Ornamental Plants Annual Reports and Research Reviews 2000

Special Circular 177-01

Disease Control Induced by Composts in Container Culture and Ground Beds

H. A. J. Hoitink
M. S. Krause
A. G. Stone

Nursery operators and landscapers have recognized for years that composts can improve plant health. Many factors must be controlled, however, to obtain consistent effects. The degree to which the raw material is heated during composting affects the potential for killing pathogens and weed seeds. The degree to which the organic matter has been stabilized plays a role in disease suppression and plant growth. Furthermore, composts do not always become colonized naturally by beneficial microorganisms, and this can lead to failures. Finally, the concentration of salts and the quantity of nitrogen released by composts plays a role. These factors are briefly reviewed here. We also provide some general information on composts widely available to the greenhouse and nursery industries and how best to use such products.

Most beneficial effects induced by composts are due to the activities of microorganisms in the rhizosphere, the area of soil immediately surrounding the roots. Some of these microorganisms produce plant growth hormones and stimulate plant growth directly. Others produce natural chelators called siderophores that keep iron at a high concentration in available form to plants in soil, even at pH 7.6.

Water-soluble fulvic acids formed during composting are the building blocks for humic acids. They also chelate trace elements and keep them in solution even at high pH. This probably explains why growers using composted biosolids can produce "acid-loving" plants such as azaleas at pH 7.4 in container media consisting of aged pine bark (60%); fibrous sphagnum peat, coir (husks), or composted rice hulls (20%); composted biosolids (10-15%); and silica sand in regions where the irrigation water is high in carbonates. This is very difficult to do in peat mixes in areas with high carbonate water because trace elements limit growth as the pH increases and their solubility decreases.

The fulvic acids and siderophores produced by beneficial microorganisms in compost-amended mixes reduce this problem. As the compost ages in containers, this beneficial effect declines, trace elements now must be applied, and root rots develop unless the plant is repotted into a fresh mix.

Beneficial microorganisms that control diseases are known as biocontrol agents. Disease control obtained with this microflora is attributed to four mechanisms. The first is competition for seed, root, or leaf exudates (e.g., sugars), which leak out of seeds during germination or root tips as plants grow through the soil. Pathogens swimming to these sources of nutrients in composts must compete with this beneficial microflora in the infection court. This reduces infections and, therefore, disease.

Second, some biocontrol agents produce antibiotics that are effective against pathogens. Third, yet another group parasitizes and kills pathogens. Microarthropods, such as springtails and mites, actually search out pathogen propagules in soils and devour them.

The fourth mechanism involves the induction of systemic resistance in plants by microorganisms present in composts. A few beneficial microorganisms can induce all four mechanisms. Trichoderma hamatum 382 is an example of a fungal biocontrol agent with all these beneficial effects.

Some of these specific microorganisms colonizing roots in compost mixes activate biochemical pathways in plants leading to resistance to root as well as foliar diseases. This mechanism may explain the often-heard statement that plants on "healthy soil" are more able to resist diseases. It has now been proved that compost can indeed support such effects.

Composts Can Induce Systemic Disease Resistance in Potted Crops

Most of the sphagnum peat sold for use in container media is of a decomposition level that cannot support the growth and activity of beneficial microorganisms. Such peat mixes are conducive (no suppression) to all diseases tested. Figure 1 shows plants produced with one half of their roots in one mix and the other half in a second. The plant in the center had both sides in a sphagnum peat mix (H4 on the von Post decomposition scale harvested from 3-8 feet deep in peat bags). The right side was in a mix infested with Pythium ultimum, a root-rot and damping-off pathogen. Note that the roots of the center plant with both sides in the peat mix were small relative to the others. The rest rotted. The plant on the right had both sides in a composted pine-bark mix with Pythium on the right side. Note the healthy root system. The plant on the left shows the systemic effect. The right side was in peat, also with Pythium, but the left was in the compost mix. When the compost was sterilized, it did not control the disease. Somehow, the microflora in the compost seemed to induce factors in the roots on the left that transferred to roots on the right in the peat mix, which made the plant resistant to root rot.

Figure 2 shows control of cucumber anthracnose on the foliage of a plant produced in a composted pine bark mix. The plant on the left, where the disease was much more severe, was grown in an H4 peat mix. Some bacterial diseases in the foliage of plants can also be controlled in this way with composts in the mix. This may have positive implications for the suppression of other difficult-to-control bacterial diseases.

Figure 1. Systemic effect induced in cucumber by microorganisms in composts against Pythium root rot. See text for details
Figure 2. Suppression of anthracnose of cucumber caused by Colletotrichum orbiculare on a plant produced in a composted pine bark mix fortified with Trichoderma hamatum 382 on right. Note severe disease on the plant on left grown in a sphagnum peat mix that does not induce systemic resistance in plants.
Figure 3. Biological control of Phytophthora collar rot of apple caused by Phytophthora cactorum in a composted hardwood bark-amended potting mix on the right and severe disease in a sphagnum peat mix on the left.
Figure 4. Rhizoctonia crown rot on alyssum in a garden mulched with freshly ground wood. Composted mulches provide natural control of this disease.
Figure 5. Systemic resistance induced by Trichoderma hamatum 382 (T382) in a composted pine bark mix against bacterial spot of radish caused by Xanthomonas campestris pv. armoraciae (Xca). Row on left not inoculated with the pathogen. Second, third, and fourth row were inoculated with Xca. Note severe disease in second row on plants not treated with the biocontrol agent. Note systemic resistance control in the third row drenched with ActigardTM, a chemical that induces resistance and in the fourth row, a similar level of control caused by inoculation of the mix with T382.
Figure 6. Natural control of Pythium root rot of Poinsettia caused by Pythium ultimum. Note control in the bottom row in a composted pine bark mix, partial control in the middle row in a light sphagnum peat mix (less decomposed peat harvested from near the surface of the bog), and severe root rot in top row in a more decomposed dark peat (H4 on the von Post scale) mix.

Plants produced in compost-amended mixes that induce systemic resistance have higher concentrations of enzymes related to host defense mechanisms. Plants grown in the peat mix that does not provide biological control do not have this elevated level of activity. In summary, plants grown in substrates rich in biodegradable organic matter can support microorganisms that induce biochemical changes in plants relative to disease control. Such plants are better prepared to defend themselves against diseases.

It is important to realize that composts usually do not provide total disease control. When all conditions are favorable, composts offer the potential to reduce many diseases to below critical threshold levels. Pythium and Phytophthora root rots are among the most easily controlled diseases. Figure 3 illustrates natural control of Phytophthora collar rot of apple in a composted bark mix (right) and root rot and death of seedlings in a peat mix (left). Some foliar diseases such as Phytophthora diebacks typically are not controlled at all by composts, particularly when high fertility levels are maintained in the crop.

The Importance of Composting vs. Fresh Wood

Self-heating during composting kills pathogens and weed seeds and also decomposes the food base in fresh materials that stimulates pathogens Some landscapers utilize fresh wood chips as mulch. The question is, can this lead to the spread of diseases? The answer is yes, and fresh mulches also increase the activity of pathogens already on the site.

It has been shown that fresh mulch, prepared from maple trees that died from Verticillium wilt, kills eggplants mulched with this material. Verticillium was recovered from the dead eggplants. This study demonstrated that pathogens introduced with freshly ground infected trees may cause problems in the landscape. Damping-off of bedding plants has been observed in Ohio landscapes mulched with fresh woody materials. Avocado trees mulched with fresh green crop debris also suffer more from Phytophthora root rot. How can these problems be avoided?

First of all, pathogens, insect egg masses, and weed seeds are killed when temperatures in compost piles exceeded 130ºF for just a few days. Turning of piles so all parts are exposed to high temperatures ensures that pathogens are destroyed. The pathogens not killed outright are weakened and are more susceptible to parasitism.

It is important to stabilize organic matter in mulches. Organic matter must be stable enough so that plant pathogens cannot utilize it directly as a food base. Otherwise, the mulch actually increases the population of the pathogen. Rhizoctonia is an example of a plant pathogen that can grow on fresh materials. Figure 4 illustrates Rhizoctonia damping-off in a flower bed treated with a woody mulch.

Another reason for partial composting of mulches is that some beneficial microorganisms grow strictly as saprophytes (growing on dead organic material) in fresh mulches. Once organic matter is partially decomposed, beneficial microorganisms begin to compete for nutrients. Some of the beneficial microorganisms then produce several types of antibiotics that lead to pathogen kill or inhibition. This does not occur in fresh wastes. Some Trichoderma isolates serve as examples of a group of beneficial microorganisms that behave in this manner.

What is the best way to compost fresh mulches to achieve these beneficial effects? After just a few weeks of composting, the organic matter in most materials is already stabilized enough for most diseases to be controlled. The best way to accomplish this quickly with fresh ground brush is to enrich it with nitrogen. Add 1 lb. urea per cubic yard, some grass clippings (10 to 20 percent by volume), or composted sewage sludge (10 to 15 percent by volume), or composted poultry manure (10 to 20 lbs. per cubic yard), or another source of nitrogen to decrease the carbon-to-nitrogen ratio to within the optimum range for composting. Be certain to add water to the pile to maintain a moisture content of 50 to 60 percent on a total weight basis. Ammonium volatilizes as gaseous ammonia out of the pile when it is too dry. The best procedure is to compost these materials for six weeks before use as mulches. The pile then should not give off ammonia odors and will begin to smell like soil.

The procedures described previously will kill pathogens and adequately stabilize most materials for use as mulches. However, depending on the material being composted, stabilization may take much longer before it is suitable for soil incorporation as compost into soil or container media.

Colonization of Composts After Peak Heating by Beneficial Microorganisms

Very few microorganisms that control diseases survive in the high temperature part of compost piles. Most survive in the outer low-temperature layer where they constantly re-establish their populations after turning of windrows - if several factors are addressed. First, the moisture content in the organic matter fraction of composts must be above 40 percent (w/w) for beneficial bacteria to colonize the substrate. They grow as biofilms on the surface of organic matter, particularly if the moisture content is maintained above 45 percent. Dusty, dry composts and mulches become predominantly colonized by fungi that cause a variety of problems. These problems can range from difficulties in wetting of the compost-amended soil because fungal masses repel water, to inhibition of plant growth due to deleterious-to-growth microorganisms (minor fungal pathogens), to development of nuisance mushrooms in mulch.

Allowing composts to cure while a moisture content of 45 percent to 55 percent is maintained reduces the potential for these problems. Plant growth-promoting bacteria and bacterial biocontrol agents that compete with the fungi naturally colonize such higher moisture content mulches and composts after peak heating because a thin layer (film) of water surrounds organic matter particles at this moisture content. Bacteria cannot readily colonize dry surfaces, whereas fungi thrive as long as the moisture content ranges from 15 percent to 34 percent.

When all factors for colonization after self heating are optimized, 20 percent of the compost batches tested still are somewhat deficient in natural biological control agents even if the moisture content of the compost is kept above 45 percent on a total weight basis. To avoid this, composts should be inoculated with specific biocontrol agents. Commercial inoculants for compost consistently providing these beneficial effects are now being registered with the U.S. Environmental Protection Agency (EPA). Figure 5 illustrates systemic control of bacterial spot of radish induced by Trichoderma hamatum 382 inoculated after peak heating into a composted pine bark mix. This biocontrol agent can help control Rhizoctonia damping-off and Fusarium wilts.

How Long Do Disease-Suppressive Effects Last?

The readily biodegradable fraction of the organic matter in composts sustains the activity of biocontrol agents. Humic substances do not support this activity; they are too resistant to decomposition to support such microorganisms. It is the concentration of lignin and of lignin-protected cellulose, the dark materials in wood, bark, and straw, that governs the duration of disease suppression. Once these materials have been decomposed in soil, the beneficial microorganisms decline in activity, the pathogen population recovers, and fungicides must be applied for sensitive crops to remain disease free.

Light sphagnum peat harvested near the surface of peat bogs (H2 to H3 on the von Post decomposition scale) provides beneficial effects lasting six to 12 months in greenhouse crops. In outdoor containers in hot weather, the length of time may be reduced 50 percent because of the higher temperature. Pine bark can support this effect for six months to one year.

Figure 6 shows natural control of Pythium root rot in a composted pine bark mix (bottom row), mild root rot in an H3 on the von Post scale peat mix (middle row), and severe root rot in the H4 peat mix, the most stabilized source of organic matter (top row). The best fungicides would not improve control over that provided by the naturally suppressive composted pine bark mix.

Pine bark aged in large piles where pyrolysis or fires have occured behaves more like charcoal and offers little disease control potential, even though it still has ideal physical properties for use in container media to provide drainage and control root rots. Hardwood bark incorporated into container media at 15 percent by volume lasts two seasons in Ohio's climate. Composted sewage sludges and cow manure last through two years (at 10 to 15 percent by volume in a mix). Composted rice hulls and coconut coir (husks) also have an effect but will undoubtedly be shown to be short-term in nature like pine bark.

Greenhouse growers typically use peat mixes, or mixes with coir, or mixes prepared with composted pine, fir, or hemlock bark. Nurserymen typically use a large volume of aged processed bark, often little peat, and increasingly a small volume of compost. The quantity of compost used is typically guided by particle size or fertility.

Effects of Salinity and Fertility in Composts on Plant Diseases

Some immature composts such as those prepared from bark or sawdusts immobilize nitrogen (N), but most composts available today have been stabilized enough to release N. Some are consistently high in salinity (dairy and hog manures) and others vary in salinity. An increasing number of compost producers with experience in this field realize that the composition of raw materials, in both the composting process as well as curing and screening, must be kept constant to produce consistent quality products. Soil analysis laboratories can predict the fertility values of composts. Nutrient inputs must be balanced against crop needs.

Composted fir, pine bark, coir, and peat, because of their resistance to decomposition, do not release significant quantities of micronutrients. Therefore, essential micronutrients must be incorporated into the mix. Composted hardwood bark immobilizes N early, but releases various nutrients, including trace elements. Typically, however, trace elements must be added.

Composted sewage sludges release 25 percent of the total N in the first few months after utilization. Therefore, adjust the incorporation rate to the fertility needs of the crop. Generally, this means do not use more than 10 percent to 20 percent of this compost by volume in a mix or mulch, depending upon fertility needs of the crop. Overloading with the nitrogen-rich composted sewage sludge may increase such diseases as fireblight, Phytophthora diebacks, and Fusarium wilts.

All trace elements typically are supplied adequately by composted sewage sludges, particularly in low pH irrigation water regions. Trace elements typically do not need to be applied in the first year. Sewage sludges that have been treated with lime or other materials that raise the pH to high levels during the treatment process must not be utilized in container media because plant growth problems typically develop in the second year after potting.

Composted leaves also supply trace elements, not much nitrogen, and significant quantities of potash. Composted yard wastes supply some nitrogen if prepared with grass clippings, and they may contain up to 1 percent available potash. All composted manures, leaves, and sewage sludges provide phosphorus, calcium, and magnesium. Most do not have to be amended with lime. It makes a lot of sense to blend low-nitrogen with high-nitrogen composts. Mixtures of composted yard wastes and composted sewage sludges increasingly are preferred in several applications. Again, lime-stabilized composted products should not be used in container media.

The water soluble concentrations of iron, zinc, and manganese are high at a pH above 7.0 in composted cow manure or sewage sludge-amended mixes. As mentioned previously, many plants, including azaleas, grow very well and without trace element deficiencies at pH 7.0 or higher in these mixes because fulvic acids produced during the decomposition of organic matter and the siderophores produced by beneficial microorganisms keep them in solution. Once again, this does not apply to lime-stabilized composted products.

All composts can be high in salinity. As composts mature, mineralization proceeds, and the concentration of salts increases. Because compost piles often do not leach, salts accumulate during curing. Always monitor the conductivity reading of a new batch. Incorporate based on salinity limits, if needed, or apply composts well ahead of planting in the field to allow for leaching.

The negative effects of high salinity in composts was discovered during the early 1980s, when composted sewage sludge was applied to soybeans in Ohio in an attempt to control Phytophthora root rot. The disease was increased in each of four years when the compost was applied directly ahead of planting. However, in plots where the compost was applied three months prior to planting (February) or in the previous fall to allow for leaching, the disease was controlled, and yields increased.

The damage done by the compost applied at planting could be mimicked by an application of salt (NaCl) directly ahead of planting. It is well known that Phytophthora and Pythium root rots are aggravated by high salinity. These factors must be considered carefully in biological control of plant diseases, particularly for composts produced from manures.

Conclusion

In conclusion, composts can provide many beneficial effects. Control of Phytophthora and Pythium root rots with composts is practiced widely in nurseries and tree production systems. Control of Rhizoctonia diseases usually requires inoculation of composts with specific biocontrol agents or application of the compost to soil at least four months before planting to allow beneficial microorganisms to increase in populations naturally. Fusarium wilts can be controlled, but this requires utilization of high-carbon-to-nitrogen composts. Control of foliar diseases with composts mostly is an academic curiosity presently. Each type of compost has its own properties that must be considered during its utilization. Without such care, composts may actually cause problems.

References

Hoitink, H. A. J., A. G. Stone, and D. Y. Han. 1997. Suppression of plant diseases by composts. HortScience 32:184-187.

Hoitink, H. A. J. and M. J. Boehm. 1999. Biocontrol with the context of soil microbial communities: A substrate-dependent phenomenon. Annual Rev. Phytopathology 37:427-446.

Hoitink, H. A. J., Y. Inbar, and M. J. Boehm. 1991. Status of compost-amended potting mixes naturally suppressive to soilborne diseases of floricultural crops. Plant Disease. Vol. 75, No. 9, pg. 869-873.