Compost Extracts and the Biological Control of Foliar Plant Disease

Objective i) Efficacy of water extracts.

To control the apple scab disease caused by Venturia inaequalis, aqueous extracts from two sources of spent mushroom substrate (SMS), anaerobically fermented for seven 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. Both extracts significantly reduced (α = 0.05) the leaf area affected by scab relative to water and spreader-sticker controls as evaluated on the Horsfall-Barratt scale (Figs. 1-2). Disease incidence was similarly decreased but to a lesser extent (Fig. 3). Extracts were not as effective in inhibiting disease as captan sprayed at the same intervals. No difference was detected between extracts with and without spreader-sticker. Higher populations of bacteria, which persisted for at least one month after the final spray, were detected on leaves treated with the extracts (Fig. 4). No differences were found in total numbers of fungi (Fig. 5). Inhibitory activity of extracts, assessed as in vitro inhibition of Venturia conidia germination, was monitored over time for extracts prepared from SMS stored under different conditions (Fig. 6). For one source of SMS, neither time nor storage conditions (outdoors uncovered, indoors in sheds) affected inhibitory activity of extracts. Decline in efficacy of the other source was apparent by 13 weeks relative to unstored compost, though not between storage regimens.

Research with other crop-pathogen systems gave mixed results on compost efficacy. SMS extracts applied for control of tomato leaf and fruit blights (caused by the fungi Alternaria and Septoria) or early blight of potato (Alternaria solani) were ineffective, whereas extracts inhibited conidial germination of Sphaeropsis sapinea (causal agent of red pine shoot blight) in vitro and gave significant reduction in disease on pine seedlings.

Objective ii) Mechanism of action and inhibitory principle.

Clarified water extracts of slurries of SMS inhibited in vitro germination of conidia of V. inaequalis by up to 98% relative to germination in water controls (Fig. 7). Inhibition of conidial germination increased with incubation time of slurries over five to seven days and persisted at least 14 days, at which time experiments were terminated. Compost slurries became anaerobic within 1 hour when incubated without aeration (Fig. 8). There was no difference in efficacy between filtered (0.1 μm) and untreated extracts (Fig. 9). Autoclaved extracts were less effective than untreated extracts but retained most of their efficacy (Fig. 9). Passage of filtrate through microconcentrators with molecular weight cut-off limits ranging from 100 to 3 kDa did not diminish activity of the extracts (Fig. 10). Aeration of slurries decreased efficacy of the resulting extracts compared to non-aerated controls. When aerated slurries were allowed to incubate without further aeration, extracts regained efficacy, becoming not significantly different from non-aerated controls (Fig. 11). Extracts produced from sterile SMS were virtually ineffective compared with those from nonsterile SMS. When small volumes of slurry from raw SMS were added to slurries of sterile SMS and incubated for an additional period, efficacy of the inoculated SMS was enhanced compared to uninoculated controls (Fig. 12). Thus, a major inhibitory principle of the SMS extract is a low molecular weight, heat-stable, non-protein metabolite produced by anaerobic microorganisms in the compost. The implications of this part of the research are that now that the mode of action is tentatively identified, it should be possible to isolate and characterize the compounds responsible and to increase production in composts or in vitro, and retention and stability on leaf surfaces.

Objective iii) Compost sample size vs predictability of efficacy.

We examined the effect of compost sample size (50 to 5,000 g) on the precision of estimates of inhibition of V. inaequalis conidial germination induced by extracts of various anaerobically-incubated composts. Composts were prepared from vegetable material; some were amended with manure. Variability due to sample size was considered as a function of extract efficacy (high, medium or low) and compost heterogeneity (high or low). Extract efficacy was characterized as ability to inhibit more than 75% of the conidia (high), 75% to 40% (medium), or fewer than 40% (low). Heterogeneity was characterized as visual or tactile presence (high) or absence (low) of parent material in the compost. To address the question of sample size-related variability, we partitioned individual extract incubations (samples) into aliquots (sub-samples), and aliquots into microtiter plate wells (sub-sub-samples). For this nested design, the largest component of variation was consistently found to be that associated with wells; aliquots were a negligible source of variation (Fig. 13). Sample size over the range examined was generally of small importance for extracts of medium and high efficacy, but not low efficacy, independent of compost heterogeneity. For previously untested composts, particularly those of high heterogeneity, statistical analyses of our data suggest that samples of at least 500 g circumvent potentially large errors and consequent difficulties to detect differences among composts or effects of experimental variables.

The importance of this objective relates to the broader issue of whether effective composts can be distinguished reliably from ineffective composts based on screening small samples. While it has long been known that sectors of a compost pile can differ substantially in physical, chemical, and biological properties, there is essentially no information on how sample size influences results. This information is necessary for researchers to efficiently and accurately screen prospective sources.

Objective iv) Formulation and storage.

To test whether there was retention of efficacy over time, the extracts from two sources of compost were assayed periodically during storage for five months at 24°C, 4°C, and -20°C. One source (SMS) lost significant efficacy after two months storage at 24°C. Inhibitory potential of the other source (cow manure/wood chips/soil) did not decline appreciably with time at any temperature, but was less inhibitory overall than SMS.

The broader context of these findings is that if composts are to be used widely in practice for disease control, it will be necessary to: (i) store them prior to application; (ii) transport them conveniently (e.g. in lyophilized form); and (iii) ensure maximum retention and efficacy on foliar surfaces.