Rhododendrons are a popular spring flowering shrub found in many Iowa landscapes. Unfortunately, they are susceptible to phytopthora root rot if not planted in the right location. Phytopthora is a water mold fungus in the soil. The most common types are:

· Phytopthora cinnamomi
· Phytopthora cactorum

The fungus attacks the roots of susceptible roots causing them to rot and die. However, there are measures one can take to decrease the possibility of getting the disease. Environmental conditions need to be considered seriously.

Environmental Factors That Influence Disease

Rhododendrons prefer a well-drained soil with high moisture holding capacity. But if planted in site with high soil moisture and soil temperatures, slow decline or infection by phytopthora will eventually occur. However, there are ways to control the disease.

Control
Soil:
Placing the plant in the right location is extremely important. The site should be well drained, but if the soil is less than perfect, one can plant the shrub slightly higher than grade to alleviate water saturation. It is also very important not to plant the shrub too deeply. The soil should be right at the root flare (the first course of roots.)

Water:
Avoid over-watering the plant. Check the soil moisture before watering. The soil should be checked at a depth of one inch because the topsoil may appear dry, but the soil beneath could be wet.

Chemical:
Chemical control of phytopthora in established plants can be difficult, but a fungicide drench can be used to prevent spread to other healthy plants. In beds where rhododendrons have died from phytopthora root rot, it is suggested that a cultivar that is resistant to the disease be planted or choose another plant all together.

More Information
Information may be obtained from the Iowa State University Plant Disease Clinic. The site gives information on how to submit samples and other topics of interest. The website is as follows: http://www.extension.iastate.edu/Pages/plantpath/pdcintro.html