Phytophthora root rot can be a major problem in nurseries and greenhouses. The following are some recommendations for contolling this disease from Rutgers University.
Phytophthora can be very difficult to eradicate once established. Successful disease management, therefore, requires an approach that utilizes preventive cultural practices. To manage this disease:
• Use only pathogen-free stock. Purchase only healthy, vigorous stock, and inspect these plants carefully for signs of disease. Imported stock should be segregated in the nursery for several months to be sure the plants are free of disease. Propagate only from cuttings taken from disease-free mother plants.
• Plant only in pathogen-free soil or potting mix. Since Phytophthora root and crown rot is easily spread through the movement of infested soil, all potting mix components should be steam-pasteurized before use. Once pasteurized, mix components should not come into contact with fresh soil or plant debris. In the propagation house, containers should be new or surface-sterilized before use. All propagation tools, benches, and beds should also be surface-sterilized. In field sites with a history of this disease, an appropriate chemical fumigant should be applied before planting.
• Select well-drained potting mixes and field sites. Phytophthora root and crown rot tends to be more severe in container plants than in the field because over-watering is a frequent problem and mix temperatures tend to be higher. The disease can also be a problem when soil, peat moss, or sawdust is used as the sole component of the container mix. A mix high in sawdust is more conducive to disease development because drainage is poorer. A good potting medium should generally contain 20 to 25% air-filled pore space. When used as a potting mix amendment, composted hardwood bark releases inhibitors that suppress disease development.
• To prevent the dispersal of spores of P. cinnamomi through splashing in the nursery, place container plants on a well-drained base, such as three to four inches of gravel. Container beds should be graded to allow water to drain away from the pots and prevent puddling. In the field, select planting sites that are most appropriate for vigorous growth.
• Maintain plant vigor. Maintain proper levels of soil nutrients, moisture, and soil pH. Avoid over-watering and cultural practices that promote overly succulent growth, such as heavy fertilization, over-crowding, and low light. Succulent plants and those under stress are more susceptible to disease.
• Utilize disease resistant plants and cultivars when possible. Many well-known azalea and rhododendron hybrids are very susceptible to this disease. Under severe environmental stress, however, even tolerant cultivars may succumb to this disease.
• Apply an appropriate fungicide when necessary. Fungicides such as Banrot, dimethomorph, etridiazole, fenamidone (greenhouse only), fosetyl- Al (drench, foliar spray, soil treatment), Hurricane (enclosed structures only), mefenoxam, phosphite (spray, drench, soil pre-treatment), propamocarb- HCl, or SoilGard are useful when applied as a preventively as per manufacturer's recommendations. Once plants become infected, fungicides may arrest further infection but do not kill the fungi within infected roots and soil. Although fungicides are useful for disease control, they cannot replace good cultural practices.
Information from Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University http://njaes.rutgers.edu/pubs/plantandpestadvisory/2008/ln0807.pdf