Composts As Inducers of Plant Disease Resistance

Sally Miller

Composts have long been known to improve soil fertility and suppress plant diseases. Microorganisms in composts of appropriate maturity inhibit plant pathogen populations through several mechanisms, including competition, antibiosis and hyperparasitism. More recently, it has been shown that components of composts induce systemic resistance in plants to diseases caused by root and foliar pathogens. We have focused on the use of composts to reduce foliar diseases of processing tomatoes in both conventional and organic production systems. Foliar and fruit diseases, especially early blight, septoria leaf blight, anthracnose, bacterial spot and speck, and bacterial canker can severely reduce yield and quality of fresh market and processing tomatoes. Extensive use of fungicides is often required to manage the diseases caused by fungi. Although the disease warning system TOM-CAST is available and has been shown to reduce the use of fungicides by 50% or more, it does not benefit organic farmers due to its reliance on synthetic fungicides. Further, it is not useful for control of bacterial diseases. Additional disease management strategies must be developed to further reduce fungicide use in conventional systems and to reduce diseases and subsequent economic losses in both conventional and organic tomato production systems. The objectives of this study were; 1) to develop consistently high quality, disease suppressive composts inducing systemic resistance to disease, utilizing farm and locally produced wastes, 2) develop a compost-amended potting mix consistently inducing resistance in tomato transplants, and 3) assess the ability of selected composts in comparative farming systems to reduce foliar and fruit diseases of tomato.

Leaf spot diseases caused by Xanthomonas campestris pathovars are effective models for studying induced systemic resistance in plants. We developed a radish bacterial leaf spot bioassay as a rapid and efficient method for assessing the ability of composts to induce resistance. Resistance to bacterial spot was induced in radishes by numerous composts, including those based on yard wastes, manures and tomato cannery wastes. However, batch-to-batch variation between composts in ability to induce resistance was observed. Similar results were observed for tomato seedlings grown in compost-amended planting mixes.

Field trials utilizing composted yard wastes and composted cow manure (conventional processing tomato production) and composted cannery wastes (organic tomato production) were conducted at the Ohio Agricultural Research and Development Center (OARDC) and at Hirzel Farms, Luckey, OH, respectively. In the organic production trial, yields were increased 27% and 42% in the two years of the study for plants grown in compost-amended soil. Low rates of compost (10-12 tons/acre) were as effective as high rates (20-24 tons/acre). Anthracnose was significantly less in tomato fruit in compost-amended plots than in non-amended controls when the disease was very serious. Yield and quality improvements for plants grown in compost-amended plots resulted in an economic gain of approximately $300 per acre. In the conventional trial, composted yard wastes hastened maturity of tomatoes by at least three weeks. Bacterial spot was reduced when bacterial disease pressure was high in compost-amended plots compared to non-amended control plots. The incidence of anthracnose on tomato fruit was not affected by compost amendments. Yield was not significantly higher in compost-amended plots compared to the control.

Compost amendments may play a valuable role in reducing disease and increasing yields in organic tomato production systems. However, their role in conventional systems is less obvious. It is possible that organic production soils may better support a microbial community playing a role in induced resistance. Nutrients provided by composts may be more important here also. Nonetheless, bacterial disease was reduced significantly in compost-amended plots in the conventional system, so it is likely that induced resistance is also occurring at some level in this conventional soil. Early ripening of tomatoes in compost-amended soils may provide an economic advantage for processing tomatoes and potentially for fresh market tomato production. Clearly, compost amendments should continue to be studied as a means of increasing sustainability in tomato production.

This work was supported by the North Central SARE program and the Ohio Vegetable and Small Fruit Research and Development Program.

Sally A. Miller and H. A. J. Hoitink
The Ohio State University – OARDC
Department of Plant Pathology
1680 Madison Avenue
Wooster, OH 44691
Tel: (330) 263-3678
miller.769@osu.edu

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