Temperatures in the Fresno/Clovis area last week shot over 100 degrees for several days in a row, wilting many plants.
One way to combat heat stress is to keep the soil around plants moist during hot spells. If you’re doing that and still the plants do not recover from wilting during the night, there may be other causes. Three possibilities for continued wilting with eventual leaf yellowing and death are round knot nematode, a microscopic worm; and verticillium and fusarium, fungal diseases that interfere with plants’ ability to draw up water and nutrients from the roots. All are common problems in central San Joaquin Valley soils.
Soil-dwelling root knot nematodes feed on plants’ roots and attack a wide range of crops including fruit and nut trees, grapes and strawberries as well as cool- and warm-season vegetables such as beets, carrots, tomatoes and squash.
There are several species of root knot nematodes and each species can inhabit various spots in the same garden. Nematodes don’t travel quickly through the soil and are mostly carried into our gardens on shoes, tools, infested plants and surface water flowing from one area to another.
The feeding damage caused by root knot nematodes results in distinctive galls or swellings that form along the roots. The galls can grow together to create 1-inch-round knots, but most are smaller in size. The galls or knots will not rub off the roots.
Fruit and nut trees and grapes may not show obvious symptoms of nematode infestation for several years. If some of your summer vegetables are showing yellowing leaves, slower growth, producing fewer leaves and fruit and wilting now in mid-season, check for the presence of nematodes by pulling out the saddest-looking plants. Galls on the roots will be the telltale sign.
Fusarium and verticillium wilts are vascular plant diseases caused by soil fungi entering into the water-conducting plant tissue and impeding water from reaching the leaves. Symptoms of both fungal diseases are similar, but fusarium wilt attacks mostly vegetables and verticillium wilt is a problem in fruit and nut trees as well as some vegetables.. Leaves turn yellow, wilt, then die – usually on the lower part of the plant first or on one side first. If you suspect that a plant may be infected by one of these wilts, cut across a stem and look for a black or brownish ring of tissue inside the stem.
Nematodes and both types of wilts can persist in the soil for years. Control includes buying resistant plants. Labels should list the identifying letters N, F, V (among other letters that identify other diseases). Tomato plants infected with nematodes are less resistant to fusarium wilt. Be sure to buy tomatoes that are labeled as resistant to both.
Sanitation can help the spread of nematodes and wilts. Clean tools and shoes after working with affected plants. Dispose of affected plants in the garbage not the green waste bin. Add them to your compost bin only if you’re certain that temperatures inside the pile will reach 150 degrees.
The most effective method of control for soil-dwelling pest insects such as nematodes and soil-born disease pathogens such as wilts is soil solarization; more on that in next week’s column.