July 14, 2008 | General

Biological Control Of Diseases Of Containerized Plants

BioCycle July 2008, Vol. 49, No. 7, p. 32
Several root diseases can be controlled effectively – or at least suppressed – by use of blended materials in potting mixes, inoculation of the mix and optimization of environmental factors.
Harry Hoitink and Dennis Lewandowski

MUCH PROGRESS has been made in the field of biological control of diseases of containerized plants. Several root diseases can now be controlled effectively, or at least suppressed adequately. It requires the use of: Appropriate blends of materials in potting mixes, which can serve as a food base for beneficial microorganisms; Inoculation of the mix with one or more specific biocontrol agents during its formulation; Optimization of environmental factors (pH, temperature, moisture content) in the mix to enhance colonization of the food base by the beneficial microorganisms; and Use of fungicides for some diseases.
Diseases of the foliage of plants also may be affected through biological control. Some biocontrol agents used in potting mixes for control of root rots, for example, can induce systemic resistance in plants. These microorganisms have been shown to decrease the severity of powdery mildews, Botrytis, Phytopthora and bacterial blights, and leaf spots. Virus and nematode diseases can be affected as well by this mechanism. Genes in tomatoes now have been identified that are involved in systemic induced resistance to disease. Several of the genes are related to stress metabolism, which may explain why biocontrol agent-activated plants are more able to defend themselves against several different types of plant diseases. As more is learned about this mechanism, better control methods undoubtedly will be identified for growers. Finally, some biocontrol agents can be applied as sprays to the foliage of plants and provide control, but this is not the main topic of this paper.
Broad spectrum biological control of root diseases, which in the field of ornamentals means control of major root diseases of several different crops, involves not just activity of the introduced biocontrol agent but also that of other beneficial microorganisms that colonize the food base naturally. Raw organic ingredients (peat, composts, etc.) used in potting mixes typically do not harbor adequate natural populations of biocontrol agents for effective disease control. Thus, to enhance opportunities for colonization of the potting mix by beneficial microorganisms, these specific strains should be added when the mix is prepared or immediately thereafter when plants are potted. It is important to emphasize, however, that biological control is but one piece of an Integrated Pest Management (IPM) program. In some cases, biocontrol strategies must be coupled with conventional management tools, including control of the environment in the greenhouse, sanitation, etc., and fungicide applications – all of which is described below in more detail.
Organic nutrients released from the roots of living plants do not provide an adequate food base for sustained activity of biocontrol agents. Therefore, energy required for activity of this beneficial microflora in potting mixes must come from the potting mix itself. The type of organic matter used in potting mixes largely determines the degree and longevity of biological control of root diseases, and this often is not considered by growers.
Dark, decomposed Sphagnum peat, bark ash and even pyrolized bark or wood (glistening, black material that has been composted for months in tall high temperature heaps where fires occasionally occur) do not adequately support biocontrol agents even for short term crops. As a result, plant pathogens such as Pythium, Phytophthora and Thielaviopsis cause the most serious losses in mixes prepared with these stabile organic and inorganic materials such as perlite, vermiculite, expanded shale, calcined clays, etc. In contrast, peat mixes prepared with light, fibrous, horticultural grade Sphagnum peat classified as H2 to H3 on the von Post peat decomposition scale (at least 50 percent on a volume basis) can provide control for up to two to five months, depending upon the decomposition level of the peat and greenhouse temperatures. OFA (Ohio Florists’ Association) Bulletin # 778, August 1994, pages 1 to 3, provides a more complete description of how the decomposition level of Sphagnum peat can be classified. It also describes the impact of Sphagnum peat on natural Pythium root rot suppression.
High soil mix temperatures result in higher rates of organic matter decomposition. Therefore, disease suppressive effects do not last long in high temperature climates. Phytophthora and Pythium root rots typically are the first diseases to reveal themselves in such decomposed potting mixes, particularly if the mix has inadequate drainage. Thielaviopsis black root rot also is most prevalent in highly stable substrates, but it occurs most commonly under stress conditions in highly porous substrates on susceptible plants.
Some batches of dark peat (H4 Sphagnum peat) may support biocontrol for a few weeks, but fungicides must often be applied at potting to avoid losses. Growers have begun to use Milorganite-derived and other concentrated composted manure by-products such as vermicomposts at low rates of 1 to 3 lbs/cubic yard (cy) of mix to enhance natural biological activity in potting mixes prepared with peat, aged (pyrolyzed) pine bark, etc. Particularly for short term crops such as bedding plants, this is an effective way to improve the potential for natural biological control of Pythium root rots.
Long-term biocontrol activity can be obtained in bark mixes and in mixes prepared with composts. Composted pine bark mixes (30 percent or more pine bark by volume) may support control for a year or more, but it depends on how it was aged or composted, and mix temperature. Mixtures of pine bark (65 percent or more) and composted sewage sludge (5 to 12 percent by volume), or other composts such as composted manures (5 to 8 percent by volume), composted leaves or green (yard) wastes, or composted hard wood bark (typically blended in at volumetric ratios of 12 to 25 percent) may provide control for 18 months or longer. These mixes have proven particularly effective against Pythium and Phytophthora root rots and Thielaviopsis black root rot in perennial and nursery crops, especially when inoculated with specific biocontrol agents capable of inducing systemic resistance in plants. Thielaviopsis black root rot is difficult and expensive to control with fungicides in long-term production cycles. In conclusion, growers must match potting mix ingredients and introduce biocontrol agents to crop production cycles if they want to maximize the potential for biological control.
Biocontrol agents suppress plant diseases by several different mechanisms. Most of those available as biopesticides (e.g. Root Shield and Mycostop; see review by Daughtrey and Benson, Annual Review of Phytopathology, 2005, vol. 43:141-169) are antagonists that inhibit and/or kill plant pathogens and thus reduce the severity of root diseases. As mentioned above, they must colonize the mix before disease control can be provided. It may take from days up to a week or more after potting for some microorganisms to fully colonize a mix and express disease control. It may take even longer for those that colonize roots and induce systemic resistance.
Some biocontrol agents produce antibiotics immediately after their application. In spite of this, most biocontrol agents are not very effective against diseases that can infect plants immediately after planting. Thus, crops highly susceptible to damping-off should be treated at planting with an appropriate fungicide if minor losses are intolerable and if a significant opportunity for infection with such pathogens exists in the production system. Metalaxyl (Subdue) is an ideal fungicide to be used in combination with biocontrol agents on crops highly susceptible to Pythium and Phytophthora (azalea, begonia, poinsettia, etc.) because it does not inhibit the beneficials. To crops highly susceptible to Rhizoctonia (New Guinea impatiens, radish, snapdragon, etc.), a spray or sprench rather than a drench with a fungicide active against this pathogen should be applied, because several fungicides that control this plant pathogen also suppress biocontrol agents such as Trichoderma and Gliocladium spp. After these initial applications have been made on these disease indicator greenhouse crops, fungicide drenches typically are not necessary for root disease control in biocontrol agent-fortified potting mixes until the crop needs to be repotted.
Best management practices (BMP) include several key factors; omission of any of these can lead to failure. Environmental conditions in the mix (pH, temperature, moisture content) must be optimum for effective colonization by biocontrol agents. These factors do not differ from optimum plant growth conditions and therefore do not bring additional costs. The temperature of the mix must be below 100°F, or most beneficial microorganisms will be killed by heat exposure. Composts often reach temperatures of 140°F or even higher in large curing piles before their utilization. Typically, however, the moisture content of these hot materials is low. Therefore a lot of water must be added during the preparation of mixes to increase their moisture content to the optimum level (above 45 percent) and avoid problems later during the crop, as described below. Because water has a high heat capacity relative to organic matter used in mixes, the temperature of the final mix after its formulation typically is lower than 95°F. This means that biocontrol agents will not be killed, and instead grow into the substrate, even under high temperature summer conditions.
The pH of the mix must be above 5.3 to allow beneficial bacteria to colonize the food base. Fungi such as Trichoderma generally are not inhibited by low pH. Thus, pH requirements, like those described above for temperature, are in line with production needs for almost all crops. The moisture content of mixes must be raised to at least 45 percent on a total weight basis, although 50 percent is better. This will stimulate bacterial growth and therefore suppress nuisance and or detrimental fungi (toad stools, fungi that inhibit root growth, etc.) but not that of introduced inoculum of Trichoderma or of other fungal biocontrol agents. Trichoderma hamatum 382 (T382), which still is undergoing registration, specifically was selected for use because it kills many of the commonly occurring nuisance fungi in potting mixes, while also providing all of the biocontrol mechanisms described above.
One final factor that must be considered is the sequence of adding various materials during mix blending. The best procedure is to add the introduced biocontrol agent and water separately, as the last step in the blending process. This avoids contact of the biocontrol agent with potentially toxic chemicals added during the formulation process.
Growers occasionally store prepared mixes long enough in large heaps so they reheat to temperatures higher than 100°F in the center of the pile. Although this will result in death of the introduced biocontrol agents in high temperature parts of the pile, the microflora will recolonize the mix within days after potting due to blending of hot, biocontrol agent-free mix with lower temperature mix where it flourished.
Potting mixes prepared as described above become colonized naturally within days after potting by the introduced biocontrol agent and plant growth promoting bacteria, which stimulate initial root development into potting mixes. Roots on plants produced in these mixes often have more root hairs than on the same plant type produced in a low microbioal biomass mix, such as a dark peat mix or in hydroponic systems where the threat of disease is much greater. Top growth often is slower initially in inoculated mixes because plants produce more root biomass early in such mixes. After 3 to 4 weeks of growth, top growth in inoculated mixes begins to surpass that in control mixes to eventually produce a stronger plant.
Control of Fusarium wilt diseases has been most effective under commercial greenhouse conditions in potting mixes inoculated with Trichoderma. For example, losses in cyclamen due to Fusarium wilt as a result of spread of airborne inoculum were reduced more effectively by T382 than by systemic fungicides. This is not surprising because fungicides generally have not been very effective against this systemic wilt disease. Several Trichoderma strains have been registered that can cause such effects.
Control of Botrytis blight on begonia and geranium through inoculation of a light Sphagnum peat potting mix during mix preparation with T382 has been compared at several different universities with weekly sprays of Daconil in several greenhouse tests. Plants in these tests were drenched immediately after potting of rooted cuttings with Subdue to control losses and poor plant establishment on this crop due to Pythium damping-off and root rot. Without this fungicide application, the quality of the crop in some experiments was reduced because Pythium inoculum frequently is introduced with the “pathogen-free” rooted cuttings of begonias and other crops. Furthermore, Trichoderma, as mentioned above, has not been very effective against Pythium infections early after potting.
The degree of Botrytis control provided by T382 in these begonia tests was not different from that provided by Daconil. Amendment of the peat mix with composted cow manure (5 percent on a volume basis) in addition to inoculation with T382 decreased the severity of the disease, while it also improved the salability of the crop. Similar results have been obtained for powdery mildew of begonia in comparison with control provided by Pipron (see Horst et al., 2005, Plant Disease 89:1195-1200 for more details). Several reports have shown that addition of small quantities of highly mature composts to light peat potting mixes typically increase the effects of introduced inoculum of biocontrol agents.
Integrated control of Phytophthora dieback of Rhododendron with fungicides and Trichoderma biocontrol agents was tested over several years in nursery container media prepared with pine bark, composted yard wastes, silica sand and/or expanded shale. The number of branches per plant and growth of individual branches was increased by inoculation of the mix with T382. In another test on Phytopthora dieback of Rhododendron, the container medium was inoculated with T382 during mix formulation just before planting of rooted cuttings. A severe Phytophthora blight epidemic developed naturally during the following growing season. The disease was suppressed by T382 throughout the season and control did not differ from that provided by provided by monthly drenches with Subdue. Subdue and T382 reduced the number of diseased plants by 50 percent. However, a drench with Aliette was more effective. The isolates of several Phytophthora spp (P. citrophthora, P. parasitica) that caused this natural epidemic had developed partial resistance to Metalaxyl (the active ingredient in Subdue) after almost 30 years of use of this fungicide in nurseries.
Biocontrol agents must be able to establish themselves on the substrate before other natural isolates that are less effective in disease control colonize the media. As mentioned above, this means that the inoculum must be added at potting or during planting of the crop in a new mix. Application of inoculum months after potting when other microorganisms already had colonized the mix, has consistently failed to provide disease control.
Several experiments have been performed over the years in which introduced biocontrol agents were traced in the environment using tools of molecular biology. In general, the introduced biocontrol agent does not spread in potting mixes beyond one to two rows of pots past the point of introduction. The reason is that other similar microorganisms occupy the same niche in the environment and many of these do not have the same biocontrol abilities. As a result, specific biocontrol agents need to be reapplied each time that a potting mix is made.
Some biocontrol agents are marketed specifically as topical applications for control of foliar diseases. These microorganisms may require repeat applications, but this topic is beyond the scope of this paper.
Biological control can be expected to contribute effectively to the control of several different types of plant diseases if plants are produced in potting mixes capable of supporting growth of the introduced biocontrol agents. The degree of control provided ranges from highly effective against Fusarium wilts and some root rots, to less effective against foliar diseases and to marginal efficacy against crops highly susceptible to damping-off. Many factors must be considered for optimum efficacy, but these factors already are included in horticultural BMPs for potted crops. The length of time that disease control in inoculated mixes lasts depends on the organic materials used in the mix. Materials used in mixes need to match the production cycle of the crop. Light Sphagnum peat should be used for short term crops. Bark mixes (not pyrolized bark) and compost-amended mixes should be used for long-term crops.
Several of the beneficial microorganisms that provide this control also increase the quality of the crop (improved root systems, flowering, keeping quality, etc.). Therefore, an effective biocontrol strategy does not necessarily lead to increased production costs. In fact, decreased pesticide use and better quality can outweigh increased costs. It is also obvious, however, that biological control of plant diseases has the best potential for success if it is applied as an integrated component of a total disease management strategy (pathogen free stock, sanitation, environmental controls, etc., and chemical control if necessary). Growers increasingly seem to recognize these needs and harvest the side benefits because sales of biocontrol agents have increased steadily in recent years.
Harry Hoitink, Professor Emeritus and Dennis Lewandowski, Assistant Professor, Department of Plant Pathology, The Ohio State University. An earlier version of this topic was published in the September/October 2006 issue of the Ohio Florists’ Association Bulletin.

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