MB fumigation is most important in intensive horticultural systems where continuous crop monoculture is practiced and losses caused by soilborne plant pathogens are most severe. As a result, profitability of these crops relies strongly on the availability of MB. Strawberries in California and fresh market tomatoes in Florida are classic examples. Tomatoes, strawberries, lettuce, cucumbers, roses and chrysanthemums are examples for the West European growing region.
However, a major problem associated with MB fumigation and other soil sterilization procedures is that most beneficial microorganisms, such as mycorrhizae, biocontrol agents and plant growth promoting microorganisms, are also destroyed. The “biological vacuum” left after sterilization typically is filled within days after treatment by microorganisms that recolonize the soil. Whether harmful or beneficial microorganisms predominate after soil sterilization is determined by any of several environmental factors prevailing at that time. Obviously, recolonization by plant pathogens in the absence of biocontrol agents can have significant negative impacts on crop yield. During the 1960s, the nursery industry encountered serious problems with Phytophthora cinnamomi because it was often reintroduced after soil fumigation with infected plants or in irrigation water. Heavy Phytophthora root rot losses occurred on a regular basis on rhododendron, azalea and other crops highly susceptible to this disease.
In the 1960s, yet another problem was associated with MB fumigation. Evidence was obtained which implicated MB as a potent contributor to ozone depletion. For this reason, it is scheduled to be phased out by 2005 under the Montreal Protocol. Germany and Switzerland have essentially banned MB over this problem as well as bromine residues in food. Bromine can also accumulate in groundwater and be taken up by plants. MB has already been phased out in the Netherlands, which at one time was one of Europe’s largest users of MB for soil fumigation. Therefore, research on the development of alternatives has become a high priority.
Emphasis On Biological Alternatives To MB
Several alternatives to both MB fumigation and the pathogen recontamination problem are now under development. One approach is to substitute MB with another less problematic but effective fumigant. A second is to inoculate sterilized soil with beneficial microorganisms after treatment. A third represents a return to an old and all but forgotten practice, which is to replace sterilization procedures with soil organic matter management that provides control. Examples are applications of animal manures, green manures, composts or biocontrol-agent-fortified composts, which as described below can provide effective control of diseases, as well as insects and weeds if combined with herbicides and specific cultural practices.
Even if the alternative chemicals to MB succeed, biocontrol strategies still need to be developed to reduce recolonization of treated soil by pathogens. Much has been learned about biological control of soilborne plant pathogens during the past 60 years to facilitate the introduction of this integrated concept. Many types of biocontrol agents and plant growth promoting microorganisms have been identified. In general, this approach to control has not been very effective, and there is a lack of literature that addresses the recolonization of soils after sterilization.
The idea of filling a biological vacuum left after fumigation with beneficial microorganisms has not been practiced widely on a commercial scale. An exception is a preparation developed in Belgium (DCM Bio Fungust) in the late 1980s that consists of several isolates of Trichoderma harzianum, a well known biocontrol agent. This product has been used on a commercial scale for over a decade on many crops. It has improved yields in replicated field trials through both plant growth promotion and improvement of plant health. Today, Belgian fumigated soils are routinely treated with preparations containing T. havzianum or Trichoderma spp. and it has become a widely accepted practice there.
Disease Suppressive Composts As Alternatives
During the past two decades, considerable pro-gress has been made in the reintroduction of cultural practices into agriculture that offer opportunities for biological control. The nursery industry first observed that composted tree bark seemed to suppress Phytophthora root rots. It was discovered that woody plants requiring mycorrhizae for growth also performed much better in soil beds treated with composted bark than in those treated with MB.
It has been shown that Pythium and Phytophthora root rots indeed can be controlled most effectively in composted bark-amended container media. Physical and chemical properties of the mixes must be ideal for this to occur. Stabilized dark sphagnum peat mixes do not become suppressive, because the microbial carrying capacity of this highly humified source of organic matter is too low to support the activities of biocontrol agents.
Rhizoctonia solani, another important pathogen of many crops, usually is not controlled during the first few weeks after potting in compost-amended media suppressive to Phytophthora root rots. Seedlings are particularly susceptible to this patho-gen, making this tempo- ral absence of natural suppression to Rhizoctonia diseases in compost-amended mixes a problem on some crops. Rhizoctonia spp. can also cause losses in field agriculture after composts are first applied. Long term curing of composts avoids these losses. The compost and the amended substrates eventually become suppressive to both types of diseases as effective biocontrol agents naturally colonize the mix or soil.
Rhizoctonia solani, as mentioned above, is most severe on young plants. Therefore, a fungicide drench has to be applied to susceptible potted greenhouse crops to prevent losses. Unfortunately, effective fungicides for control of R. solani destroy some of the biocontrol agents responsible for the naturally suppressive effects to species of Phytophthora and Pythium. Therefore, two fungicides have to control both types of pathogens (R. Solani as well as species of Pythium and Phytophthora) until the mix has become naturally suppressive due to colonization of the compost-amended mix with the right diversity of biocontrol agents. This integrated approach to root disease control (fungicides and natural suppression in compost-amended mixes) has become quite effective as a substitute for MB in the nursery industry.
During the 1970s, substitution of MB for control of diseases in field soil with compost was reported from the Nagano Valley in Japan, where Fusarium crown rot of Chinese yam was suppressed in a sandy soil amended with composted larch bark. This compost effectively replaced MB if a spray of benomyl was also applied to the soil at planting. The suppressive effect of the composted larch bark by itself against Fusarium crown rot was significant and also attributed to the activity of isolates of Trichoderma spp.
In spite of the beneficial effects obtained with composted tree bark, several factors prevent wide spread introduction of this concept into agriculture. First of all, composted bark is too costly for most agricultural applications, even though it can offer broad-spectrum disease control. Secondly, the supply of bark cannot possibly meet the demand in agriculture. This in itself does not pose a problem, however, because many other types of composts, such as composted manures, offer the same potential. Perhaps the most limiting factor, as explained below, is that composts do not consistently provide biological control of diseases caused by soilborne plant pathogens unless many factors are considered. Furthermore, most weeds and insect pests also are not controlled by composts.
One of the main reasons composted manures are not used widely in field agriculture is that the animal producing industries which generate manures have become separated almost completely from vegetable and fruit farms, and from nurseries and landscape sites where the greatest potential for utilization of these amendments exists. Transportation costs are too high for many potential applications. Chemical agriculture, for reasons mentioned above, allowed this spatial separation of farming systems to occur throughout much of the 20th century.
Today, farmers increasingly must distribute their manures off the farm because of inadequate availability of land on the farm for environmentally sound utilization of available nutrients in these products. Additionally, exposure of humans to pathogens in animal wastes, contamination of surface and ground waters and foods with these pathogens and nutrients has become an issue. Farmers are facing increasing pressures to develop better waste management procedures. Utilization of these resources by farmers not producing manures would offer a solution to this waste problem and turn a potential liability into an asset. In some farming systems, composted manures are already recognized as an additional source of revenue. However, full utilization of composts in most agricultural crops where MB is used requires that these treatments cost less than $1,000/acre. This problem is not easily resolved in all situations, mostly due to costs associated with transportation of composts.
Another limiting factor is that composts prepared from manures may vary in concentrations of essential plant nutrients and contain considerable amounts of sodium and chlorides. Salts in composts must be applied in quantities well below those stimulating species of Phytophthora and Pythium causing root rots. It has been shown in field trials that high-salinity products must be applied in the fall or winter well ahead of planting to allow for leaching and thus avoid an increase in Phytophthora root rot of soybean. Proper timing of the application provided control of the disease and increased soybean yields over that provided by the fungicide metalaxyl.
Compost Impact On N Fertility
The impacts of composts on nitrogen fertility must also be taken into consideration to avoid severe epidemics caused by plant pathogens. Phytophthora dieback of rhodododendron, Fusarium wilt of cyclamen and fireblight are examples of diseases that are increased in severity as a result of excessive N fertility introduced into container media with composted biosolids. The opposite effect may occur when composts are produced from products with a high carbon to nitrogen ratio, such as wood residues. Most high C:N ratio composts (>70:1) immobilize nitrogen. Thus, plants grown in such products suffer from chronic nitrogen deficiency resulting in lack of growth and increased susceptibility to stress pathogens or insects. The solution to these problems is to produce composts of consistent quality. The knowledge required for production of such composts is available but compost quality standards have not yet been implemented in many parts of the world.
The foregoing discussion reveals that nutritional factors need to be considered in formulating biocontrol strategies with composts. A major problem is that the fertility of composts cannot be predicted easily unless specific guidelines for the specific compost in question are available. Much of this information is not readily available to farmers. Furthermore, many composts placed on the market today are prepared from variable inputs, making predictability even more difficult, if not impossible, at this time.
Fumigants such as MB typically are applied as a precautionary measure yearly or every other year. MB can be used successfully when pathogens have reached populations that cause major losses. The same strategy unfortunately cannot be adopted if manures or composts are to be used for disease control. Unlike MB, which kills pathogens almost instantly, composts typically suppress or eradicate pathogens slowly and over a long period of time. Therefore, these amendments must be applied well before pathogens reach populations capable of causing losses, and this requires more management. More careful monitoring for particular pest problems, with greater emphasis on and attention paid to pest biology, will be required than is currently practiced in conventional agriculture and horticulture.
Utilizing Biocontrol Agents
We mentioned earlier that biocontrol agents do not consistently colonize all composts to induce biological control. One of the reasons is that most biocontrol agents are destroyed by heat treatment during the composting process. They must recolonize composts during the curing process and this does not always occur. Composts produced near a forest are much more likely to become colonized by effective microorganisms than the same compost produced in an enclosed system. Inoculation of mature composts with biocontrol agents has improved the consistency as well as the spectrum of disease suppression, particularly for R. Solani. Isolates of several Trichoderma spp. can provide effective control of this disease, and even better control if applied in combination with any of several bacterial biocontrol agents. This controlled inoculation strategy promises to improve efficacy and thus stimulate the adoption of this approach to biocontrol into every day agricultural practices.
In the potting mix industry, biocontrol agent-fortified composts seem to offer the greatest opportunities for commercialization. For example, a composted pine bark mix fortified with Flavobacterium balustinum 299 and Trichoderma hamatum 382 has been very effective for control of Fusarium wilt of cyclamen, Rhizoctonia diseases, in addition to Phytophthora and Pythium root rots of potted greenhouse crops. Drenching of potting mixes with a combination of soil fungicides that control the most important root diseases caused by species of Rhizoctonia, Pythium and Phytophthora costs approximately $10 per application per cubic yard of mix. Although most crops do not have to be treated more than once, some, such as poinsettia, Easter lily, azalea and rhododendron produced in conducive peat mixes, may have to be treated several times. Biocontrol agents can be inoculated into compost-amended mixes for less than 50 percent of the cost of a single combination fungicide drench, and this presents an opportunity.
A factor that is often overlooked by specialists working with biocontrol agents is the decomposition level of the organic matter in the potting mixes or soils. Fresh organic matter does not support biocontrol, even when inoculated with the best strains. High concentrations of free nutrients (glucose, amino acids, etc.) in fresh crop residues repress the production of enzymes required for parasitism by biocontrol agents such as Trichoderma spp. Composts must be stabilized well enough and colonized to a degree that microbiostasis prevails. As mentioned earlier, excessively humified organic matter, such as dark sphagnum peat, cannot support the activity of biocontrol agents. Organic matter with properties in between these to extreme degrees of decomposition level supports biological control. The rate of hydrolysis of fluorescein diacetate defines this property of composts and of amended soils. In this field too, however, much remains to be defined.
The loss of MB and our increased awareness of environmental problems caused by inadequate solid waste practices promises to provide a boost to the utilization of organic amendments in agriculture. This, in turn, will reduce the potential for soilborne plant pathogens to cause epidemics that cause major losses. In many applications, however, suitable alternatives to MB have yet to be developed. Especially in intensive horticultural systems where continuous cropping is practiced, MB may well be irreplaceable. No single alternative management technique is likely to replace MB in all its applications. Rather, groups of alternatives, integrated within various combinations depending on the needs of the grower, will be required to replace MB.
This integral pest management approach will also require careful monitoring for pest problems. Factors such as climate, soil type and structure, time of year, and crop will need to be considered. While this may prove to be inconvenient in the short run, over the long term it will lead to more sustainable agricultural practices while improving the environment on, as well as off, the farm. The nursery industry in many parts of the world already has taken advantage of these ideas. The same is occurring in other crops as farmers become more aware of these soil organic matter quality issues.
By Tom J.J. De Ceuster and Harry A.J. Hoitink