In this country we basically face three soil wilt diseases. They range from sudden death to a gradual decline depending on the wilt and on conditions. Bacterial wilt is a sudden-death type of wilt, where we arrive at the land and see plants wilting so fast that the first inclination is to check for cutworms. This wilt is more or less confined to moist, warm climates. The classic test for it is to take about 15cm of wilted shoot with most of its leaves removed and suspend it in a glass of water, with the cut stem halfway down the glass.
Be sure to take from a plant with advanced symptoms. In about half an hour, a milky ooze will stream from the cut stem down to the base of the glass. This ooze is comprised of millions of bacteria. bacteria is extremely persistent in the soil and can survive for many years without a host crop. Many weeds can also harbour it, including Datura spp and other species of the nightshade family. Black nightshade (nastergal), is very common in many tomato-growing Lowveld areas.
The bacteria is also very easily spread by implements, muddy feet and water from dams and furrows. It usually starts in patches and spreads down the rows quite rapidly. This bacteria likes warm conditions and goes into a dormancy in the winter in our warm areas, where winter tomatoes are planted. Farmers tend to plant lands with known infection only in the coldest part of winter. bacteria can be greatly reduced by desiccation in areas which experience prolonged dry spells. Deep-ripping the ground and allowing it to dry out greatly reduces the population. best approach is to select tomato varieties with resistance.
As there are various kinds of wilt some imported varieties may not be resistant enough. We have biovar 2 here; some imported varieties have biovar 1 resistance. I’ve seen cases where most of one variety is in a state of wilt, while the resistant variety in the next row is totally unaffected. Genetic resistance breaks down above certain temperatures. In hot areas in summer, choose the coolest land possible for tomatoes, and cover the soil with a mulch to keep the soil temperature down.
It may seem like a lot of work and expense but it’s much cheaper than losing part of the crop. The mulch will hold back the disease and also improve growing conditions, as soil that’s hot enough to break down resistance also stresses the crop. conservation of moisture is an added benefit. – Bill Kerr ((016) 366 0616 or e-mail [email protected]). |fw
