Prevent Blight at the Beginning

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By Scott Aker
Special to The Washington Post
Thursday, September 4, 2008

Q: My tomato plants have been afflicted this summer with a fungus or wilt. Does the soil retain these diseases year to year or were the problems caused by this season's rain and humidity?

A: It might be that early blight is killing your tomatoes, caused by a fungus named Alternaria solani. It is a common disease and starts as the plants begin to hit their stride in the early days of summer. Rain or irrigation splashes the spores upward from the soil surface, and the disease progresses up through the plant with each rainfall.

Preventing the initial infection is the key to managing the disease. Remove all traces of your tomato plants after the last harvest. That should remove the source of any spores. When you plant your tomatoes next year, apply a two-inch layer of mulch to the soil surface, which will keep any spores that might be in the soil from reaching the leaves when it rains. When you irrigate, be careful to keep the water off the foliage.

Another fungus, Fusarium, might be the cause of your problems. Afflicted vines exhibit wilting, and if you cut through an infected stem, you should see dark staining in the water-conducting elements. Because fusarium is in the soil, you will need to grow your tomatoes elsewhere for a few years or pick varieties bred to be resistant to Fusarium. A lot of heirloom varieties are not resistant, so it is important to rotate those varieties into a fresh bed next year.

Last fall, we planted four crape myrtles, each with six to eight stems. We have noticed that crape myrtles are pruned back to a branching point in late fall. Do we wait for the stems to start branching before pruning or does the pruning promote the branching?

Also, are there any pests or diseases to worry about?

Unless you need to remove branches that are abrading one another or dead branches or want to develop a single trunk, you shouldn't need to prune at all. When removing dead branches, wait until new growth appears in the spring to make sure the branch is dead. Crape myrtles are one of the last plants to leaf out.

You see this butchering of crape myrtles because the chosen varieties are too large for their locations. Cutting them back to the same size each year doesn't affect flowering because they bloom on new wood, but the practice is harmful nevertheless. Vigorous new shoots will sprout, but they will tend to be weak and prone to splaying and bending. Often one of the crape myrtle's most attractive attributes, the bark, is lost or compromised if they are pruned this way.

Careful plant selection is necessary to make sure that you have the right variety for your garden. Most crape myrtles are small trees, even though they are often marketed as shrubs. If you have a limited space and need a smaller plant, I'd suggest you grow Tonto, Caddo, Hopi, Zuni or Pocomoke, which are on the low end of the spectrum.

Crape myrtles are remarkably pest resistant. A weevil sometimes feeds on the margins of some of the leaves, and there is an aphid that is specific to crape myrtles, which is nearly always controlled by ladybird beetles and other predators. Powdery mildew is the main crape myrtle disease, but modern cultivars found in most nurseries are resistant.

Scott Aker is a horticulturist at the U.S. National Arboretum.


© 2008 The Washington Post Company

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