Overcome the odds of mold, mildew and plant disease

August 11, 2005
Victoria County
Master Gardener

The number of pests and diseases that can attack the plants we place in our yards can intimidate gardeners of every level. Sometimes we wonder if we can overcome the odds of growing anything.

Plants, though, are amazingly resilient and grow healthily in spite of adverse conditions. But every once in a while things do get out of hand. To keep the plants healthy we need to watch for the signs of problems.

From the Master Gardener Handbook we learn that problems are from three major categories: 1. insects, mites and insect-like pests; 2. infectious diseases - caused by fungi, bacteria, viruses, nematodes, etc.; 3. cultural and environmental disorders.

Another rule we learned as master gardeners is that for a disease problem to occur, three things must exist: 1. the disease pathogen itself; 2. a susceptible host; 3. environmental conditions favorable to disease. If any one of these is missing, there will be no disease problem.

So it is simple. Learn about the possible diseases. Understand which plants can be hosts. And lastly, watch for weather conditions favoring diseases.

According to Joe Janak, Victoria County Extension Agent, diseases reproduce from spores that are kind of like the seeds of plants. In many instances the spores are nearly continuously around, waiting for an opportunity to grow. As a result, fungal diseases are a way of life for plants in our warm, humid weather. Poor air circulation, bad watering habits and crowding contribute to the molds and mildews on our plants.

To minimize plant disease problems, first try to obtain disease-resistant varieties if they are available. Remember that the host plant must be susceptible. Agricultural research scientists continue to try to identify and develop disease resistant plant varieties. So take advantage of their work.

Then follow recommended cultural practices such as those from soil test results that will help to balance plant needs - since over-fertilizing (and even over-watering) will hinder healthy growth and encourage disease. This is where we as humans affect the environment. When we over-fertilize or over-water, a lush environment that is favorable to many diseases is created.

The same occurs when spraying the plant's foliage with water, which primes the plant for disease disaster. Try to water early in the morning or even late afternoon so plants will have time to dry before nightfall. Learn to water at the base of plants to keep water off the leaves. Too many gardeners seem to feel the need to water the foliage of plants. This is not only unnecessary, but also a sure way to increase the incidence of disease problems. Apply water only where the plants can use it - in the soil for root uptake.

If plants continue to have problems, consider moving them to another location possibly with less shade, better drying conditions due to increased air circulation and/or less crowding. Remember, most likely the disease spores are already there. You have to interrupt the cycle by either not having the host or altering the environment.

One of the common diseases currently being identified in local landscapes is powdery mildew, primarily on crape myrtles, but also on other plants. The warm, humid days and cooler, foggy nights we have experienced are ideal for this fungus to grow. Although hot and dry summertime conditions limit fungal growth, the disease can start just before it gets really hot and continue as we have cloudy and cooler rainy spells.

Powdery mildew on crape myrtles appears as a whitish-gray powdery looking fungi that grows on foliage. It causes leaf, stem and flower bud deformation resulting in unhealthy growing and looking plants. Once again, certain plant varieties are more susceptible to it than others and planting a resistant variety is the first step in minimizing the disease.

Since the disease spores are around almost continually, next we need to consider altering the environment immediately around the plant. This means no unnecessary wetting of the foliage, improving air circulation and, in severe cases, moving the plant to a sunny, drier location.

So what do you do if disease still takes over? As a last result, there are several kinds of fungicides available for both disease prevention and control. Fungicidal soaps, sulfur-based fungicides, neem-based or copper-based sprays as well as other fungicides are available.

In an article written in the Texas Extension Horticulture newsletter about fungicides for disease control in crape myrtles, Alfredo Martinez, pathologist from the University of Georgia, states: "Systemic fungicides including fenarymol (Rubigan), myclobutanil (Sisthane), propiconazole (Banner), thiophanatmethyl (Cleary 3336, Domain), triadimefon (Bayleton, Strike) and triforine (Triforine, Funginex) provide good control.

"Additionally, protectant fungicides such as chlorothalonil (Daconil), piperali (Pipron), trinumizole (Terragard) or wettable sulfur provide good powdery mildew control."

When using fungicides, always read directions carefully and heed all warnings.

Plants need continuous surveillance - so, as often as possible, check for insects, disease and growth habits. Learn the signs - leaf spots, blights, yellow blotches on leaves; these are all signs of disease. Catching a problem early will often save a plant, a tree or a crop - and help overcome the odds in growing healthy plants.