next via tiny resting structures called microsclerotia or as thick-walled hyphae. The fungi usually infect through wounds in roots, although Verticillium may also infect potatoes through shoots and leaves. Because the fungi are present in soil, any mechanism which moves soil, such as water or wind, farm equipment or transplants, can spread the fungi. Additionally, Fusarium can be introduced to new areas in infected seed potatoes, or even on the surface of uninfected tubers. The fungi are returned to the soil when infected plant debris is tilled in after harvest.
The Fusarium species which infect potatoes are quite specific to potatoes, so there are no alternate hosts for the Fusarium wilt diseases. However, many crop, ornamental and weed species are hosts to Verticillium, including other potato-family ("Solanaceous") plants such as eggplant and pepper, and nightshades. The strain of Verticillium albo-atrum which causes wilt in alfalfa does not cause disease in potato.
Control of both Fusarium and Verticillium relies heavily upon cultural methods. If an area is known to be contaminated with either of these fungi, but especially with Fusarium, avoid planting potatoes in that area. Crop rotations are not especially effective in reducing the populations of Fusarium in the soil, since the fungus can live saprophytically in the absence of a host. Land infested with Verticillium should be planted to non-susceptible crops (e.g. cereals, legumes, cole crops, leafy vegetables) for at least one year before being returned to potatoes.
Do not use seed tubers from plants or from soil known to be infested with either fungus, or showing symptoms of vascular discoloration. Since many weeds are hosts for Verticillium, good weed control is important for reducing this disease. Some fungicides applied to the soil have been effective in controlling Verticillium wilt in potato, but they are not readily available to home gardeners. These fungicide treatments, as well as some fumigants and seed treatments, may be useful for commercial potato growers.
There are no potato varieties resistant to Fusarium wilt, but some commercial varieties show resistance to Verticillium wilt. For example. Russet Burbank has moderate resistance and Targhee, Houma, Ontario, Russet Rural, Sequoia, Yampa, Cariboo, Hunter, Ona, Red Beauty, and Shoshoni are resistant. Reliance is tolerant of Verticillium wilt. The effectiveness of this resistance is somewhat dependent upon location, population of the fungus, and the strain of the fungus. The use of resistant varieties does not preclude the possibility of having the disease. When conditions are highly favorable for the fungus, these diseases will occur despite the resistance. Therefore, use of resistant varieties should be combined with other prudent control measures. Kennebec is very susceptible to Verticillium wilt and should be planted where the disease has been a problem.
For wilt diseases in potatoes, it is extremely important to prevent movement of contaminated machinery and tools, irrigation water, soil, seed or transplants to areas not already infested. In addition, crop debris should not be returned to the soil, but should be discarded or burned, in an area which will not be threatened by the pathogens. Infected debris can be sanitized by composting, only if all portions of the compost pile heat to 130 F. This procedure is not generally recommended if the compost is to be returned to the garden or field.
Other practices which reduce the severity of wilt diseases include the use of nitrogen, side-dressed in mid-season, to reduce stress to the plant; and avoidance of drought stress to the potato plants.
Precautionary Statement: All pesticides have benefits and risks, however following the label will maximize the benefits and reduce risks. Pay attention to the directions for use and follow precautionary statements. Pesticide labels are considered legal documents containing instructions and limitations. Inconsistent use of the product or disregarding the label is a violation of both federal and state laws. The pesticide applicator is legally responsible for proper use. This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Vice President for Extension and Agriculture, Utah State University.