1993 Annual Report for ANC93-016
Compost Extracts and the Biological Control of Foliar Plant Disease
Summary
Rationale:
Apples are among the top five of the major food commodities in the USA. This country ranks
second internationally in apple production (about 8 billion pounds valued at $1 billion). The
apple scab disease is distributed worldwide and has significant economic impact, especially in
moist, temperate areas, where it is considered to be the most important pest problem of the crop.
Growers in Wisconsin and climatically similar regions routinely apply 11-15 fungicide sprays
each season. Apples rank third nationally in percentage of acres treated with fungicide (78
percent) and third in total fungicide expenditures ($23.5 million). These sprays thus represent an
appreciable input cost to growers and, additionally, they can have substantial indirect cost to the
environment.
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 sustainable philosophy which emphasizes alternative, low-cost
inputs, alternative cultural practices, and use of recycled on-farm wastes. It is even possible that
home owners in urban areas could use compost extracts prepared from food and yard wastes to
control disease in their gardens. If so, this would reduce a significant pesticide source to the
environment and one often associated with a misuse.
Objectives:
1) To test water extracts of composts for seasonal and overwintering control of apple scab
disease.
2) To determine the mechanism of action as direct vs indirect, and microbiologically-based or
chemically-based.
3) To determine how mass of compost samples for bioassays relates to predictability and
reproducibility of assay results.
4) To determine optimal formulation of compost or compost extract for storage.
Methods:
To control the apple scab disease caused by Venturia inaequalis, aqueous extracts from two
sources of spent mushroom substrate (SMS), anaerobically fermented for 7 days and amended
with spreader-sticker, were applied at weekly intervals to apple trees (cv. McIntosh) from green
tip to petal fall and biweekly thereafter. Trials were conducted for three seasons at two locations
in Wisconsin.
Results:
Extracts significantly reduced the disease but were not as effective as a fungicide sprayed at the
same intervals. Evaluation of the extract potency, based on laboratory assays for inhibition of
germination of the pathogen's spores, suggested that the major active principle was a small,
heat-stable, non-protein metabolite produced by microorganisms in the compost. In studies
conducted on various composts to examine the effect of compost sample size (50 to 5,000 grams)
on the precision of estimates of extract potency, we found that samples of at least 500 g were
needed to avoid large errors related to sample heterogeneity and efficacy. Experiments to test the
effects of aging and storage conditions (indoors vs outdoors) of compost on efficacy of extracts
showed that the effects of these factors vary with compost and may be negligible or significant.
Similarly, storage conditions of extracts (5 months at 24 C, 4 C, or -20 C) influenced efficacy of
extracts from one source tested but not the other.
Farmer Adoption and Direct Impact:
This approach to disease control is still considered experimental and has not yet been
promoted for adopted by farmers. Growers attending our meetings showed enthusiasm for the
method and would likely adopt it if consistent efficacy were shown.