Compost Extracts and the Biological Control of Foliar Plant Disease

• 1991 annual report

[Note to online version: The report for this project includes tables and figures that could not be included here. The regional SARE office will mail a hard copy of the entire report at your request. Just contact North Central SARE at (402) 472-7081 or ncrsare@unl.edu.] 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 three or more days generally increased efficacy. Germination of Venturia conidia in compost extract (1:2 w/w compost:water) ranged from 3%-101% 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% vs. water (0%) or captan (99.9%) 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.

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; 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.