Compost Extracts and the Biological Control of Foliar Plant Disease

Objective 1: Preparation of composts. Some 32 composts were assessed for efficacy (Table 1). In general, they were obtained from commercial sources or composting facilities in Wisconsin. This approach was considered to be more efficient than constructing experimental piles ourselves, and also to be representative of the products that are available to the public. Most of the composts were manure-based and in general these were more effective in disease suppression than the entirely plant-based mixtures. Maturity (state of decomposition; Table 1) varied from days to months and C:N ratios were estimated to range from relatively high (e.g. wood chips; no. 6) to low (e.g. chicken manure, no. 15) (Table 1).

Objective 2: Disease control efficacy of compost extracts. Composts were incubated in distilled water without agitation for 0-21 days, usually at 1:2 w/w compost:water. In general, incubation for >3 days enhanced efficacy (Fig. 1) as determined by suppression of germination of Venturia conidia and disease reduction under controlled conditions (assays described below). The amount of compost sample incubated in water was generally 500g. This assay size was established based on experiments with a range of quantities from 50g to 1kg, which showed that amounts less than 500g were not reliable predictors of efficacy.

The spore germination (in vitro) assays were conducted as follows: Following incubation in water, composts were stirred and filtered through cheesecloth and centrifuged at 16,000 x g for 10-30 sec to clarify the extract. Extracts were then added to chambers of multi-well plastic plates (40 μl per well) to which 10μl of a suspension of Venturia inaequalis conidia at 50,000 spores/mL were added. Germination of the Venturia spores in the extract was expressed relative to that in water and used as a measure of the inhibitory efficacy of the candidate composts. Representative data are shown in Fig. 2.

The second measure of efficacy (in vivo assays) involved evaluating disease development and degree of sporulation of the pathogen on apple seedlings grown in growth chambers and inoculated at approximately the 6-8 leaf stage with Venturia, with or without the candidate compost extracts clarified by centrifugation at 16,000 x g for 10-30 sec and applied as sprays. After 10-12 days incubation at high humidity (85-98%) and approximately constant temperature (15° C), disease severity on a 1-5 scale (1=healthy plants; 5=severe disease) was rated. The amount of spores produced by the pathogen per unit of plant tissue was determined by washing the leaves, examining the washings for conidia, and expressing the counts on a per gram fresh weight basis of infected tissue. Both positive (captan fungicide sprays) and negative (water sprays) were included. For representative data, see Figs. 3 and 4. Correlation between the in vitro and in vivo assays was evident but low overall (Figs. 5 and 6). Thus, the laboratory assay based on spore germination gives a modest indication performance of the composts in the plant assay. As expected, the amount of sporulation is closely related to the amount of disease (Fig. 7).

In the summers of 1992 and 1993, field experiments were conducted that involved 12 apple trees (cv. McIntosh) at a university research station and six McIntosh trees in a commercial orchard. Two composts (nos. 1 and 5; see Table 1) were assessed in 1992 and two others (nos. 27 and 36; Table 1) in 1993. Some control of apple scab was apparent (Figs. 8 and 9) in both years but the composts did not perform as well as the captan fungicide. In vitro performance of composts applied in the field was assessed on each date of application (Fig. 10). In 1992 the season was abnormally dry so that disease pressure was unusually low, whereas the 1993 season was much wetter than usual, so that disease was more severe than normal.

The composts were further field-tested in 1992 (compost no. 5) and 1993 (compost no. 36) in collaboration with Dr. W. Stevenson, (Plant Pathology Department, UW-Madison) for the control of early blight (Alternaria solani) of potato. Compost no. 5 did not perform significantly better than the water controls in 1992 but no. 36 gave significant control in 1993, ranking intermediate between unsprayed plots and fungicide (Bravo) treatments (not significantly different from either).

Leaf washing experiments in 1992 and 1993 from orchard trees showed that the epiphytic fungal and bacterial populations were changed by the compost and fungicide sprays (Fig. 11).

Objective 3: Mechanisms. To determine whether the suppressive activity resides in the microbial fraction, or is entirely or partly accounted for by the cell-free chemical component of the compost, extracts were either heat sterilized by autoclaving, or passed through bacteria-exclusion microfilters, then assayed in vitro and in vivo as described above. Data from two experiments with two of the most effective composts (nos. 27 and 36) suggest that the principle passes filters and is heat-stable, indicating that the presence of living microbes in the extract on leaves is not necessary for antagonism. However, this work is being repeated because disease severity in the water control treatments was not sufficient for a rigorous test of the sterilization hypothesis.