Verticillium wilt is caused by the soilborne fungus Verticillium albo-atrum. The fungus overwinters in soil or plant debris as dormant mycelium or black, specksized bodies (microsclerotia). These microsclerotia can remain viable in the soil for many years. Under favorable environmental conditions, they germinate and produce threadlike fungal structures (hyphae). Hyphae can penetrate root hairs directly or through breaks or wounds in the rootlets. Once inside the root, the fungus invades and destroys the water-conducting tissue. The destruction of water-conducting tissue results in reduced water uptake by the plant; thus, the plants wilt and wither. As fungal colonies get older they produce microsclerotia in infected host tissue. As these infected tissues die and return to the soil, the disease cycle is completed." (Quoted from Ohio University Plant Pathology Dept.)
Symptoms of Verticillium wilt in new strawberry plantings often appear about the time runners begin to form. In older plantings, symptoms usually appear just before picking time. Symptoms on above-ground plant parts may differ with the susceptibility of the cultivar affected. In addition, above-ground symptoms are difficult to differentiate from those caused by other root infecting fungi. Isolation from diseased tissue and culturing the fungus in the laboratory are necessary for positive disease identification.
On infected strawberry plants, the outer and older leaves droop, wilt, turn dry and become reddish-yellow or dark brown at the margins and between veins. Few new leaves develop, and those that do tend to be stunted and may wilt and curl up along the midvein. Severely infected plants may appear stunted and flattened, with small yellowish leaves. Brownish to blueish-black streaks or blotches may appear on the runners or petioles. New roots that grow from the crown are often dwarfed with blackened tips. Brownish streaks may occur within the decaying crown and roots.
If the disease is serious, large numbers of plants may wilt and die rapidly. When the disease is not so serious, an occasional plant or several plants scattered over the entire planting may wilt and die.
Take this next statement very seriously: Do not plant susceptible strawberry cultivars (or resistant cultivars for that matter) in soil where tomato, peppers, potato, eggplant, melons, okra, mint, brambles, stone fruits, chrysanthemums, rose or related susceptible crops have grown for the past five years
Having said that, I encourage you to start with fresh, rich in organic matter soil when planting strawberries. Plus, and this is very important, purchase resistant plants that are healthy - don't see sick looking plants on the "quick sale" rack and think you are saving by taking them home. You may be taking verticillium wilt or some other deadly disease home with these plants and it will spread throughout many of your prized vegetables. And, it is guaranteed that sick plants will always attract pests - not beneficial insects - but nasty pests. So start with the best soil, totally healthy (more expensive maybe) plants and chances are good that you will receive many times over abundant fruit to enjoy.
Talking about these diseases will hopefully not deter gardeners from planting fruits and vegetables, but instead should help all of us avoid the problem by enriching our soil and planting only strong healthy plants in order to have the best harvest possible.
My column this week explains how to plant strawberries - so please read and get ready to start that strawberry patch this year. You will be glad you did.