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Gardening: Avoiding plant diseases by doing things differently

Watering Tomatos

Careful and timely watering of plants also helps keep leaves and fruit dry, which prevents activating pathogen spores that land on the plants. Dreamstime

I recently surveyed 77 local gardeners and found that 31 percent did not use either conventional or organic fungicides. Most gardeners used both types of fungicides (42 percent), which I also do, while 8 percent used only conventional and 16 percent used only organic products.

Gardeners who prefer not to spray any fungicides on their plants can use other strategies to manage plant diseases. One of the most important strategies is avoidance.

Fruit Rot

Rhizoctonia fruit rot on tomato can be avoided by staking plants to keep fruit from touching infested soil. Anthony Keinath/Provided

There are two ways to avoid diseases. The more straightforward way is to physically separate a plant from its pathogens. The other way is to alter the plant’s microenvironment to make it unfavorable for pathogens.

Separating plants and pathogens can be done easily by changing planting dates. Some pathogens grow during summer and early fall when soil is warm, at least 85 degrees. One of these pathogens is the fungus that causes the disease yellows in cabbage family vegetables, particularly collard and kale. Planting these greens during late fall to spring but avoiding summer plantings will prevent yellows.

Note that some cabbage varieties have resistance to yellows, designated FY in the name or the description. "Morris Heading" collard is partially resistant to yellows because it was crossed or naturally cross-pollinated with cabbage.

Another example of a warm-season pathogen is the root-knot nematode. Cool-season vegetables that are susceptible to root knot should not be grown when nematodes are active. Vegetables in the celery family, or Apiaceae, like carrots, parsley and cilantro, are very susceptible to root-knot nematode. Growing these vegetables and herbs during the summer will allow root-knot nematodes to proliferate in soil.

However, growing celery family vegetables in late fall, over winter and in spring when root-knot nematodes are dormant (literally asleep) is not a problem even in soil which harbors nematodes.

Experienced gardeners probably use techniques to physically separate plants and pathogens without realizing it. Preformed wire “cages” can be purchased and placed in the ground around young plants, one cage per plant.

Staking or tying tomatoes, peppers and eggplants also keeps fruit off the ground, which harbors fungi and water molds that rot ripe fruit. Commercial growers prune off all but two lower branches before staking and weaving plastic twine around tomato stems and stakes to hold tomato plants upright, so that rows of tomatoes look a bit like hedges with main branches running parallel in the row.

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Tomato Plants

Gardeners who prefer not to spray any fungicides on their plants can use other strategies to manage plant diseases. One of the most important strategies is avoidance. Anthony Keinath/Provided

Growing tomatoes, other fruiting vegetables and annuals in potting mix, either in large pots or in wood-framed raised beds, provides a soil environment free of pathogens. A well-drained potting mix should be used. Some commercial potting mixes with large amounts of organic matter may hold too much water after heavy summer downpours when they are used in plastic pots.

Modifying the microenvironment includes several standard gardening practices recommended to reduce the chances of disease starting or spreading in the garden and yard. Planting vegetables, herbs and annual flowers in raised beds of garden soil improves drainage and so reduces damping-off of seedlings and root and stem rot of older plants.

Careful and timely watering of plants also helps keep leaves and fruit dry, which prevents activating pathogen spores that land on the plants. The rule of thumb is to water the soil around the plant instead of wetting the plant while watering it. Directing water to the roots instead of the leaves is one advantage of drip irrigation systems that use soaker hoses or in-ground emitters to deliver water.

Irrigation systems with pop-up sprinklers, typically installed to water shrubs, should be timed to run early in the morning before sunrise, so that the irrigation water is not rewetting foliage after the dew has dried.

One drawback to avoidance techniques is that pathogens are not eradicated from soil or the environment. Pathogens are still present and may attack plants. Spores of soil pathogens may splash up onto tomato fruit if soil is left unmulched. Staking does not protect plants from pathogen spores that blow into the garden from external sources.

Raised beds may not improve drainage enough to prevent root rot. Even without supplemental watering, dew periods in the fall are long enough to promote disease.

However, the practices I’ve recommend are more effective than doing nothing. The time and care spent avoiding diseases will yield healthier plants in most growing seasons.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at tknth@clemson.edu.

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