Verticillium and Fusarium wilts don’t cause the sudden wilting bacterial wilt does. These fungal diseases develop gradually from the base of the plant upward. Fusarium wilt was one of the first resistances bred into commercial varieties. There are a few races of this disease. The two most common in South Africa are Fusarium oxysporum lycopersici races 1 and 2. Some countries also have race 3, which hasn’t yet appeared in this country as far as I’m aware.
Another race does occur to a limited extent – F. oxysporum sp radisis-lycopersici, which is called fusarium crown and root. It’s usually confined to tomatoes grown in protected cultures and most resistant varieties are usually greenhouse-adapted. E very time an additional resistance has to be included in a variety, the base from which to select varieties with the best commercial traits becomes smaller. Good commercial varieties are available with F1 and F2 resistance, requiring fusarium crown resistance as well limits the range. To include F3 resistance limits it even further, although there are varieties with an all-in-one package.
Fusarium wilt is a soil-borne, heat-loving fungus. It starts at the base of the plant, where the leaves start to go yellow, often on one side first. wilting is slight, unlike the dramatic wilting of bacterial wilt. When stems are cut, there will be a reddish-brown discolouration of the conducting tissue which may extend right up the plant. Roots will also have areas of dark discolouration.
This disease will gradually kill the plant. Fusarium crown rot stunts the plants. Chlorosis starts from the plant base were the stems become very dark, almost chocolate brown, and brown vascular staining is seen when the skin is removed. What sets this fusarium apart is that this browning is restricted to the lower 25cm of the stem. The roots also become discoloured. Verticillium wilt is also a gradual wilt. first symptoms are yellowing of the leaves to either side of the midrib in a “V”, and dying off.
Again there’s a discolouration of the vascular tissue when the stems are cut across, in this case a lighter colour that’s more pink than brown. lthough the symptoms can be confused with fusarium, depending on the disease’s progress, verticillium wilt is a cool season disease requiring soil temperatures below 25ºC. resistant gene confers immunity to this disease. Finding out what diseases your variety is resistant to, as well as the climatic conditions, are good starting points before closely inspecting physical symptoms. – Bill Kerr ((016) 366 0616 or e-mail [email protected]). |fw