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How to Prevent Plant Diseases

Plant Disease Basics

Growing healthy plants is the first step toward a great garden. To achieve this, it's important to prevent disease by paying careful attention to plant selection and plant care. The following suggestions should help keep your plants free of disease.

Plant Selection

Some plants are just naturally more hearty and healthy than others. It's a good idea to choose these types of plants for your garden -- especially if you don't have tons of time to devote to keeping weeds, pests, and diseases at bay.

Choose disease-resistant cultivars whenever possible. They are bred to resist infection -- an ideal way to avoid diseases. Growing disease-resistant vegetables prevents chemical tainting of your food. Disease-resistant plant varieties of popular flowers such as roses save you time, trouble, and expense.

There are varying levels of protection available:

  • Some cultivars have multiple disease resistances for maximum protection. The Big Beef tomato, for instance, resists various types of wilts: tobacco mosaic virus, nematodes, and gray leaf spot. Little is left that can harm it.

  • Some cultivars resist only one disease. But if that disease is a problem in your area, then these plants will be worth their weight in gold.

  • Other plants are disease-tolerant, meaning they may still get the disease but should grow well despite it.

To find out more about disease-resistant cultivars for your area, consult your local Cooperative Extension Service or a knowledgeable professional grower. Or get your name on the mailing list for nursery and seed catalogs that describe disease-resistant cultivars.

Many tomatoes are specifically bred to resist disease.
© 2006 Publications International, Ltd.
Many tomatoes are specifically bred to resist disease.

Plant Care

Diseases can creep into your plants when you least suspect it. Fortunately, you can prevent some diseases before they strike by following these tips:

  • Spray plants susceptible to foliage fungus with wilt-proofing solution. This product is a pine oil modified to spread into a film coating that protects evergreen foliage from drying out during winter. An unexpected side effect of the film is that it keeps fungus spores from penetrating into susceptible leaves. Mix according to label directions and try it on phlox, bee balm, cucumbers, watermelons, tomatoes, and apples.

  • Experiment with baking soda sprays to prevent fungus diseases. Mix 2 teaspoons baking soda in 2 quarts water with 1/2 teaspoon corn oil. Shake well, put in a sprayer, and go to work. Spray susceptible plants often and always after rain to help keep diseases such as powdery mildew from getting started.

  • Thin the stems of disease-prone plants to improve air circulation. Mildew-susceptible phlox and bee balm, for instance, can grow into clumps so thick that they block airflow. This encourages fungus attack, but it is easily corrected. When new growth is coming up in the spring, cut out every third stem, targeting those that are weak or in areas of the thickest growth.

Weeds can be just as damaging to healthy plants as diseases can be. They are quick to germinate and grow rapidly. As soon as they're brought to within an inch of the soil surface (through digging), they'll begin to sprout. Here are a few ways to keep weeds under control:

Scatter granules of a pre-emergent chemical on soil to prevent weed seeds from sprouting.
©2006 Publications International, Ltd.
Scatter granules of a pre-emergent chemical on soil
to prevent weed seeds from sprouting.

  • After digging deeply in early spring to ready the bed for planting, wait a week or more before doing any planting or seed sowing. Every three days, stir up the top 1 inch of soil with a scuffle hoe or cultivator, leaving the lower soil undisturbed. This will expose several cycles of young sprouted weeds to dry out and die in the sun and air. This approach can be used when seeds are going to be planted either in addition to, or instead of, bedding plants.

  • In places where only bedding plants will be used, the use of a pre-emergent product such as corn glutin is recommended. This is sprinkled on the soil around the already planted annuals. It's important that the annuals be at least 3 to 4 inches tall before the product is applied. In the case of seedlings, the pre-emergent can be applied once the young plants have grown to this 3- to 4-inch size.

  • The most popular way of dramatically reducing weed problems is by using some kind of mulch. Mulch is a layer of organic or inorganic material laid on the soil surface to shade out weeds, retain soil moisture, and have a moderating effect on soil temperature.

    Many materials can achieve these results, but some are more practical, less expensive, easier to handle, and more attractive than others. The list of organic mulches includes: pine needles, leaves, straw, dried seaweed, tree bark strips, bark chunks, old newspapers, sawdust, wood chips, cocoa bean hulls, and cotton seed hulls. Inorganic mulches include "blankets" made from solid sheets of porous landscape fabric; and pieces of new or used carpets.

    Perhaps the choice of which kind of mulch to use isn't as important as the decision to use some kind of mulch. Mulching cuts down dramatically on weed problems, conserves soil moisture, keeps soil warmer in cool weather and cooler on hot days, and, if it's an organic mulch, will improve the quality of the soil as it breaks down and adds to what gardeners call "soil tilth."

    The mulch on this fountain garden helps prevent weeds and gives the landscape a neat, defined appearance.
    © 2006 Publications International, Ltd.
    The mulch on this fountain garden helps prevent weeds
    and gives the landscape a neat, defined appearance.

    Some mulches are not very attractive-looking. Other organic mulches may alter the soil chemistry as they break down (annual soil tests will detect these changes so you can adjust fertilizer applications to compensate). Still other types have an odor when they're fresh or may prove too expensive for use in large quantities.

    Organic mulches are the best choice for annual and perennial beds. The finer the texture, the better. Coarse mulches are unattractive and may interfere with the growth of delicate plants. In addition, coarse mulches take larger amounts of nutrients to decay. Use a balanced organic fertilizer along with the mulch to assure that plants are not robbed of precious nutrients.

    A 1- or 2-inch layer of fine-textured mulch is sufficient. Do not put down too thick a layer or you will smother plants. Avoid mulching right up to the crowns of plants. Pull the mulch back a bit from each plant so it can expand freely.
Some Disease-Resistant Cultivars
If given the choice, why not choose plants that are resistant to disease? Here are some of the most common:

Apples: Liberty, Jonafree, MacFree, Freedom

Florence, Buttercrisp, Jade

Park's All-Season Burpless Hybrid, Fancipack, Homemade Pickles, Tasty King, Sweet Success, Salad Bush

Peas: Super Sugar Snap, Sugar Pop, Maestro, Green Arrow

Roses: The Fairy, Red Fairy, rugosa roses, Carefree Delight, David Austin English Roses, Town and Country Roses, Meidiland roses

Strawberries: Surecrop, Cavendish, Redchief, Allstar, Guardian, Scott, Lateglow, Delite

Tomatoes: Celebrity, Better Boy, LaRossa, Enchantment, Sunmaster, Mountain Delight, Big Beef, Beefmaster, Sweet Million, Viva Italia, Roma

Zinnias: Star Gold, Star Orange, Star White, Crystal White

Now you know which disease-resistant plants you'll want to begin your garden.

Publications International, Ltd.