University of California Cooperative Extension
County Square Drive, Ventura CA 93003
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.
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.
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
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.
If the organic product is applied as a mulch to the soil surface it does
not contact the roots of susceptible plants.
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.
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
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
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.
Baker, K.F. and R.J. Cook. 1974. Biological control
of plant pathogens. W.H. Freman and Co., San Francisco. 433pp.
M. 1998. Compost as an alternative weed control method. HortScience 33:938-940