Mulching: Influence on the soil environment and tree health

 Jim Downer

 University of California Cooperative Extension

 669 County Square Drive, Ventura CA 93003 8050645-1458

 The integrated waste management act of 1989 brought both challenges and opportunities to Californians. The challenge of meeting diversion requirements of this law was tough; to divert 25% of recyclable materials by 1995 and 50% by 2000. The opportunities have also been a challenge. New business ventures have been started to handle wastes, small companies have sprung up to receive, stockpile, compost, grind and sell yard trimmings diverted from land-fill disposal. An unsuspecting public is now sold compost, of variable quality that is not always beneficial to plant growth. Science has also found however, that the organic fraction of soil can suppress root diseases and that additions of organic materials can make a disease conducive soil suppressive (Baker and Cook, 1974). There are obviously pros and cons associated with yardclippings wastes. Over the last decade, the University of California has conducted a tremendous amount of research on the use of these recycled organic materials. There are benefits, detriments and misconceptions about the role of recycled organics in both the cause and cure of plant diseases. 

Plant Maladies Associated with Yard-Trimmings Composts

Organic materials are a natural part of the soil environment. Soils that support plant life have as a consequence of plant growth, an organic layer or horizon. Litterfall is an especially important event in forests where nutrients are recycled in the organic phases of decomposing mulch layers. Now that recycling of organics is mandatory in California we are faced with an onslaught of various mulch products. Composting is a man made process that reduces the volume of feedstock dramatically and generates a product that can be used in horticulture. Unfortunately composting is a complicated process that must be regulated carefully. A compost pile 25 feet high is not natural. The byproducts of large pile composting process are often phytotoxic. Partially degraded compost from anaerobic piles contain short chain organic acids that drop the pH of the compost dramatically (Ozores-Hampton, 1998). These acids are both volatile and highly water soluble. Thus they will only remain in the organic material for a short time after spreading. However, I have seen plants whither under mulch and amendment applications of these immature yardclippings composts. Immature compost that has been somewhat anaerobic from stockpiling will have a characteristic "landfill" smell. Avoid products with this odor unless you can wait on planting where the product will be applied for a couple of weeks. Leaching also reduces the risk of damage from these chemicals.

 Imported plant pathogens

It has been a major concern for users of recycled yard trimmings that they would encounter plant pathogens in the green materials. There have been no technical papers written on the spread of fungal pathogens in yardclippings composts. We only can infer the fate of such pathogens as they enter the waste stream. Since diseased plant materials are being introduced into the waste stream, it is possible that disease propagules could be spread in this manner.

 Scenario 1

Most people are concerned about receiving organics with Armillaria mellea in it. Armillaria is the oak root fungus that leads to the death of many oaks and other woody plants in Southern California gardens and groves. There is no fungicidal `cure' for this pathogen once it has invaded a plant's vascular system. If an old stump or trunk of a tree is disposed of at a recycling center, it could be ground in the tub grinder and spread all through the stockpile. The fear is that this inoculum will infect healthy plants after application. This is very unlikely for the following reasons:

 l.) The inoculum is greatly diluted by all the other wastes

2) The inoculum is chopped into small pieces decreasing its energy reserves

3) The inoculum is immediately attacked by other fungi that grow in the stockpile or compost.

4) If the organic product is applied as a mulch to the soil surface it does not contact the roots of susceptible plants.

 Scenario 2

A homeowner purchases a nice `Tam' juniper in a five gallon container and brings it home to plant. The plant which was infected at the nursery with Phytophthora cinnamomi appears healthy because of fungicide applications that halt disease progress without killing the pathogen. The avid gardener plants the juniper and a few months later is mystified by the now brown landscape plant. He pulls the plant and promptly disposes of the dead organic matter in the greenwaste recycling container. The root ball which is loaded with chlamydospores of the fungus enters the waste stream and is ground with all the curbside collected green materials from the gardener's city. For much the same reasons in the oak root fungus scenario, the pathogen is limited or killed. However, since water molds such as Phytophthora have resistant resting spores, it is conceivable that they may slip through the processing that occurs in the waste stream and survive. We have no data to substantiate this, however if the diseased root ball ended up on the edge of a compost pile that was not well turned, it is possible for the spores to survive.

 Microbial degradation of plant pathogens is rapid and usually complete. Fungi eat fungi and so disease propagules usually have a short life span in the waste stream. If aerobic composting is used and the piles are turned on a regular basis, 100% of the pathogens can be eradicated. In most cases the use of organic materials is beneficial to plants and can even be helpful in controlling plant diseases in landscapes or orchards.

 Use of Organic Materials to Control Root Disease or Improve Root Health

 Pesticidal control of disease is becoming less practical as fewer fungicides are available now than in past years. In my research on the effects of mulch on Phytophthora cinnamomi, I discovered a complicated web of plant-soil and microbe interactions. This is nothing new. In their classical text on biological control Baker and Cook (1974) point to some of the complexities of biological control in the following passage:

 "The idea has become generally accepted that large changes are necessary to produce the desired control. Chemical treatments produce large abrupt, brief changes, and the pathogen population may fluctuate wildly. Biological control is more subtle, operates more slowly and on a smaller scale, and is generally more stable and longer lasting. Man, an inhabitant of the more violent aerial habitat, is inclined to think that large rapid changes are necessary. With a better developed worm's eye view of events, he may come to see that gentle "nudging" of the epiphytic and soil microflora can be far more effective in the long run than overkill or "dynamite" treatments. "

 I view mulching as a "nudging" process, slowly changing the chemical, physical and biological properties of soil to achieve a goal. Much of the time we try to achieve the goal rapidly by amending-applying vast quantities of amendment to backfill before planting trees. This practice may have merit in some cases but I think the passage from Baker and Cook about chemical control can also apply to use of organics. Mulching is a subtle process that takes years to achieve some of its desired effects. One of those effects is biological control of soil-borne diseases. A noted example was the Ashburner system devised in the 1940's in Australia (Broadbent and Baker, 1974). Control of Phytophthora cinnamomi was achieved with mulches of green plant materials and lime. I have studied a modified version of the Ashburner system for California soils that used tree trimmmings and gypsum.

Know Your Mulch: The feedstock counts!! What is in the mulch? We have used fresh 100% Eucalyptus globulus chips. Freshly chipped tree trimmings produce a high microbial activity mulch layer which is suppressive to disease.

 Gypsum is an essential part of the mulching system. An annual application of 15 pounds per tree has given excellent control of root rot. An understanding of moisture conservation under mulches is essential to controlling root rots.

 I have also found that fungi residing in mulch produce an enzyme system which is detrimental to the root rot fungus. These enzymes destroy the cell walls and spores of P. cinnamomi. High C:N ratios in the mulch support populations of cellulytic fungi, i.e. wood rotters or white-rot fungi common to mulch chip piles. We have identified two species of Basidiomycete fungi common to eucalyptus chips that appear to be important in this suppression.

Although biological control holds promise for help in controlling difficult root diseases of ornamental trees, an integrated approach using fungicides, cultural methods, resistant varieties and biological methods should give the best results.


 Broadbent, P. and K.F. Baker. 1974. Behaviour of Phytophthora cinnamomi in soils suppressive and conducive to root rot. Aust. J. Agric. 25:121-137.

 Baker, K.F. and R.J. Cook. 1974. Biological control of plant pathogens. W.H. Freman and Co., San Francisco. 433pp.

 Ozores-Hampton, M. 1998. Compost as an alternative weed control method. HortScience 33:938-940 .