Hot weather effects on plants and people

By By Steve Strong / area horticulture agent
July 17, 2002
When the humidity level and the thermometer each hit 95 degrees, the stresses put upon gardeners and their plants are obvious.
As the outside air becomes so thick with moisture that it prevents sweat evaporating from human skin, plants also have a hard time "transpiring" water away from leaves and fruit.
This excess moisture can produce some strange effects in the yard and garden, as in the case of slime molds on turf grass. Homeowners have watched with alarm as shiny black and gray splotches appear on their prized St. Augustine lawns, looking as though the next-door neighbor dumped their oil change in the front yard.
What folks are actually witnessing is the spore production of slime molds, harmless fungi that spend most of their life cycles in the soil, feeding on the thatch of decayed turf grass. High temperatures combined with high humidity stimulate slime molds to reproduce, causing spore growth on grass blades like a blanket of dark shiny dust.
While slime mold can grow a spore layer thick enough to block needed sunlight from turfgrass leaves (causing a slight yellowing), the unsightly fungous spores can be easily brushed away with an old broom. A high pressure spray with the water hose can also wash away the "oil-slick" appearance, but be aware that extra moisture will likely just promote more spore growth.
Slime molds can be managed with treatments of an approved lawn fungicide, but this option is not necessary since these fungi cause no disease problems. The reproductive phase lasts only a few short weeks, and the fungi will again return to their invisible lives inside the turf thatch.
Other fungous problems are not so harmless, such as the mildews that attack dogwoods, roses and other favorite landscape plants. Air circulation is vital for plants prone to mildew problems, especially sun-loving shrubs like roses. A number of fungi (powdery mildew, black spot) thrive during periods of frequent rainfall, and one of the few control measures is a regular fungicide spray program.
The same holds true for peaches and the ever-present brown rot fungus that attacks the fruit during ripening. This pathogen, like the mildews, over-winters each year on the plants and will remain a problem throughout every growing season. Begin a spray program early, just after the bloom drop as a prevention measure rather a cure.
Other garden nuisances seem to have no cure, like the wet rot affecting okra pods. There is no fungicide known to work on the fungus that causes wet rot, and proper plant spacing to provide air circulation is the best way to prevent it.
One other garden oddity occurring recently is the mysterious black rot found inside tomato fruit (this one is different from the blossom-end rot caused by calcium deficiency). Normally seen only on plants grown in greenhouses, this rot problem is not a disease and has no known cause other than the weather thus there is no known method of control.
Sometimes gardeners simply have to put up with the stuff that goes on in their lawns and landscapes, but it's at least nice to know what you're dealing with. Contact the Lauderdale County Extension Service at 482-9764 for more information about gardening, or try out the university Web site at http://www.msucares.com.