Humidity a problem for some plants

By By Steve Strong / MSU extension service horticulture agent
July 14, 2004
Lantana ranks among the most popular shrubs planted in southern landscapes, touted for its heat tolerance and resistance to most garden pests. However, when the cruel "dog days of summer" set in for the duration, even tough plants like lantana may succumb to a form of garden "meltdown."
Humidity is the culprit, as many savvy gardeners have already guessed, and there is little that can be done to combat three solid weeks of daily rainfall. Combine all that moisture with heat indexes topping the 100-degree mark, and the result is a recipe for plant diseases, like the mildew fungus currently attacking the foliage on lantana and other garden shrubs.
Mildews thrive in hot steamy landscape environments just like in a bathtub or shower stall, and problems arise when there is not enough air circulation to dry things out quickly enough. Lantana and other drought tolerant plants rely on dry foliage and flowers for healthy growth, and when the air around them remains thick enough to cut with a knife, the extra moisture stays stuck to the plants.
Most plants survive in extreme heat by "transpiring" water out through stomata on the undersides of their leaves. Stomates are tiny openings that draw moisture up through a plant's vascular system from the roots, and then "push" out the water similar to the way pores on our skin produce sweat to keep us cool.
When the outside air is so humid that it prevents the water from evaporating, the excess moisture collects along the edges of leaf surfaces, and that is where the mildew starts to attack. On lantana, the leaves begin to look gray and crispy at the tips, followed by dieback of entire stems and reduced blooming.
Few gardeners really care about all of this technical plant physiology stuff, they simply want to know what they can do to cure their plants. The answer is one of two things (or try both) first cut back and destroy all of the unsightly diseased foliage, then give them a light dose of your favorite fertilizer to stimulate new growth.
Fungicides can be applied on weekly basis to help protect new leaves as they emerge, but be aware that fungicides will not "cure" diseased foliage. Pruning back is still the best bet in reducing the amount of mildew spores that could remain to infect the plants again during the growing season.
Trailing forms of lantana such as the award-winning "New Gold" variety appear to have the worst problems with mildew, because they grow lower to the ground where the humidity is highest. Maximizing exposure to drying sunlight and improving air circulation are a couple of cultural ways to reduce mildew outbreak, and you could also try planting more upright varieties.
Cultivars such as "Miss Huff" or the newer "Son" series of lantana (Sonrise, Sonset, Samson) can reach heights of 3 to 6 feet, and seem to be fairly mildew resistant. Another plus is that these varieties along with New Gold' produce very few berries, which are deadly poisonous and also work to reduce flowering as the fruits mature.
Finally, Lacebugs attack lantana, too, and may cause damage similar to mildew (like lacebugs on azalea, river birch), but insect injury normally shows up earlier in the growing season. On a side note, many of our turf grass diseases also thrive in the same weather conditions as lantana's mildew, and the best preventions are minimal watering and reduced doses of nitrogen fertilizer.

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