Microorganisms that damage plants are collectively called diseases, and every garden has them. The good news is that these troublesome microbes require specific host plants and are incapable of jumping family lines. Tomato diseases can’t bother beets, and squash diseases can’t spread to peppers. The bad news is that moist, organically enriched soil hosts a huge entourage of fungi, and while diversity provides basic safeguards, it does not exclude strains that cause serious problems.
People set the stage for trouble by growing the same food crops year after year. Each time a certain plant is grown, microbes accumulate in its root space in anticipation of a repeat food supply the following season. The first year you might not notice the slight damage from Pythium root rot of carrots, for example, but if you grow carrots in the same soil three years in a row, Pythium fungi may be mighty enough to overwhelm the crop.
The same goes for a long list of soil-borne diseases that affect tomatoes, potatoes, and cabbage family crops. Rotating the space to different botanical families interrupts this cycle, so it works as both a prevention and cure for diseases that live in the soil. Because plants vary in their nutrient uptake patterns, crop rotation also prevents the depletion of micronutrients and minerals in the soil.
1. Use the GrowVeg Crop Rotation Tool
One of my favorite features of the GrowVeg Garden Planner is the crop rotation tool, which uses a glowing red background color to tell you when you are placing a vegetable where a close relative was grown during previous seasons. If you are new to the Planner, start by creating a new plan that shows the layout of last year's garden. Once you’re finished and everything is in place, create another new plan by clicking the New Plan button in the upper left corner. In the dialog box, under Type of Plan, select the Follow-on Plan option. The new plan will look like a blank slate, but as soon as you click on a vegetable you want to grow, for example potatoes, the spaces where potatoes and other related plants such as tomatoes or peppers were previously grown will pulse with red, telling you to choose another bed or row.
The crop rotation tool simplifies space management, and also reminds you of obscure family relations, like that celery is related to carrots, or that chard and beets are close cousins. It will guide you through up to five years of rotations, which is usually more than enough time for soil to recover from too many of the wrong microbes, such as the fungus that causes sweet potato scurf.
2. Choose Resistant Varieties
Some soil-borne diseases are naturally present in healthy soil, and cannot be rotated into oblivion. For example, tomato gardeners in cool climates may see plants wilt and die from verticillium wilt, while in warmer climates fusarium fungi and rootknot nematodes often are persistent problems. When you know a soil-borne disease is present in your garden, seeking out and using resistant varieties will insure successful crops, while denying the disease microbes suitable host plants.
Varieties with the highest levels of disease resistance are often traditionally-bred hybrids, the result of planned crosses by plant breeders. For example, yellow-fleshed ‘Yukon Gold’ potato was crossed with ‘Brodick’, a disease-resistant Scottish variety, to create ‘Yukon Gem’, which has good resistance to blight and scab. Tomatoes with resistance to fusarium and verticillium wilts are widely available, noted by the letters VF after the variety name, such as ‘Roma VF’. Other soil-borne diseases like black rot of cabbage and onion pink root rot can be prevented with resistant varieties, too.
3. Keep Soil Healthy With Compost
Many studies have shown that rich, bioactive compost can help suppress soil-borne diseases by introducing microbes that antagonize the soil’s current residents. If you use enough compost, say 20 percent of the soil’s volume, it turns the existing soil microbial community on its head. When you see a new disease problem affecting plant roots, compost therapy is part of the cure. Think of it as first aid for troubled soil.