The recent rainfall and cloudy weather has certainly been welcome from the standpoint of water and moisture supply for our lakes, trees, lawns and gardens.
This type of weather pattern, though, can result in certain plant diseases. Many fungal diseases require a period of prolonged leaf wetness for spores to germinate and penetrate the host tissue. You can think of spores germinating in much the same way as plant seeds. When you sow seed, or seed falls from a ripe flower in the field, it will not germinate until there is sufficient, prolonged soil moisture to start and sustain the germination process. A film of moisture on a leaf surface, whether through rain, an irrigation system, or even heavy dew, is what spores of most fungal organism that cause leaf spots need to germinate.
Also, like plant seeds, the temperature must be right. Many wildflowers, like bluebonnets, will just sit there and not germinate if the soil and air temperature is too high, even if there is sufficient soil moisture. Some fungal diseases need cool or mild temperatures, while others need warmer weather for spores to germinate.
Once germination takes place, the fungal organism grows into the tissue and eventually causes disease symptoms like leaf spots, chlorosis or defoliation.
GRAY LEAF SPOT
One common problem we often see in early summer on St. Augustine grass is a leaf spot disease called gray leaf spot. Gray leaf spot can develop rapidly with abundant moisture and warm temperatures. It can be more prevalent in shaded areas that remain damp for prolonged periods of time, or in low-lying spots or where water runoff flows regularly. Under these conditions, the disease can cause serious thinning of the turf.
Leaf spots first appear as tiny brown to ash- colored spots with dark margins that enlarge and become elongated or diamond-shaped. Sections of the leaf blade will also turn yellow. In severe cases, lesions develop on stems and the leaves wither and die. Turfgrass may have a burned or scorched appearance resulting from death or spotting of the leaf blades. In a mild case, a general yellowing of a patch of grass may be what is noticed first.
Avoid applying high rates of soluble nitrogen fertilizers on moderately shaded lawns during early summer months. Water the lawn early in the morning, and only when supplemental water is needed. Avoid evening waterings that keep the leaf surface wet for long periods. Catch and remove grass clippings where gray leaf spot is a problem.
Several fungicides are recommended for gray leaf spot control, including azoxystrobin (not readily available for homeowners), propiconazole, and thiophanate-methyl.
Another weather-related turf phenomenon is called slime mold. You would think by the name we are talking about a slimy, gooey condition on the grass. Actually, slime mold often appears as if gray or black ashes or oil were deposited on the grass. Closer examination usually reveals a granular, crusty or powdery material moving up the blades of grass from the thatch and soil surface. They are "slimy" when the first appear, but quickly dry out.
The good news is that slime mold is not harmful to grass, but because it is so visible, it often causes concern. Slime mold lives most of its life cycle unseen, breaking down and feeding on organic matter in the thatch and on the surface of the soil. When environmental conditions are just right, it goes from its vegetative state into a reproductive mode. This is when the light to dark gray to black spore masses become visible. They slowly moves up any type of support, in this case grass blades, and turn into a large mass of spores that are then blown in the wind and transported in runoff water. The spores easily rub off on your shoes or hand when touched.
Besides looking unsightly, slime mold does no harm, and can be washed or swept off the grass. No fungicides are required.
A similar slime mold is often seen in flower beds with compost from tree products, or bark mulch. This one can really get folks attention. It looks like a dog threw up in the flower bed, hence the common name dog vomit slime mold. Sorry about the graphic description, but that is what it is frequently called. When it appears, it is usually bright yellow or sickly orange and is soft and moist. Later, it dries to a light brown crust, and eventually will release lots of spores if disturbed. The mold is living on the decaying woody organic matter and usually stirring up the mulch will help aerate and dry out the area.