1Department of Plant Pathology, University of California at Davis, CA 95616, USA; 2Department of Biology, University of Toledo, OH 43606, USA; 3Department of Nematology, University of California at Davis, CA 95616, USA

Background and objectives
To promote sustainable agriculture, plant diseases and pests have been compared in conventional and alternative (organic, biodynamic and reduced-input) farming systems. Soilborne diseases and pests are generally less severe in alternative than in conventional systems. The mechanisms of root disease control in alternative systems vary with location and environment. For example, corky root (Pyrenochaeta lycopersici) and Phytophthora root rot (P. parasitica) of tomato were less prevalent in organic than conventional farms in central California. These differences were associated with differences in soil biological properties for corky root and soil physical properties for Phytophthora root rot [3]. Corky root suppression was correlated with higher diversity and abundance of actinomycetes after incorporation of cover crops (CCs) in organically managed soils, but effects of longer rotations at organic farms could also contribute to the observed differences. We recently compared soilborne diseases caused by fungi and nematodes in a long-term field experiment with conventional 2-year rotations and conventional, low-input and organic 4-year rotations. The objectives were to determine if rotation and/or other farming practices affected soilborne diseases in tomato. Additional short-term CC experiments were conducted to document successions in bacteria, fungi and nematodes and infection by soilborne plant pathogens.

Results and conclusions
Crop rotation was a more important factor determining root disease severity than farming system when conventional 2-year rotations were compared with conventional and alternative 4-year rotations. Symptoms of corky root, Pythium and Fusarium root rot were significantly more severe in 2- than in 4-year rotation plots [1]. Verticillium dahliae was isolated more frequently from roots in both conventional treatments than in the alternative treatments. Plant-pathogenic nematodes, especially Pratylenchus and Tylenchus spp., were less abundant in organic or low-input tomato plots following beans and a vetch/oats CC than in conventional plots, regardless of rotation [1, 2].

In short-term CC experiments, differences in root disease suppression between organically and conventionally managed soils were temporarily minimized after addition of fresh organic matter. Despite a surge in bacteria and fungi and hydrolytic activity immediately after CC incorporation, this activity did not coincide with Pythium or Rhizoctonia suppression, which occurred only after hydrolytic activity declined. This suggests that nutrients were not limiting for saprophytic growth of these pathogens immediately after CC incorporation, and that competition for nutrients could be an important factor in pathogen suppression later. Both C and N concentration and N form also played an important role in the ability of Pythium spp. to grow and infect roots. Plant-pathogenic nematodes were not affected by short-term effects of CC debris.

In longer-term experiments, repeated amendments in organic soils were probably responsible for differences that developed between these and conventional soils, i.e. significantly greater populations of P. thomei and Tylenchus in conventional than in organic treatments [1, 2]. Maturity and diversity indices for all nematodes were greater (CA) or similar (NC) in conventional compared to organic systems. The smaller diversity and maturity index values in organic systems reflected the predominance of opportunistic species resulting from repeated organic matter amendments. There were no significant differences in omnivorous and predatory nematodes; their numbers were very low in these disturbed soils. Thus, smaller plant pathogenic nematode populations in organic farms could not be explained directly by higher nematode diversity or populations of predatory nematodes.

1. Clark MS, Ferris H, Klonsky K, et al., 1998. Agricultural Agrosystems and Environment 68, 51-71.
2. Ferris H, Venette RC, Lau SS, 1996. Applied Soil Ecology 3, 161-175.
3. Workneh F, van Bruggen AHC, Drinkwater LE, Shennan C, 1993. Phytopathology 83, 581-589.