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Harry A. J. Hoitink, The Ohio State University, Ohio Agricultural Research and Development Center, Plant Pathology; Daniel A. Herms, Pierluigi Bonello, |
Nuisance Fungi in Mulches
Several factors contribute to growth of nuisance fungi on mulches. The four most important factors are:
Most nuisance fungi grow naturally on the cellulose or the lignin in the mulch while it decays. Others such as slime molds (dog vomit fungus) grow on microorganisms that decay the mulch.
Landscapers often apply mulch from high temperature (120 to 160F) piles directly into the landscape. The beneficial microorganisms in these hot composts cannot grow after the mulch cools (60 to 80F) in the landscape. They cannot compete with dominant soil microorganisms; thus, they die and this leaves a "biological vacuum." The microorganisms that fill this vacuum become the predominant initial colonizers.
If the hot mulch is wet (45 to 60% moisture content), then bacteria as well as fungi colonize the food base. If it is fresh bark or woody mulch, then a lot of food is available for organisms to consume as opposed to a stabilized compost that resists colonization by microorganisms. Certain bacteria produce antibiotics, and this suppresses colonization by fungi. However, if the moisture content of the hot mulch is low (less than 34%, i.e., dusty), then fungi become the predominant colonizers.
This produces a moldy product over a two-week period. After heavy rains when this material becomes wet, bacteria and other microorganisms colonize the fungi on the mulch. The fungi produce fruiting structures thereafter which we see as toadstools, spore masses from the shotgun fungus, the beautiful birds nest fungi, and more. Thus, high temperature, dry, relatively fresh, woody mulch applied to the landscape produces fungal problems. As decomposition progresses, mulches have a higher nitrogen content, and this also favors bacteria over fungi. Therefore, problems are less severe on partially composted mulches.
The pH of mulch plays a role as follows: Bacteria in general do not colonize mulches if the pH is below 5.0. Fungi predominate on low pH mulches. Thus, sour mulch, which has fermented to produce vinegar and other organic acids in anaerobic (no oxygen), large, compacted storage piles, inhibits bacteria even when its moisture content is high. Dry, sour mulches seem to cause the worst problems. In bagged products, the same problems develop when the moisture content and pH are low. Water from biological activity condenses along the edge of bags. After some time, enough water is present there to stimulate fruiting of fungi induced by bacterial growth.
How does one avoid these problems? Maintain a moisture content greater than 45% on a weight basis in the mulch. Soak
it with water during mulching to set up competition for fungi. Avoid using sour mulches. Finally, use mulches that have been partially decomposed and avoid high wood fiber content materials.