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Why do rhododendrons droop and die? Suspect phytophthora. That's root rot.

When a well-fed, well-watered rhododendron looks shabby or suddenly wilts and dies, suspect Phytophthora (fie-toff-thora) root rot. This parasitic fungus attacks the water-conducting tissues of rhododendrons as well as azaleas, junipers, petunias, pines, and many other garden plants.

Probably the most destructive disease to rhododendrons, it is far easier to prevent than to cure. Now--at spring planting time--is when to head it off.

Phytophthora is common in Western soils. Even if it isn't already present in your garden, it can ride in on infected nursery stock or on plants from friends. During warm, wet weather, its swimming spores spread through very wet soil and attack roots.

Healthy plants in fast-draining soil may resist the disease. Under stress, however, infected plants rapidly succumb. Foliage becomes dull, yellow, wilted. Individual branches on an otherwise healthy shrub may wither. On azaleas and rhododendrons, infected branches die back from the tips toward the trunk. Sapwood at the base of many infected plants turns brown. Roots become blackened and stunted; fine white feeder roots are destroyed. How to head off Phytophthora

To minimize the chances of root rot becoming established in your garden, you can take the following steps before you set out your plants.

Select plants suited for your site. Choose heat- and drought-tolerant types for sunny spots. For shady areas, choose plants tolerant of shade and damp soil.

Select resistant species and varieties of susceptible plants. Resistant but hard-to-find varieties of rhododendrons are 'Caroline', 'Professor Hugo de Vries', and 'Red Head'. Resistant varieties of junipers are Juniperus virginiana 'Prostrata' and J. scopulorum 'Table Top Blue'. But remember, resistance lessens in a garden teeming with the disease.

Buy clean, vigorous nursery stock with plenty of white feeder roots. If you are in doubt, your nurseryman might agree to knock the plant out of the container so you can see its roots. You can't be certain that good-looking plants are disease-free, but they are a safer bet than pale, wilted plants with rotting roots.

Plant in well-drained soil. If necessary, add organic material such as composted redwood sawdust and manure to slow-draining soil, or build raised beds. In well-aerated, fertile soil with adequate organic matter, beneficial fungus may block the spread of Phytophthora.

Keep susceptible species apart. A seemingly healthy rhododendron could infect your favorite avocado or oak tree.

Group plants with similar water requirements, or plant them on the same irrigation line. Avoid planting trees that don't need summer water in a lawn that does. Space plants far enough apart so that air can circulate between them and dry excess moisture that might otherwise spread disease.

Set the crown (the base of trunk) of all but the most water-loving plants slightly above the soil line. If you're going to build a berm of soil around a plant, set the plant high enough that its crown will remain above the standing water line. Some varieties can stand root rot, but few can live with crown rot.

Splash from overhead sprinklers can spread disease: so can flooding. Drip irrigation helps avoid some fungus problems. However, if you leave drip systems on too long, they'll leach nutrients from the soil and encourage disease. Misting heads help keep up humidity for rhododendrons. Water in the morning and let the soil dry a bit between waterings, but don't underwater, since thirsty plants are also susceptible to Phytophthora.

Keep planting beds clean of fallen fruit, leaves, stems, and flowers. Phytophthora spores can overwinter in garden debris. What to do for infected plants

For aid in identifying Phytophthora, consult your cooperative extension service or a soil-testing laboratory.

Thin aboveground plant parts to enhance air circulation and to compensate for any root damage. Between cuts, disinfect pruners.

Some retail nurseries sell a fungicide called metalaxyl (subdue) that can be surprisingly effective in abating Phytophthora. However, this chemical is not packaged for the home gardener, and it is so concentrated that it's difficult to mix properly (even a quart-size container is too large for home garden needs). It's also expensive--about $40 and up per quart.

Metalaxyl is only effective on Phytophthora and the related Pythium fungus. Indiscriminate use encourages the diseases to develop resistance. Nor can it overcome poor drainage and overwatering; under such conditions, the disease returns as the chemical wears off.
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No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Date:Apr 1, 1985
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