A friend planted several heirloom tomatoes in containers several weeks ago. One of the plants is not doing well. The lower leaves have turned brown and fallen off and the browning is progressing up the stem on one side. These are signs of a wilt problem. Either verticillium (best guess without inspection) or fusarium wilt – they’re hard to distinguish since symptoms are so similar.
Both wilts are caused by soil born fungal pathogens that remain in the soil for years, even after diseased plants are removed. Fortunately, the contaminated soil can be sterilized this summer using the heat of the sun to kill or destroy most disease pathogens (bacteria and fungi), weed seeds and seedlings, pest insect eggs, and nematodes within the top 12 to 18 inches of soil.
During the solarizing process which takes four to six weeks during the hottest summer months, temperatures in the top 6 inches of garden soil can reach 140 degrees and around 90 degrees at 18 inches. If our summer temperatures are really high, the soil will be sterilized within 3 to 4 weeks. Some pathogens will survive at lower depths and nematodes which are mobile can move down to cooler soil levels. (Earthworms also move away from the heated soil). Contaminated potting soil from containers placed into double black plastic bags can be completely sterilized in just one hour since temperatures inside the bags can reach 160 degrees or above.
Solarization is a simple four-step method.
▪ First, clear the area to be solarized of all weeds, plants, debris and rocks. Rake the soil level if possible to promote even heating. Solarization works best in areas larger than 3 feet square.
▪ Second, soak the soil to a depth of one foot. Wet soil conducts and holds heat better than dry soil.
▪ Third, cover the area with clear plastic sheeting at least 1 mil thick. The thicker plastic will hold up better to deterioration caused by UV rays. A double layer of plastic, separated by plastic water bottles, pvc pipes or even soda cans, will increase temperatures and speed up the solarization process.
▪ Fourth, seal the edges of the plastic tightly to prevent heat from escaping. You can dig a shallow trench around the perimeter and use the trench dirt to hold down the edges of the plastic or use a combination of dirt, bricks or rocks.
If solarizing container soil, moisten the soil, double bag it and place the bags on some sort of rack, pallet, or slotted bench to allow air to circulate around the bag. Flip the bag at least once during the short solarization time so that the soil inside is evenly heated.
In addition to ridding the soil of pathogens, weed seeds, and pest insects, solarization has another great benefit-the populations of mycorrhizae and other beneficial fungi and bacteria remain stable during the solarization process and rebound even higher after solarization is finished. Beneficial micro-organisms feed on dying and decaying disease pathogens and solarized soil provides a full buffet of decomposing organic matter.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).