PLANT DISEASE CONTROL
Composts from different feedstocks are tested on crops to evaluate their effectiveness in suppressing Fusarium wilt and damping off disease.
PAIN is one of the largest producers of horticultural crops and flowers in Europe. Many of these crops are intensively grown in plastic greenhouses, either in the field or containers. The practice of disinfecting soil by fumigation with methyl bromide (MeBr) or through the application of other chemicals is widespread. MeBr is used to control Fusarium oxysporum, Phytophthora spp. and Verticillium spp. and the root-knot nematode Meloidogyne spp. In 1998, Spain ranked fourth in the world, after U.S.A., Japan and Italy, in the use of methyl bromide (MeBr) with a total of 4,191 metric tons applied as a soil fumigant to 9,000 ha of cultivated soil. It was used principally for strawberry (33 percent), cucumber (29 percent), ornamentals (9 percent) and cucurbitaceae (9 percent). As a member of the European Union (EU), Spain is compelled to reduce these levels by 2005, with a gradual reduction of 60 percent in 2001 and of 75 percent in 2003. (In the U.S., methyl bromide use is also being phased out.) For this reason, an interregional project was initiated to test the feasibility of using suppressive composts as an alternative to chemical control. The natural suppressive effects of composts prepared from tree barks and some municipal solid wastes against soil-borne plant diseases are well established. However, little is known about effects of composts prepared from other byproducts. Spain produces a large amount of agricultural residues from cotton fiber, plant oil, rice, cork, grape, cotton and mushroom production. These industries annually generate approximately one million tons of residuals from the oil industry, 320,000 tons from the wine industry, 150,000 tons of spent mushroom compost, 45,000 tons of rice hulls, 18,000 tons of industrial cork wastes and 29,000 tons from cotton fiber. Thus compost produced from residues of these industries are of special interest. COMPOSTING AND
DISEASE SUPPRESSION EXPERIMENTS The agricultural materials studied were: cork bark from cork production; grape marc which consists of grape skins, seeds and stems from the wine industry; oil residues which consist of olive cake (marc) and olive mill wastewater from the oil industry and cotton gin trash. The cork and grape marc were each composted without amendments. The olive marc and cotton gin trash were combined at two volumes of olive marc to one volume of cotton gin trash. All materials were composted according to procedures shown in Table 1. Mineral nutrients were added to the compost piles initially and periodically depending on the recipe to maintain a balance of nutrients during composting (particularly nitrogen). Physical, chemical and biological characteristics of the composts were determined for each compost.
The disease suppressive properties of the composts were tested for two specific pathogens, Fusarium oxysporum fsp. lycopersici race 1 (FOL-1), which causes Fusarium wilt, and Rhizoctonia solan isolate AC-4, which is associated with damping off disease. The level of disease control was determined through bioassays growing susceptible plants in the composts and other plant growth media inoculated with the test pathogens. The other media used in the experiments, peat moss and vermiculite, served as experimental controls. Rice hulls were added as needed to some of the composts to improve the physical characteristics of the growing media. To determine the role of the composts' natural microflora in the disease suppressive effects, some of the compost and peat treatments were heated (60 deg C, 6 days) to destroy the microflora. For comparison, other treatments used the natural substrates. All bioassays were performed at least three times. For both experiments, pots were placed in a growth chamber, irrigated and fertilized according to the needs of the plant. The pH of the media was 6.5.
For the Fusarium wilt bioassay, the plant growth media were infested with the pathogen (FOL-1) at three inoculum densities - 0, 1x10^sup 4^ and 1x10^sup 5^ cfu cc^sup -1^ (colony forming units per cubic centimeter of substrate). For each treatment, five pots with four tomato plantlets (2-3 true leaf stage), initially grown in sterilized vermiculite, were transplanted into each pot. Disease progress was determined every two days after wilt symptoms first appeared on the tomato plants until 25 days after planting. Disease severity was judged on a symptom severity scale where: 0 = no symptoms; 1 = weakly infected plant (<50 percent of chlorotic or wilted leaves); 2 = highly infected plant (>50 percent of wilted leaves but where plants were still alive); and 3 = dead plant. The final disease severity per pot was determined graphically, by calculating the area under the disease progress curve. Disease was also evaluated as length of xylem tissue that showed browning at the end of the bioassay. Also, at the end of the bioassay, the population of the pathogen was determined by accepted cultivation methods.
For the Rhizoctonia damping-off bioassay, plant growth media were inoculated with Rhizoctonia soil inoculum prepared according to standard techniques. Five pots of each (330 ml volume) were used per treatment with 15 cucumber seeds per pot. Control treatments were not infested with Rhizoctonia soil inoculum. Disease incidence (D.I.) was expressed by the proportion of affected seedlings relative to the total number of healthy plants (i.e. number infected + total number of plants). The limits are 0 (minimum) and 1 (maximum). A lower value corresponds to less disease (i.e. more suppressive compost).
EXPERIMENT RESULTS AND DISCUSSION
Composting of the agricultural residuals proceeded without problems. Typical composting temperatures were achieved in all cases. The time of exposure to high temperatures during composting of cork, grape marc and olive marc/cotton gin trash ensured that materials were hygienized properly.
The physical properties of the composts and other media produced are shown in Table 2. All composts produced were near to the optimum range for physical properties for plants grown in plastic bags or in containers. Proper crop management and minor adjustments in irrigation practices, container size, type or size of plants, would allow the composted cork and grape marc to be used as container media without amendments. The addition of rice hulls to olive marc, before the composting process, improved its porosity, but water availability still was low. Addition of peat or cork to the olive marc composts would improve water content.
The compost which proved most suppressive to Fusarium wilt of tomato was natural composted grape marc (Table 3). No plants were affected by wilt at the inoculum concentration of 10^sup 4^ cfu cc^sup -1^. Very few plants showed browning in xylem vessels or disease symptoms at 10^sup 5^ cfu cc^sup -1^. Heat treating the grape marc compost substantially reduced its ability to suppress disease. The disease severity was significantly greater in the heat treated compost. The compost made from cork was not as effective in suppressing diseases as the grape marc compost and about equal to the heat-treated grape marc media. In general, heat-treated composted cork, natural vermiculite and peat were about equally conducive to Fusarium wilt, even though the pathogen populations were low in these media and similar to those recovered from natural composted cork.
The biological vacuum produced by treating the composted cork with heat allowed development of a higher pathogen population, but this did not occur in the grape marc compost for both pathogen concentrations tested. The concentrations of essential plant nutrients in soils as well as soil pH may affect the severity of Fusarium wilt diseases. Therefore, when the Fusarium wilt bioassays were completed, all plant growth media were analyzed for these factors. Natural and heat treated grape marc composts had the lowest concentrations of available nutrients and also the highest pH. The high pH values and lower availability of microelements observed in the grape marc medium in comparison to the moderately suppressive cork medium may explain why disease control in the composted grape marc was best.
In contrast to the Fusarium wilt results, the compost which proved most suppressive to Rhizoctonia dampingoff of cucumber was the natural cork mix. As shown in Table 4, composted grape marc and peat were conducive to the disease. However, when cork was composted with rice hulls (2:1, v/v), the resulting compost almost lost its suppressive effect (data not shown). Similarly, the compost from olive marc plus cotton gin trash was only moderately suppressive to Rhizoctonia damping-off.
In our experiments, we found that composts produced from several different types of agricultural residues to be suitable materials for container media or in field soils. In addition, these composts may potentially alleviate disease problems and provide high quality crops. Promising results were obtained in terms of the disease suppressive effects of the composts.
However, the disease control spectrum varied depending on the medium used. To some degree, results could be explained on the basis of their chemical and microbiological properties. Further studies should be done to: Develop predictably suppressive biocontrol agent-fortified composts; Determine the best raw materials to be mixed for preparation of disease suppressive compost; Elucidate key chemical factors that affect the activities of pathogens and antagonistic microorganisms and; Identify methods most suitable for predicting suppressiveness.
The use of composts in plastic bags or containers can replace soil systems in many areas of Spain. This practice would eliminate the routine need for soil fumigation with methyl bromide. In Mediterranean areas, solarization and biofumigation may also reduce the need for application of pesticides and soil fumigants in field agriculture. Suppressive composts promise to become an alternative to chemical disease control in this part of the world.
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Suppressive composts promise to become an alternative to chemical disease control in this part of the world.
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PREDICTING THE ABILITY OF COMPOST TO BE DISEASE SUPPRESSIVE
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SESSIONS at the 2002 International Symposium on Composting And Compost Utilization presented re
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search results on the role of compost in suppressing plant diseases. Following are brief summaries of findings that were discussed:
Suppressive Effects of Composted Yard Wastes - "Our results clearly demonstrate that use of high quality composts such as yard waste compost, which are sufficiently cured and of a low nutrient status, can result in a suppression of pathogens or at least in a reduced risk of infection," conclude Christian Bruns and Christian Schuler of the Department of Ecological Agriculture at the University of Kassel, Witzenhausen, Germany. "This is an important compost quality criterion which needs to be investigated further."
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According to Bruns and Schuler, the production process has to be defined and a quality assessment program must be developed to result in composts with consistent properties. "Although knowledge on microbial mechanisms governing Pythium suppressive composts are in principle understood, procedures which assess the curing state of specific suppressive composts and predict the interactions of plants, pathogens and beneficial microorganisms still need to be developed," they point out.
Using Composted Biowastes in Potting Mixes to Suppress Disease - Studies at Wageningen University and the
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Centre for Terrestrial Ecology in the Netherlands show that when biowaste compost is added to peat-based potting mixes at a 20 percent volume rate, it can strongly increase disease suppressiveness against Pythium ultimum, Phytopthora cinnamomi and Rhizoctonia solani. Level of suppression varied considerably between compost batches. Contributing factors to compost effectiveness were cited as general microbial activity, microbial biomass and organic matter quality.
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Differences among composts, among young and old batches of the same compost, among compost-amended potting mixes and between the latter and nonamended potting mixes were revealed by the research. There was evidence that not one single, dominant population was responsible for disease suppression provided by these biowaste composts. The Dutch scientists also concluded that more work - including stronger "multivariate techniques and sequencing of bands" - is needed to more fully understand the relation between microbial composition and disease suppressiveness. The research team included Wim J. Blok, Trudie C.M. Coenen, Agata S. Pijl and Aad J. Termorshuizen of the Biological Farming Systems Group at Wageningen University and the NIOO-Center for Terrestrial Ecology.
Organic Matter-Mediated Suppression of Root Rots in Field Soils - Alexandra Stone of Oregon State University dis
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cussed the influence of organic matter (OM) quality and quantity on suppression of root rots in her presentation at the Symposium. One of the most notable examples of commercially viable OM-mediated disease suppression which she cited occurred in the organic avocado orchards in Australia. A grower, Guy Ashbutner, reported that his orchard plowed out of the native Eucalyptus forest - was experiencing severe losses to Phytophthora root rot while the native Eucalyptus rainforest was not. In trying to recreate the rainforest soil conditions in his orchard system, he amended his soils with chicken manure and oat straw mulches and maintained a continuous legume cover crop understory. After several years, his soil suppressed root rot. Suppressive soils were characterized by high levels of microbial activity, organic matter and calcium.
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"Harry Hoitink and Michael Boehm of The Ohio State University have argued convincingly that active organic matter drives suppressiveness to root rots caused by Oomycete fungal pathogens in peat or compost-amended potting mixes," sums up Stone. "In this review, I have attempted to demonstrate that these same processes are likely to work in field systems; lightly decomposed organic matter (derived from plant residues or organic wastes) may drive suppressiveness in both natural and managed field soils. An understanding of the dynamics of organic matter
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aggradation and degradation in field soils should help scientists and growers better manage these types of root rots in field agricultural systems. OM-mediated suppression of Pythium, Phytophthora and Aphanomyces root rots is a potentially powerful tool for plant health management in field soils."
Future of Compost Use for Turf Grass Disease Control - As sources of peat continue to diminish, the use of alternative organic components of topdressings and construction mixes will continue to grow. In particular, explain Eric Nelson of Cornell University and Michael Boehm of Ohio State (see June and July, 2002 BioCycle for the full report), industrial and municipal organic residuals are being viewed as amendment sources.
However, they also point out, one of the greatest obstacles to widespread use of compost amendments for turf grass disease control has been the inconsistent performance from site to site, batch to batch, and year to year. Much of the unpredictable nature of compost amendments can be attributed to our overall lack of understanding of the microbiology of these materials. This understanding is critical for determining the suppressive properties of and microbial responses to amendments when incorporated into turf grass soils or when applied as topdressings. Increased research efforts in this area will eventually make compost amendment use more predictable and manipulable.
"An important consideration in the use of organic amendments in turfgrass disease control is the compatibility with other management inputs such as fungicide, insecticide, and herbicide applications. No information is currently available on
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the compatibilities of organic amendments or other microbial components of organic amendments with pesticide applications," they observe. "Although much remains to be understood about the efficient use of organic amendments in turf grass management, it is clear that the benefits of such amendments far outweigh any negative aspects of their use."
Effects of Dried and Composted Manure on Plant Health - The impact of adding raw and composted dairy manure on crop, weed, disease, nematode, soil and microbial variables are being examined in a vegetable rotation by an Ohio State research team. Differences in plant disease pressure in the tomato plots indicated that compost amendments may contribute to enhanced host resistance. The researchers are Brian McSpadden Gardener, Sally A. Miller, Matthew D. Kleinhenz, Doug Doohan, Parwinder Grewal and Deborah Stinner.
Organic amendments affected the experimental system in multiple ways, the team concluded. The C:N ratio of the amendments may explain the response observed in the plant and microbial data. Soil microbes depend on carbon and nitrogen for growth, but they are thought to be less competitive for nitrogen than plants. "The manure amendment had a C:N ratio close to the level associated with immobilized plant-available N but, conversely, would be nearly optimal for microbial growth," write the authors. Indeed, increases in microbial abundance were indicated both in the rhizosphere mid-season, and, to a lesser extent, postharvest. Because the community profiles were qualitatively very similar, differences in composition of the amendments had little effect on which microbial popu
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lations responded to the amendments. This may indicate a high degree of nutritional competency and generalist feeding among such communities. Nonetheless, differences in composition might play a role in biological control of plant disease. In this first year, we observed a trend towards less foliar and fruit disease in the tomato plots amended with compost, but not with manure. It is unclear at this point whether such an effect is chemically or biologically mediated."
Compost-induced Disease Suppression in Vegetable Crops - The effects of compost amendment in both conventional and organic systems were assessed in studies by S.A. Miller, H.A.J. Hoitink and M. D. Kleinhenz of The Ohio State University. The researchers reported the following observations: Bacterial spot, caused by Xanthomonas spp., was significantly reduced in compost amended tomatoes in both greenhouse and field evaluations, when disease pressure was high. Anthracnose fruit rot, caused by Colletotrichum coccodes, was suppressed in organic but not conventional ly-produced tomatoes under environmental conditions favorable to disease development. Organic tomato yield was higher in compost amended than nonamended soil. One or two years of compost amendment did not increase conventional tomato yield. Initial evidence from studying the effects of multiple seasons of compost amendment on disease suppression and plant growth and yield in conventionally-produced tomatoes and peppers indicates that compost amendment may lead to shifts in biomass accumulation and partitioning, especially under low moisture and high temperature stress. - J.G.
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Composts produced from several different types of agricultural residues proved suitable for container media and field soils.
[Author Affiliation]
Isabel Trillas is in the Facultat de Biologia, Universitat of Barcelona, Barcelona, Spain; Manuel Aviles and Jose Ordovas are in Escuela Universitaria Ing. Tec. Agricola, University of Sevilla, Sevilla, Spain; Antonio Bello is in the Centro de Ciencias Medioambientales, CSIC, Madrid, Spain; and Julio C. Tello is in the Escuela Politecnica Superior, University of Almeria, Almeria, Spain. The authors acknowledge Micoteca del Instituto Nacional de Semillas y Plantas de Vivero del MAPA for providing isolates of the pathogens and FITO SA. for providing tomato and cucumber seeds. We also acknowledge Prof H. Hoitink and Dr. D. Sant for critical discussions and R. Rycroft for reviewing the manuscript
