Vegetable Seedling Diseases Associated with Earthworm Castings Contaminated with Phytophthora capsici and Pythium attrantheridium

Earthworms and worm castings have been recommended for their beneficial effects in increasing yields and suppressing soilborne diseases. However, in a few cases, earthworm castings have been shown to harbor soilborne pathogens. The research documents that earthworm castings used as an amendment in soilless potting mixes at several organic farms in North Carolina were contaminated with Phytophthora capsici and several Pythium species. Phytophthora capsici and P. attrantheridium were subsequently isolated from rotted roots of vegetable seedlings grown in the potting mix. Commercial producers of earthworm castings should only use clean plant material to maintain earthworms and earthworm castings should be ascertained as pathogen-free before incorporation into plant growth media. Introduction Earthworm castings are an odorless, nutrient-rich, organic medium that supports a diverse microbial community (2,3,5). Castings are also rich in calcium humate, a binding agent that reduces desiccation of the castings and promotes incubation and proliferation of beneficial organisms such as Trichoderma spp. , Pseudomonas spp. , and mycorrhizal fungi (6,8,10,11). Worm castings have been shown to suppress Verticillium wilt of eggplant (7) and Fusarium wilt of cyclamen, iris, and tomato (8,10). Earthworms disperse propagules of both beneficial and plant pathogenic soil microorganisms (4), but less is known about the potential of earthworm castings to serve as a source of soilborne pathogens in agricultural systems (5,13). Here we report the presence of Phytophthora and Pythium species in earthworm castings and their association with disease in soilless potting mixes. In spring 2010, four growers in North Carolina experienced problems in their organic vegetable production. The first grower noticed disease symptoms in cucumber, tomato, and pepper transplants soon after seeding. Seedlings were stunted with marginal leaf necrosis and some eventually died. Two weeks later, vegetable transplants at three other grower facilities exhibited damping-off. All growers had used the same lot of custom-blended soilless mix in their transplant product. The mix was composed of 50% peat and 50% perlite, and did not contain synthetic wetting agents or fertilizers; moreover, there were no pathogens detected in these soilless mix. The mix had also been amended with worm castings produced and purchased from same source. 21 April 2012 Plant Health Progress A sample of the soil was initially sent to the Soil Testing Laboratory of the North Carolina Department of Agriculture (NCDA), Raleigh, NC (www.ncagr.com/agronomi/sthome.htm), for chemical analysis. The report indicated satisfactory nutrient levels. Therefore, soilborne pathogens were investigated to determine if the earthworm castings were a source for pathogens causing damping-off of the vegetables at these organic farms. Isolation from Diseased Plant and Worm Casting Traditional techniques such as media isolation and Rhododendron baiting were employed. Molecular approaches involving DNA sequence analysis and Blast search (1) were used for species identification of the pathogens. Root lesions from five diseased plants were excised and surface-sterilized in a 0.5% sodium hypochlorite solution for 5 min, placed on potato dextrose agar (PDA), and incubated for 3 days at room temperature in the dark. Isolates were initially obtained from all diseased plant roots. The representative isolates from cucumber, tomato, and pepper were named Phytophthora735, 738, 739, and Pythium735, 736, 739, respectively. An indirect isolation method was used to recover oomycetes from the worm castings. Rhododendron leaves were surface-sterilized in 0.5% sodium hypochlorite for 3 min, air dried, and used as bait by cutting the leaves at the mid-vein in a herringbone pattern. Three to four cut leaves were placed into worm castings for 48 h, then set on moistened paper inside a container (length 30 cm, width 26 cm, height 10 cm) and incubated at room temperature in the dark for 3 days, to allow for potential growth of oomycetes. Each rhododendron leaf was examined daily and pieces of leaf showing discoloration were placed on PDA. Putative isolates of Phytophthora and Pythium, based on morphological characteristics, were consistently recovered from diseased roots and from the worm castings (Table 1). Table 1. Species identification based on the Blast search of the isolate sequences. All the isolates for DNA sequence analysis listed in the table are from one farm. x The GenBank accession numbers of Phytophthora and Pythium sequences deposited from this research. y The GenBank accession numbers of Phytophthora and Pythium sequences in GenBank match to the sequences from this research. Type Isolates Plant Accession no.x Species Similarity