Preventing Fungal Disease

Gardening by the Yard : Episode GBY-401 -- More Projects »
Fungal diseases are the athlete's foot of the plant world. You can take action to prevent them from spreading, but no amount of spraying or dusting will eliminate certain fungal diseases once they've taken hold.
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Figure A
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Figure B
Fungal diseases appear on leaf surfaces as discolorations such as black, white or rust-colored spots or as powdery or moldy growths (figure A). They may also cause leaves to become distorted (figure B). In some cases they cause weird-looking growths such as golf-ball-sized galls. Bacterial diseases can cause similar symptoms but generally kill plants rather quickly. Fungal diseases tend to cause a slow death. The best defense against fungal diseases is prevention:
  • Choose resistant varieties of plants. Many plants, both ornamental and vegetable, are naturally resistant to fungal diseases or have been genetically engineered to resist them. To find these, read the tag, ask the nursery staff or read the latest literature.

  • Pick up leaf litter. Fungal spores, the diseases' reproductive bodies, may overwinter in leaves, so rake them as they fall and dispose of them in the trash, not the compost pile.
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Figure C
  • Disinfect pruning tools (figure C). Use a mild solution of bleach and water to disinfect pruning shears, loppers and saws between cuts and between plants when you prune.

  • Provide adequate air circulation. Numerous fungal diseases thrive in cool, damp areas, where there's little air circulation.
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    Figure D
  • Mulch. A thick layer of mulch (figure D) may serve to smother overwintering fungal spores and prevent rains or water from the hose from splashing spores onto leaf surfaces.

  • Water properly. Early-morning watering will prevent certain fungal diseases because leaf surfaces have a chance to dry out during the day. Watering the base of plants rather than the foliage can help too, no matter when you water.
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    Figure E
  • Control insect populations (figure E). A number of insects, especially aphids, can cause more damage to plants by spreading fungal diseases than by sucking or chewing on plant parts.

    Despite your best efforts, chances are you'll still have to battle a fungal disease at some point. Here's an explanation of popular fungicides:

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    Figure F
    • Synthetic fungicides. Garden centers offer a number of several synthetic fungicides, many of which are systemic (figure F). These are absorbed by the plant, where they remain for weeks or months. Some are broad-spectrum formulations, meaning they control all kinds of fungal diseases, whereas others act only on specific diseases, so read the label carefully.
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    Figure G
  • Barriers (figure G). A light spray of vegetable oil or highly refined horticulture oil coats leaf surfaces and acts as a barrier to fungal diseases, especially rusts and mildews. Antitranspirants or antidessicants, such as Wilt-Pruf, whose primary function is to control moisture loss in plants during the winter, may also reduce the likelihood of certain fungal diseases.
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    Figure H
  • Plant products. In addition to being a potent broad-spectrum insecticide, garlic appears to have fungicidal properties when blended with water (figure H), strained and applied to leaf surfaces. The same is true of compost tea. Several companies offer extracts from the tropical neem tree to control various insects and fungal diseases.

  • Baking soda. It may come closer to destroying fungal diseases than any conventional fungicide on the market. It's also nontoxic. Mix two teaspoons per gallon of water, adding a few drops of liquid dish soap to help the solution stick to plants.
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    Figure I
  • Sulfur (figure I). It works great but can cause a number of problems. It's mildly toxic to soil organisms and can cause respiratory problems in humans if inhaled. And at temperatures above 85 degrees it can burn plants.
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    Figure J
  • Copper. Copper is a powerful fungicide (figure J), but it can cause more damage to plants than sulfur. And it too can be toxic to gardeners. The most familiar copper-based fungicide is Bordeaux mixture.
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    Figure K
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    Figure L
    Apply fungicides thoroughly to leaf surfaces, including the undersides (figure K). Unless the product is a systemic fungicide, be prepared to spray often, at least every 10 days or so during the growing season (figure L). And if you find that even with repeated spraying a particular plant or plant group continues to be plagued by one or more fungal diseases, consider getting rid of it and growing something else.
    Also in this Episode
    Gardening by the Yard : Episode GBY-401
    •Preventing Fungal Disease
    •Topsoil
    •Fruit Trees
    •Freeze Damage