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Survival of Plant Pathogens in Static Piles of Ground Green Waste

First, third, fourth, and seventh authors: University of California Cooperative Extension, 669 County Square Drive, Suite 100, Ventura 93003; second author: University of California, Department of Soils and Environmental Sciences, Riverside 92521; fifth author: University of California, Department of Nematology, Riverside 92521; and sixth author: University of California, Department of Plant Pathology, Riverside 92521.

Ground green waste is used as mulch in ornamental landscapes and for tree crops such as avocados. Survival of Armillaria mellea, Phytophthora cinnamomi, Sclerotinia sclerotiorum, and Tylenchulus semipenetrans was assessed for 8 weeks within unturned piles of either recently ground or partially composted green waste. S. sclerotiorum survived at the pile surface and at 10, 30, and 100 cm within the pile for the entire 8 weeks in both fresh green waste (FGW) and aged green waste (AGW). A. mellea and T. semipenetrans did not survive more than 2 days in FGW, while P. cinnamomi persisted for over 21 days in FGW. AGW was less effective in reducing pathogen viability than FGW, most likely because temperatures in AGW peaked at 45°C compared with 70°C in FGW. Survival modeling curves based on pile temperatures indicate the time to inactivate 10 propagules of pathogens was 11, 30, 363, and 50 days for A. mellea, P. cinnamomi, S. sclerotiorum, and T. semipenetrans, respectively. Sclerotia-forming pathogens pose the greatest risk for escape; to ensure eradication of persistent fungi, green waste stockpiles should be turned intermittently to mix pile contents and move pathogen propagules to a location within the pile where they are more likely to be killed by heat, microbial attack, or chemical degradation.

Additional keywords:avocado root rot, citrus nematode, compost, lettuce drop, mulch, oak root rot.