Over the summer, the Plant Clinic received a number of samples and questions about this fungal disease. Refer to issue No. 11 of this newsletter for details about the disease and the pathogen. Report on Plant Disease No. 664 is also a good reference. The fungus is a water mold, and water is required to infect plants. Most problems occur in wet or poorly drained areas, and especially on clay sites. Logically, one of the most essential management tools for rhododendron disease control is to improve drainage within the soil as well as away from the site. A well-drained planting site in a clay soil with no provision for water flow away from the plant will likely result in problems with Phytophthora root rot.
We try to emphasize site improvement for disease control, but fungicides are often necessary to stop the spread of Phytophthora. The Commercial Landscape & Turfgrass Pest Management Handbook lists several fungicides registered for use on rhododendron, including Aliette T/O (active ingredient fosetyl-aluminum) by Terra, Banol (propamocarb- hydrochloride) by AgrEvo, Chipco Aliette (fosetyl-aluminum) by Rohm & Haas, Prodigy (fosetyl-aluminum) by Lesco, Pythium Control (metalaxyl) by Scotts, Subdue (mefenoxam) by Novartis, and Terrazole (etridiazole) by Uniroyal.
The Illinois Homeowners’ Guide to Pest Management states that there are no fungicide options available to the homeowner.Why is nothing listed in the manual? These chemicals are not restricted-use products, and they could be used by homeowners. But they are packaged in quart-sized--or even larger--containers that would last the average homeowner a lifetime. The cost for such containers is usually prohibitive for homeowners. Because these products are not usually on the shelf in retail outlets, availability is also a problem.
The bottom line for managing rhododendron disease is that you must improve soil drainage, remove badly infected plants, and use fungicides if they are available. Look in your local retail outlets for the products listed. You might also ask a reputable lawn-care company in your area if they could apply one of these products. You will have to check the label for timing and repeat applications. We always suggest that poorly drained areas be renovated. Badly infected plants serve as an inoculum source for healthy plants, so consider removing them. Horticulture fact sheet LH 6-82 discusses amending landscape soils with sand, and other products can also be used to provide a well-drained medium. Again, make certain the well-drained soil drains away from the plants.