Beware of late blight, and report damage

By Joel M. Lerner
Saturday, June 12, 2010; E04

More than in most years, there is acute concern associated with pest control in the Washington area. On May 7, late blight was detected in a greenhouse in St. Mary's County. Measures have been taken by the grower to eradicate the disease, but plant scientists don't know whether it will spread or whether this outbreak is related to last year's.

Plant pathologists are monitoring the situation to determine whether fungicides should be recommended to treat tomatoes and potatoes this year. You can help by being vigilant in scrutinizing your plants and by providing plant samples or photos to agricultural extension agents, even if you only suspect a problem. The symptoms of late blight on tomatoes and potatoes will present themselves in various ways. The most common symptoms: The leaf turns brown; plants do not just get spots like with many other fungal diseases. Disease can appear on the stem with large brown lesions. Or brown areas may appear that will eventually cover the entire fruit. At that point, it is too late to treat, so as soon as you see any hint of potential damage, report it to the extension or to the University of Maryland's Home and Garden Information Center (HGIC).

In May, the office of Jon Traunfeld, director of the HGIC, issued an alert about the early onset of late blight (Phytopthera infestans). Treatment can be recommended if necessary, but common sense dictates that you shouldn't spray for a disease that hasn't been diagnosed.

A strain of this fungal disease caused the 1845-51 potato blight in Ireland. Late blight is capable of destroying the tomato and potato crops in this region for the 2010 growing season, which could be ruinous to commercial growers. According to Traunfeld, last year's late blight outbreak devastated many backyard and community gardens in Maryland, whose infected tomato and potato plants rapidly declined and died.

Three fungicides -- chlorothalonil; mancozeb; and for organic growers, fixed copper fungicides -- are labeled for late blight and will provide fair control. The tricky part is that they work to prevent the disease; plants that show symptoms generally do not respond to treatment. These should not be sprayed into the environment unless it's absolutely necessary.

Although late blight is the most serious threat gardeners may encounter this summer, it isn't the only disease, insect or mammal problem that may cause trouble. I have already been asked about aphids, ants, flea beetles, slugs, deer, powdery mildew and winter kill.

Here are some potential problems you may encounter this season and commonsense suggestions for managing them:

-- Powdery mildew Theophrastis wrote of this problem on roses in 300 B.C. Spots can occur on dry leaves, and warm temperatures with shady conditions encourage the fungus to spread. Select resistant varieties, grow them in full sun with good air circulation and discard plants with severe mildew.

Powdery mildew is commonly seen on peony foliage late in the season after the plants bloom. Cut them back after they lose their ornamental value and discard the foliage. It is not a serious problem; they should grow back next year, barring more serious problems.

Native dogwoods showing signs of mildew do not require treatment with a fungicide. The white cast on leaves is temporary unless there is a summer drought. With cooler weather and soil moisture, the common dogwood will still display maroon leaf color in fall.

-- Gypsy moths This is the first time in 20 to 30 years that I have seen the characteristic yellow-tan egg masses in such large numbers (100 to 1,000 per mass) on the trunks of oaks, this caterpillar's favorite. Fortunately, gypsy moth problems are cyclical, with populations reaching outbreak levels every five to 10 years. This means trees that defoliate have time to renew their sugar stores before future outbreaks. Gypsy moths aren't native insects, but they have been kept in check because of more than 20 natural predators, parasitoids and diseases that attack them in North America.

-- Dogwood anthracnose After 15 to 20 years of dying trees, with sprays only superficially holding this fungus at bay, Ethel Dutky, University of Maryland plant pathologist, reports that fewer native dogwoods are developing anthracnose symptoms throughout this area. An untested theory is that they, or their offspring, developed resistance and the disease lost its environment.

-- Insect control One state-of-the-art mode is to find animals that feed on problematic insects. Another approach employs hormones called pheromones that attract predators and parasites that will kill the harmful pests. One product is ladybug lures. Ladybugs can eat up to 1,000 aphids in their lifetime.

-- Flea beetles These pests can affect vegetable crops and render them inedible. They can also pierce the leaves on perennials. If they return next year, you might need to treat. The recommended treatment is pyrethrin, used when leaves first show damage.

-- Weeds Hoeing, tilling or pulling weeds have been common methods of control for more than 2,000 years. In the 1950s, commercial weed killers were developed. To have the least impact on the environment, try applying vinegar for safe, instant knockdown and corn gluten powder for a preemergent weed killer in spring and fall. If this fails, tolerate some lawn weeds and continue to hand weed and mulch your beds as necessary throughout the season.

Joel M. Lerner is president of Environmental Design in Capitol View Park.

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