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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.
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.
Economic losses due to foliar disease induced biotically or abiotically are difficult to estimate, but are on the order of $1 billion annually in the USA for frost injury alone. Moreover, any direct costs must be viewed within a broader context that includes environmental costs and constraints, increasing cost of pesticide research, and the increasingly frequent revocation of registered (or failure to reregister) pesticides by regulatory agencies.
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. In moist, temperate areas 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%) 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 costs in impact on the environment. Fungicides also pose the threat of adverse effects on health. For example, about 90% of all fungicides used in agriculture are animal oncogens. Although these chemicals constitute only ca. 10% of all pesticides applied to food crops, they account for roughly 75% of the oncogenic risk associated with consumption of processed foods and nearly 60% overall (versus 27% from herbicides and 13% from insecticides).
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 that 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.
Finally, the urgency of finding alternative control measures is evident from the recent controversy regarding withdrawal of the ethylenebisdithiocarbamate (EBDC) fungicides. Because of concern about the carcinogenicity of ethylenethiourea, a breakdown product of these chemicals, the leading manufacturers decided in September 1989 to modify the registrations to eliminate use of EBDC fungicides on all but 13 of the 55 crops for which they are registered. Subsequently, the EPA has proposed to cancel use of EBDCs on 45 of the crops. One of the principal uses of these chemicals has been on apple, and the EBDCs (mancozeb, maneb, manzate, zineb, metiram) are now no longer available to orchardists. Although other fungicides are available for the time being, it may be only a matter of time before they are withdrawn, because most are oncogenic (see above). Those that are not, such as copper and sulfur, are either relatively poor protectants, or can be phytotoxic, or both. This situation is typical of the fungicide dilemma.
Educational & Outreach Activities
The following presentations were made:
27-30 July Attended and presented a paper entitled “Compost Extracts and the Biological Control of Apple Scab” at the Canadian Phytopathological Society Meetings, Charlottetown, PEI. Attendance: approximately 80 persons.
10 September Attended and presented a poster on disease control by compost extracts at the UW Sustainable Agr. Field Day, Arlington, WI. Attendance: approximately 50 persons.
28 July-6 August Attended and presented poster entitled “Prediction of Compost Fungistatic Activity by Small Samples” at the 6th International Congress of Plant Pathology, Montreal, Quebec. Attendance: approximately 200 persons viewed poster.
5 August R. Voland (postdoctoral researcher on project) attended and presented poster on predicting compost fungistatic activity at UW/DATCP Sustainable Agr. Field Day, Arlington, WI. Attendance: approximately 70 persons viewed poster.
22 October Participated in and summarized our work at the annual field day of The Michael Fields Agr. Institute, East Troy, WI. Attendance: approximately 15 persons.
Areas needing additional study
1. A primary biocontrol mechanism operating in compost extracts is antibiosis.
2. The scientific basis for the enhanced biocontrol efficacy achieved by incubating composts in water is production of a metabolite(s) by anaerobic fermentation. When the extract is applied to plants, this metabolite(s) is used as a substrate by facultative aerobes that produce the antibiotic.
3. Compost samples of > 500g are needed to reliably predict efficacy of composts in disease suppression.
4. Compost extracts will work in practice to control foliar diseases if: (1) the antibiotic(s) is stable or can be stabilized and (2) the extracts adhere to leaf surfaces sufficiently to withstand weathering.
5.The best means to store composts to retain efficacy is to freeze-dry the extracts.