Compost Extracts and the Biological Control of Foliar Plant Disease
If compost extracts can be used successfully they will provide, for conventional farmers, a
needed alternative to fungicides and, for organic farmers, a nonchemical means to control
disease. For all farmers, compost would facilitate the movement away from high input, synthetic
chemical practices towards a more sustainable ones, which emphasize alternative, low-cost
inputs, alternative cultural practices, and use of recycled on-farm wastes.
1) To prepare composts of animal-based and plant-based origin.
2) To prepare water extracts of composts of various ages; to spray these onto apple leaves and
leaf litter at a university research station and at a commercial orchard; and to assess the efficacy
for control of apple scab and potato early blight.
3) To make a preliminary determination, under controlled conditions, of the mechanism of
action as direct vs. indirect, and microbial or cell-free microbial fractions.
Aqueous extracts of 32 plant- or manure/straw-based composts were bioassayed for their ability
to inhibit the apple scab pathogen Venturia inaequalis or signs of the apple scab disease.
Composts were incubated in water without agitation for 7-12 days. Incubation for 3 or more days
generally increased efficacy.
Germination of Venturia conidia in compost extract (1:2 w/w compost:water) ranged from 3-101
percent relative to water controls. When compost suspensions were sprayed onto apple seedlings
in a growth chamber prior to inoculation with the pathogen, the resulting relative disease
severity (Scale 1-5 units; 1=healthy, 5=severely diseased) after 10-14 days ranged from 1.0 to
3.5 units compared with water (1.9-3.5 units) and captan fungicide (1.0-1.7 units) controls.
Sporulation of the pathogen on diseased plants was reduced up to 98 percent vs. water (0
percent) or captan (99.9 percent) controls.
The active principle in the composts in at least one instance appears to be a heat stable, filterable
compound. Inhibition in vitro was not highly correlated with suppression of disease in the
seedling assays, though extent of sporulation was highly correlated with disease severity. Field
trials in two orchards for two seasons showed that the four composts tested showed some
activity but, overall, suppression was not statistically significant. The composts and captan
fungicide sprays influenced the numbers of bacteria and fungi on the treated leaves.