March 18, 2004 | General

Vermicomposts Suppress Plant Pest And Disease Attacks

Clive A. Edwards and Norman Q. Arancon
BioCycle March 2004, Vol. 45, No. 3, p. 51
The application of commercial vermicomposts that are produced through interactions between earthworms and micro-organisms in the mesophilic degradation of organic wastes – using a range of technologies – is expanding rapidly. It has been well-established in both greenhouse and field experiments in our Soil Ecology Laboratory at the Ohio State University that even small substitutions of vermicomposts into plant growth media and soil can produce dramatic increases in germination, growth, flowering and yields of crops, independent of their nutrient supply. Since nutrients are involved only minimally, we have hypothesized that these increases are due to earthworms causing greatly increased microbial populations that produce plant growth hormones which become adsorbed on to the humates produced during the vermicomposting process. Since most plant growth hormones are very soluble, they may account for the reported plant growth effects of aqueous extracts from vermicompost or commercially-produced vermicompost teas.
During the last three years, our laboratory – with research support from USDA grants – has been studying the effects of vermicomposts on the incidence of plant diseases and pests such as plant parasitic nematodes, insects and mites in both greenhouse and field experiments. As with plant growth studies, we have based our greenhouse research on a range of substitutions of vermicomposts into a soilless plant growth medium (Metro-Mix 360), at dilutions ranging from 10 percent to 40 percent, and soil amendments of one to four tons per acre of vermicomposts in the field. In all of these experiments, the plants were supplied with all nutrients they needed and we usually included a traditional thermophilic compost, produced from the same organic waste, as a comparison standard treatment.
It is well-known that attacks by some soil-borne plant diseases, such as Pythium, Fusarium and Phytophthora, can be suppressed significantly by thermophilically-produced composts and other organic soil amendments. Various mechanisms have been suggested to explain this suppression of pathogens, most of which are based on some aspect of microbial antagonism. There have also been a few reports in the scientific literature of suppression of soil-borne pathogens such as Plasmodiophora brassicae, Phytophthora nicotianae, Fusarium lycopersici, F. oxysporium and Rhizoctonia solani by vermicomposts. In our laboratory, we have researched the effects of relatively small applications of commercially-produced vermicomposts, on attacks by Pythium on cucumbers, Rhizoctonia on radishes in the greenhouse, and by Verticillium on strawberries and Phomopsis and Sphaerotheca fulginae on grapes in the field. In all of these experiments, the vermicompost applications suppressed the incidence of the diseases significantly. These responses were clearly based on a form of microbial antagonism, since the pathogen suppression was almost eliminated if the vermicomposts were sterilized prior to use. Two mechanisms of pathogen suppression have been suggested: a “specific suppression” which is mediated by competition from a relatively limited range of species of microorganisms and a “general suppression” based on a much wider diversity of microorganisms. Vermicomposts usually support a much greater variety and size of microbial communities than thermophilic composts, hence they probably have a much greater potential for general pathogen suppression based on microbial competition. We are currently extending this research to investigating the effects of vermicomposts on attacks by Phytophthora capsici and Fusarium oxysporium on tomatoes and peppers.
There are reports in the scientific literature of various kinds of soil amendments of organic matter amendment, including thermophilic composts, often tending to decrease populations of plant parasitic nematodes. There have also been a few reports of vermicomposts suppressing plant parasitic nematode populations, particularly the root knot nematode Meloidogyne hapla. The Soil Ecology Laboratory has been investigating the effects of low application rates of vermicomposts on populations of plant parasitic nematodes in the field. Vermicomposts – prepared commercially from paper waste, food waste and cattle manure – were applied to soils in field experiments, planted with tomatoes, peppers, strawberries or grapes, at rates ranging from 2 to 8 tons per acre. In nearly all of these experiments, independent of crop, type of vermicompost, or application rates, there was consistent and significant suppression of populations of plant parasitic nematodes (Figure 1). In the same experiments, populations of fungivorous nematodes increased significantly, but there were only small changes in numbers of bacterivorous nematodes.
These responses to vermicomposts may be because earthworms derive much of their nutrition from fungi and promote fungal activity in soils through their casts. There are several possible explanations for these significant changes in nematode community structure. It may be that populations of nematode-trapping fungi or fungi that attack and destroy nematode cysts, could have built up in the vermicomposts due to earthworm activity. Further experiments are under way in our laboratory to elucidate the nature of these interactions between nematode trophic groups. We are also studying the effects of vermicomposts on Meloidogyne hapla attacking tomatoes and peppers in greenhouse experiments.
There have been only a few reports in the scientific literature of vermicomposts influencing populations of arthropod pests, and the damage that they cause. In the Soil Ecology Laboratory, we have been investigating the influence of the substitution of small proportions of a range of different vermicomposts (20 percent or 40 percent), into a soilless plant growth medium (Metro-Mix 360), on damage by aphids (Myzus persicae), mealy bugs (Pseudococcus sp.), two-spotted spider mites (Tetranychus urticae), to tomatoes and peppers and cabbage white caterpillars (Pieris brassicae) to cabbages in greenhouse experiments. In these experiments, batches of ten plants were grown in pots in fine mesh cages and either 100 aphids, 50 mealy bugs, 100 two-spotted spider mites or four cabbage white caterpillars released into each cage. Some of the cages contained plants with 20 percent or 40 percent vermicompost substitutions into MM360 and others had only MM360 and received no vermicomposts. All cages were infested with pests, and treatments were replicated four times. Populations of pests on plants in each treatment were counted after 20 days and the amounts of plant tissues lost to the pests by the plants were assessed by weighing the above-ground parts of the plants in each treatment. In all of the experiments, there were statistically significant decreases in populations of the arthropod pests and consequent decreases in plant damage, in response to substitutions of 20 percent or 40 percent of vermicomposts, into the soilless growing medium (Metro-Mix 360) compared to Metro-Mix 360 only.
At this stage, it is only possible to speculate on the mechanisms of arthropod suppression, but it seems most probable that it may be due to changes in the nutrient status and/or the chemical composition of the plant tissues, which could affect their palatability to the arthropod pests. Research on the suppression of other pests by vermicomposts including tomato hornworm (Maduca guinguemaculata) on tomatoes, cucumber beetles (Acalymma vittatum), and squash bugs (Anasatristis) on cucurbits and caterpillars (Spodotera spp) on peppers and also into identifying the mechanisms of pest suppression is under way in the laboratory.
Clive Edwards and Norman Arancon are in the Soil Ecology Laboratory of The Ohio State University in Columbus, Ohio.

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