Gardening Columns

Focus on Fungus: Managing Garden Diseases

First published on garden.org on June 19, 2008, by Suzanne DeJohn

Gardeners in our region no doubt welcomed the recent rains. I know I did. But although they provided only a drop in the bucket when it comes to replenishing wells, lakes, and aquifers, the rains did bring something else: garden diseases. Or at least they created the conditions many diseases need to flourish.

How Diseases Spread
Most garden diseases are caused by fungi, microscopic relatives of the common garden mushroom. Mature fungi release millions of spores that are then carried on the wind or otherwise get transferred to our plants. And if the conditions suit them, the spores germinate and penetrate leaf tissue, creating spots, lesions, or other symptoms. Most fungi need moisture to germinate, so damp leaves are an open invitation. Some diseases are soil-borne and are transported to foliage when raindrops splash soil onto plants.

Fungi aren't the only disease-causing microorganisms. Some diseases, such as fire blight, are caused by bacteria. Others, such as tobacco mosaic, are caused by viruses. Bacterial and viral diseases are difficult, if not impossible, to control in the home garden. Fortunately, there are cultural techniques and effective products to help us control the more common fungal diseases.

Types of Diseases and Symptoms
Leaf spots. The most familiar disease symptoms are leaf spots. You can often identify the particular pathogen by the shape, color, and margins of the spots. But it really isn't necessary. The same general prevention and control measures apply whether you're dealing with black spot, anthracnose, downy mildew, early blight, late blight, septoria, or any of the other leaf spot diseases.

Powdery mildew. Aptly named, powdery mildew gives leaves the appearance that they've been dusted with talcum powder. Unlike the diseases above, powdery mildew spores do not need moisture to germinate. The disease, which attacks tall phlox, bee balm, lilacs, and cucurbits like squash and cucumbers, is worst in hot, dry weather.

Vascular diseases. Some diseases, such as fusarium and verticillium wilts, penetrate plant tissues and take up residence in the veins. Once inside, they clog the veins, preventing water transport which, in turn, causes plants to wilt.

Diseases on fruits. Many of the diseases that cause leaf spots will also affect fruits of all types -- apples, strawberries, tomatoes, cucumbers. Common symptoms include spots, corky areas, sunken patches, and discoloration.

Flower diseases. If flowers wilt or fail to open, gray mold, caused by the botrytis fungus, may be the culprit. It's especially prevalent during periods of high humidity and cool temperatures. Affecting foliage and flowers, gray mold begins as water-soaked spots which eventually grow a fuzzy gray coating.

Root problems. "Root rot" is a collective term for diseases that attack roots. In some cases, the problem is caused by overwatering, which inhibits the availability of oxygen to roots, causing them to die and decay.

Abiotic diseases. Herbicide damage, fertilizer burn, drought, chilling, and nutrient deficiency can cause symptoms similar to fungal diseases. Rule out abiotic diseases (those caused by something other than a living organism) before spraying for pests.

Disease Prevention
Simple cultural controls can go a long way toward preventing fungal diseases.

1. Choose resistant varieties. Plant breeders have created varieties with built-in resistance to certain diseases. For example, look for tomato varieties with V, F, or T after the names; these indicate resistance to verticillium, fusarium, and tobacco mosaic diseases. The apple variety 'Liberty' is less prone to apple scab disease than many other varieties. And roses in the 'Knock-Out' series resist black spot disease.

2. Space plants properly. Adequate spacing allows good air circulation, which helps keep foliage dry.

3. Water the soil, not the foliage. Drip irrigation and soaker hoses are better than overhead sprinklers.

4. Apply mulch. Soil-borne diseases can be thwarted with a layer of mulch, which prevents soil from splashing onto leaves. Apply fresh mulch annually under disease-prone plants.

5. Rotate crops. Plants in the same family are often attacked by the same diseases, so rotate annual crops. For example, avoid planting solanaceous plants, including tomato, pepper, potato, and eggplant, in the same garden bed year after year. Rather, rotate with plants in a different family, such as cucurbits (squash, cucumber, pumpkin).

6. Remove and destroy diseased plant tissue. If only a few leavs are affected, pick them off and discard (don't compost; composting may not kill the disease organims).

7. Don't overfertilize. Plants pumped full of fast-acting nutrients (especially nitrogen) grow quickly, but this succulent growth is very attractive to disease organisms. Aim for slow, steady growth by providing slow-release nutrients.

Deciding When and What to Spray
Consider the plant, the time of year, and the degree of damage before resorting to pesticide sprays. A small amount of damage on tomato foliage at the end of the season may be tolerable, while the same damage in early summer may warrant control. Diseases can spread quickly if not contained, so observe plants frequently. Note that most plant diseases are host-specific; that is, they attack similar plants or plants in the same family. For example, the powdery mildew species that attacks tall phlox is different than the one that attacks cucumbers.

Synthetic fungicides have proven to be some of the most toxic lawn and garden chemicals available. Indeed, several have been taken off the market, including once-common benomyl, captan, and daconil. Fortunately, there are now organic options and these, combined with cultural controls, are very effective. Many organic sprays act by making plant surfaces unfavorable for disease spore germination. Here's a rundown.

Copper-based fungicides. A staple for organic gardeners for generations, copper fungicides are effective but must be used judiciously because excess copper can harm soil life.

Sulfur-based fungicides. Another important fungicide for organic growers, sulfur is effective but it comes with some cautions. It is incompatible with some insecticides, especially oils, and can harm plants if applied in hot, dry weather. Repeated use can lower soil pH.

Neem oil. Derived from the neem tree, this botanical extract is a multipurpose insecticide, miticide, and fungicide. Use with care because the oil can harm some beneficial insects.

Biocontrols. Serenade is the trade name for a new biocontrol for several fungal diseases and is based on the bacteria Bacillus subtilis. It works by boosting the plants' natural immune system as well as by inhibiting fungal germination and growth. Sonata, another new product, contains a naturally occurring patented strain of Bacillus pumilus and is especially effective on rusts, as well as downy and powdery mildews.

Bicarbonates. Gardeners have been using homemade baking soda (sodium bicarbonate) sprays for decades, but recent research has shown that potassium bicarbonate is more effective at controlling plant diseases. Potassium bicarbonate-based products are sold under various trade names, including GreenCure and Kaligreen.

Compost teas. In addition to providing nutrients, some research is showing that compost tea sprays can inhibit plant diseases.