Researchers at the Department of Plant Protection of the University of Bonn, in Germany have obtained promising results in trials using compost extracts to protect crops from plant diseases. Using a simple and inexpensive technique, the researchers have significantly reduced the degree of damage caused to crops by various pathogens.

Compost, made from animal manure and cereal straw amended with topsoil and/or old compost, is the key component of this disease control technique. Thoroughly decomposed 8-12 month old compost is mixed with water at a ratio between 1:5-1:8 and allowed to ferment. After a soaking period referred to as "extraction time" (usually 3 to 7 days) the solution is ready for application. It is first strained through cheese cloth, diluted with water and then applied using an ordinary sprayer.

Experiments were conducted to test the effect of this compost extract on plant diseases in different pathogen-host relationships (Table 1). In indoor trials, whole plants were sprayed with or leaves were dipped in compost extract and later inoculated with pathogens. In outdoor trials, plants were periodically sprayed with watery compost extract. Different manure sources (horse, cattle, hog, goat, earthworm) and extraction times were also compared. In one experiment, compost used as a soil amendment was tested for its role in disease prevention.

In all of the pathogen-host relationships listed in Table 1, compost extract treatments had a significant beneficial impact on plant health. In one experiment, grapevines sprayed with compost extract in the greenhouse and then inoculated with powdery mildew ator)had a level of disease incidence equal to only 8% of that of the control. This result is comparable to that of a treatment with wettable sulphur, which measured 12%. Outdoors, grapevines treated with compost extract had half as much incidence of leaf blight (tracheiphilarol with no treatments while treatments with standard recommended fungicide applications had about a third as much.

In another experiment, primary leaves of barley were treated, then inoculated with conidia (spores) from powdery mildew minis f. sp. hordei) included compost extracts made from different manure sources, and extracted for different lengths of time. The compost extracts reduced infection levels by an average of 55%, regardless of manure source or extraction time.

The effect of compost as a soil amendment was also tested by growing barley in substrates composed of soil and compost in 7 different ratios(varying from equal parts, to 1:5 soil:compost and 5:1 soil:compost.) In some treatments, compost extract was also applied to the leaves. Primary leaves were inoculated as above and used to determine the degree of infection. The results showed that increasing the percentage of compost in the soil resulted in decreasing levels of disease. Combining compost in the soil with the extract treatment resulted in further decreases in disease intensity. For example, plants grown in a substrate of equal parts soil and compost had 54% of the level of disease of those grown in soil with no compost. Plants grown in the same substrate mixture with a compost extract application had only 7% of that level.

The mechanisms by which compost extract works are not well known but seem to vary depending on the host/pathogen relationship and the mode of application. Sterilizing the compost extract made the resulting broth rather ineffective, which indicates that the microorganisms of compost are essential to the control process. If the leaves are rinsed two days after treatment but before inoculation, the effect remains, indicating that induced resistance is at least partly responsible. Studies of the effects of compost extract on sugar beets and cucumbers sought to identify the stage of fungus development at which inhibition occurs. In these studies conidia germination in the treatments was always equal to that of the control, but the formation of secondary hyphae was reduced by more than 50%. The effect of compost as part of the soil substrate on disease development suggests the possibility that the mechanisms of control are transported in the plant. When compost extract is also sprayed on plants grown in this substrate, the effect is cumulative, giving evidence that different mechanisms are operating. More research is being carried out in this area.

The consistent and significant results of these experiments give hope that compost extracts could be a safe, economical and effective means of protecting plants from diseases.

WELTZIEN, H.C., 1989. Some Effects of Composted Organic Materials on Plant Health. Agriculture, Ecosystems and Environment 27, 439-446.

For more information contact:

Dr. Heinrich Weltzien, Director
Institut feur Pflanzenkrankheiten
University of Bonn
Nussallee 9, D-5300 Bonn 1, West Germany