GARDENING

Understanding landscape disease


Published: Saturday, July 5, 2014 at 6:01 a.m.
Last Modified: Tuesday, July 1, 2014 at 3:04 p.m.

'Everybody knows that” may be an advertising phrase you are familiar with. In some ways, it is the feeling I get when I discuss the fact that since last November, North Central Florida has received a great deal more rain than normal.

The last time we experienced a wet winter, in 1997-98, it was followed by a period of drought that began in March 1998 and lasted almost two years. With this in mind, it has been a relief to see the rains continue into summer, where by mid-June, we had already exceeded our normal average rainfall total for the entire month.

Weather is a significant factor in managing landscapes, and wet weather, combined with warm temperatures, can lead to problems for plants. Plants that sit in water-logged soil long enough may display signs of wilt, might lack vigor, and can develop light green or pale yellow-green areas in leaves because of root oxygen deficiency. The majority of roots are in the top 24 inches of soil for a reason; oxygen-starved roots do not function properly and are unable to withdraw water and nutrients from the soil. If these conditions persist, certain plant cultivars become stressed and may die.

Stress also presents opportunities for disease. There are three types of microorganisms that cause plant disease: fungi, bacteria and viruses. Disease symptoms may be described as spots on leaves or browning of part of a leaf or, eventually, death of an entire segment of the plant.

Most diseases (up to 85 percent) are fungal. Usually, I think of these as a mushroom-type growth, which is the fruiting body of the disease. These may be seen as “conks” which grow on decaying limbs or trunks of trees. Sometimes, mushrooms appear in yards and are the result of fungi feeding on organic matter, often old tree roots beneath the grass. Fungal pathogens often are dispersed by wind, which may carry spores for miles. Water splashing from rainfall or irrigation also is responsible for moving fungal spores from plant to plant. Fungi that live in the soil can move from plant to plant by growing along intermingled roots or out from infested plant debris in the soil. Some fungi can live on their own for long periods of time without a host, surviving in plant debris or soil. Human activity, through movement of already diseased plants or the use of gardening tools, is another means by which fungal diseases are spread.

A second type of disease, seen much less often than fungal disease, is bacteria. These are extremely small, one-celled microorganisms. While it is possible for plant pathogenic bacteria to survive in the soil in decaying plant material for a time, they usually need a host to survive. Splashing water through wind-driven rain or irrigation is the primary means by which bacteria are spread, but pruning tools also can be responsible for bacterial disease movement.

Viruses are the smallest and least frequent of the three pathogens, and usually are spread from diseased to healthy plants by insects, with aphids and whiteflies the prime vectors.

An ounce of prevention is worth a pound of cure is another thing everybody knows. In regard to plant disease, sanitation and prevention are key management tools. Right after pruning, rake out cuttings and after the leaf drop that occurs in fall or winter, any old leaves that may overwinter; these can provide harborage for disease. This material may be buried in a properly constructed compost pile or, where available, placed in garbage that will be incinerated. Periodically, clean pruning shears using a 30 percent solution of household bleach in water. This works well in a spritz bottle, and is especially important to do after trimming any material you suspect may be diseased. If buying any new plant material, look plants over carefully to ensure you are not bringing diseases onto your property. Avoid buying plants with leaf spotting, distorted growth, stunting or off-color leaves. Consider soil characteristics at the site, and match plants accordingly. Manage water carefully, watering only when necessary, based on recent and forecast weather conditions. When we are receiving abundant rains, as of late, irrigation systems should be off. As we move into fall and the need to water returns, direct irrigation away from leaves to root systems. Water early in the morning so that plant leaves have time to dry before sunset. Enhance air movement by reducing dense plantings, and thin plants through infrequent pruning, where possible.

We are grateful for the rains, and this is certainly preferable to drought. During this period of greater-than-normal rainfall, manage landscapes carefully to reduce stress on plants.

David Holmes is Marion County extension director. Contact him at david.holmes@marioncountyfl.org.

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