The arrival of warm weather in spring brings plant disease fungi out of hibernation. It is highly unlikely that all the diseases mentioned in this article will appear in the same yard. It is possible, though, that someone will see most of these diseases one or more times in a decade.
Powdery mildews show up in spring on woody ornamentals, like crepe myrtle, hydrangea and rose, and on mealy-cup sage, verbena, common zinnia and cosmos. The best solution to powdery mildew depends on the type of plant attacked. Pinwheel zinnia and most salvias are resistant, so these species can be substituted for the more susceptible ones.
Organic sprays that contain potassium bicarbonate, sulfur or horticultural oil will eliminate powdery mildew. Potassium bicarbonate is the safest product to use in the heat of summer. Organocide, a product made of fish emulsion (92 percent) mixed with sesame oil (8 percent), also works well. Roses, however, should be sprayed with a conventional fungicide labeled for use on roses.
The lawn disease, now called large patch, of centipede and St. Augustine is caused by a soil fungus named rhizoctonia in combination with too much nitrogen. (This disease previously was called “brown patch,” but now brown patch refers to a similar disease on cool-season grasses.)
To eliminate large patch, regular sprays with fungicides such as Headway or Pillar are needed. Although cheaper fungicides are marketed to treat large patch, they do not contain the most effective ingredients.
Large patch will not be a problem if the right amount of fertilizer is used. See “Fertilizing Lawns” at https://bit.ly/2Gn4N7L.
Extra nitrogen fertilizer makes grass more susceptible and “super-charges” the fungus, which prefers a high-nitrogen diet.
Many annual ornamentals are susceptible to root and stem rots. Among the plants that will develop rots are vinca, dianthus, scabiosa, yarrow, lamb’s ears, thyme, and mealy-cup, autumn and Russian sages.
There are several rot options to manage rot, including plant substitution. Annuals less likely to develop root rot include angelonia, pentas and ageratum for summer and alyssum and wallflower for fall.
Soil drainage can be improved by making raised beds and adding compost. Potassium phosphite (e.g. Agri-Fos) can be sprayed on and around plants to prevent root rot. Note that some plants, such as snapdragon, may be burned by potassium phosphite. The lowest rate should be tried first.
Rust disease on snapdragon shows up around this time of year as pale-yellow spots on leaves. The rust pustules are found directly below on the leaf underside.
When rust is noticed, snapdragons should be replaced with summer annuals. Snapdragons should not be planted in the same spot for one year to ensure that any rust spores that land in the soil die.
Blossom-end rot appears in early summer on fruiting vegetables, especially tomato, but also pepper, watermelon and summer squash. This nutritional disorder affects the blossom end of fruits when plants cannot take up enough calcium to supply the rapidly growing fruit.
By the time the deficiency is seen, it is too late to use calcium sprays. The remedy includes watering deeply but less often to encourage a larger root system. The amount of fertilizer applied should be cut in half, as too much nitrogen promotes blossom end rot. Before replanting a susceptible crop, the soil should be tested to make sure it has enough calcium in it.
Downy mildew of basil usually is seen in late summer to fall in South Carolina. It can be difficult to recognize, as the main symptoms are indistinct yellow blotches on the leaves. If downy mildew is causing the yellowing, the bottom of the leaf will be covered with a thin to thick layer of purplish gray spores. The spores are easily blown to other basil plants miles away.
Potassium phosphite is the best material to control downy mildew on basil. Diseased plants should be destroyed, not consumed.
One “pseudo-disease” that appears in spring is actually not a problem. Gray slime mold on turf is a fungus-like organism growing on the outside of grass blades. The slime mold does not infect grass; it is living on the dead thatch layer. Slime molds tend to show up during wet periods in late spring and are gone within a week.
Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at firstname.lastname@example.org