Garden Know-how: Disease Prevention Basics

Image Gallery You can prevent many potential garden diseases by using these strategies: wide spacing and trellises, mulching and applying aged compost. ELAYNE SEARS

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It happens in the best of gardens. Your plants are growing beautifully, and then you notice some of them are being consumed by pathogenic (from the Greek pathos, meaning suffering) microorganisms. The good news is you don’t have to be a plant pathologist to prevent many of the diseases that threaten a food garden, because disease-resistant varieties that are grown in soil enriched with organic matter usually stay healthy when the going gets tough.

Cured compost helps plants prime themselves to better handle challenges from diseases while improving the soil’s tilth. Providing enough water to avoid drought stress helps, too. Many gardeners include seaweed sprays in their garden’s preventive health care program, which provide nutrients for both plants and beneficial microorganisms.

Still, some leaves will shrivel and entire plants may sometimes suddenly collapse. To offer the best help to troubled plants, first you’ll need to know how different types of diseases tick. Seeing is believing. In late summer, most gardens offer some examples of common garden diseases, such as leaf blemishes and fruit rots, stem and root infections, and viral diseases spread by insects.

FUNGAL FOLLIES

Let’s start by looking at your tomatoes, especially the leaves closest to the ground. That area stays damp longer than the plant’s high branches, and the leaves down there are getting old — two factors that make them prime victims for early blight (dark brown patches) or several other leaf spot diseases, including gray leaf spot and Septoria leaf spot. All are caused by fungi that busily release millions of spores, which spread to new leaves by the time the colonies become big enough to see. If those new leaves are damp and temperatures are right, the spores germinate and penetrate the leaf using enzymes to melt entryways into plant cells, and a new leaf spot is born.

Leaf blemishes come in a variety of colors. Among your squash, you may see some white patches of powdery mildew, which is caused by spore-producing fungi that weaken plants by robbing leaves of their ability to perform efficient photosynthesis. You might see streaks of cinnamonlike rust in your corn, or patches of orange rust powder on bramble fruits or beans — more examples of spore-producing parasitic fungi. If you grow fruits, the velvety brown patina on your shriveled peaches or plums is caused by brown rot and Botrytis. Other spore-producing fungi turn strawberries and grapes into moldy mummies.

Resistant varieties are available for many of these diseases — especially powdery mildew of squash family crops and various blemishers of beans. (There are no highly resistant varieties to help prevent tomato early blight, grape powdery mildew, or brown rot of peaches, plums and cherries.) Once an outbreak is underway, you can slow its spread if you move in during a period of dry weather and clip off affected leaves, fruits or branches, but only if the foliage is dry. Fungal spores usually arrive in the garden on the wind or on insects’ feet, but nothing spreads spores faster than a gardener mucking around in damp, diseased foliage or fruits.

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