Removal of diseased plants and plant parts reduces the amount of spores that spread disease. Commercial nurseries should dump their crops if the disease is found in the seedling stage. If nurseries attempt to salvage their crops, surviving plants will have Phytophthora root rots and the pathogen will be spread to many locations. Gardeners and landscapers should check each batch of plants before purchases. If any plants are wilting or have collar rots, avoid all of them.
Fungicides are available that reduce disease levels.
However, no chemical will eliminate this pathogen and the pathogen will always be present once it is introduced among susceptible plants. Mancozeb, sold as Dithane M45 is a good contact fungicide and can reduce the number of leaf spots. Metalaxyl, sold as Subdue is an effective systemic fungicide that will protect leaves and new shoots. Follow label directions when using any fungicide. Aragaki, M. and J. Y. Uchida. 1978.