Harry A. J. Hoitink and Steven T. Nameth, Professors, Department of Plant Pathology, Ohio Agricultural Research and Development Center, The Ohio State University; and James C. Locke, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Application Technology Research Unit, Wooster, Ohio.
Control of Dieback
One principal objective of nursery production is to keep the cropping cycle as short as possible by providing high-fertility conditions. Therefore, plants in containers are maintained under conditions that make them highly prone to development of Phytophthora dieback throughout high-temperature growing seasons. This places more importance on disease management in nurseries through the use of fungicides, appropriate irrigation methods, and disease-suppressive container media. In arid climates, plants dry quickly enough after irrigation to avoid infection by Phytophthora spp. on the foliage.
Proper management of irrigation is basic to disease management, especially for plants produced in containers. Irrigation should be applied early in the day so that foliage dries quickly. Trickle irrigation is preferred for crops highly susceptible to Phytophthora dieback, because it keeps the foliage dry and avoids splash dispersal of pathogens. Flood irrigation should not be used because it can severely enhance the spread of Phytophthora root-rot pathogens in particular. Unfortunately, Ohio summers often are humid and high in temperature. Therefore, fungicide sprays may have to be applied frequently.
Phytophthora dieback and root-rot epidemics generally occur from early summer to early fall and mostly on container-grown stock when temperatures range from 75 to 95°F, particularly when the relative humidity is high. In some parts of the world, low-temperature Phytophthora diseases are active during mild winters as well. The authors have not isolated such Phytophthora spp. in Ohio. Sprays generally need to be applied thoroughly to all above-ground plant parts because all leaves, buds, and meristematic tissues protruding through resistant bark tissues on branches are susceptible.
Examples of fungicides that can provide effective control of dieback are mancozeb (e.g., Dithane M-45), Stature, chlorothalonil (e.g., Daconil Ultrex), and copper fungicides such as Kocide 2000, Champ DP, Champion WP, Camelot, Phyton 27, and Bordeaux mixture, in addition to fosetyl-Al (Chipco Aliette) and mefenoxam (Subdue MAXX). Subdue MAXX and Chipco Aliette are narrow-spectrum systemic fungicides. They should be applied once per month, but follow the label. Depending upon irrigation method, temperature, and rainfall, some fungicides may have to be applied weekly or even more frequently. Again follow labels precisely!