Biological control of Phytophthora cinnamomi in avocadothrough the use of mulches was identified by an Australian grower and later described as the "Ashburner Method" by Broadbent and Baker. The technique uses large amounts of organic matter as a mulch along with a source of calcium. Control of avocado root rot in the Ashburner method was attributed to the presence of Pseudomonas bacteria and Actinomycetes. Multiple antagonists are more likely the cause of biological control, since no single organism has been found to be consistently associated with soils suppressive to P. cinnamomi.
The use of organic mulches has multiple effects, such as altered soil nutrient and water status and improved physical structure. Any improvements in plant status resulting from improvements in the growing environment can improve plant health. The effect of organic amendments on soil physical and chemical properties can vary considerably depending on soil texture and the environment. One of the most consistent effects of organic amendments is an increase in biological activity. Increases in organic substrate lead to increased fungal and bacterial populations. In numerous cases, this increase in biomass has been associated with disease suppression. This biological control can be ascribed to several mechanisms: competition, antibiosis, parasitism, predation and induced resistance in the plant.
The microbial biomass is responsible for release of enzyme products and polysaccharides in soils. The microbially-produced enzymes cellulase and glucanase have been demonstrated to have a significant effect on Phytophthora populations. This mechanism of antibiosis is possible because the microbes are releasing these enzymes to solubilize organic matter. Unlike other fungi, Phytophthora have cell walls that are comprised of cellulose and in the process of decomposing organic matter with enzymes, an environment is created that is also hostile to the pathogen.
In order to see if there might be potential differences in organic materials being better at combating avocado root rot, a little field trial was established with 23 different types of materials (see types on Graphs 1 and 2). The mulch materials were obtained from nearby hedges and chipped or obtained from commercial sources of mulch. Some of these materials would be difficult to get in large amounts, such as manuka (Leptospermum scoparium), but others are commercially available chipped greenwaste. The materials were then spread on the ground to a depth of five inches, in separate plots that were 36 X 36 inch squares. Decomposition was measured over a two year period and then cellulase was measured in the mulch, at the soil / mulch interface and at a two inch depth in the soil at the end of 2 years.
Since cellulase production is part of the decomposition process, the rate of decomposition should be a partial indicator of the amount of cellulase present. Graph 1 shows the depths of various materials at the site after one and two years of decomposition. After a mulch application there is generally settling due to rainfall-caused compaction, but much of the decline by the second year is due exclusively to decomposition. The more recalcitrant materials, such as bark, wood chips and sawdust have barely lost half their depth after two years, while others such as shredded eucalyptus, manuka, avocado and willow are less than 20% of their initial depth. Much of the shredded/chipped material, such as eucalyptus had a significant fraction of leaves in the mulch. The wool disappeared a little after one year. The greenwaste + chicken manure compost is nearly the same depth as the wood chips, since it is a material that had gone through a decomposition process prior to its application and much of the easily digestible materials had already been decomposed.
The rate of decomposition has some bearing on the rate of cellulase production (Graph 2). Eucalyptus and manuka had the two greatest rates of decomposition and show the highest levels of cellulase production. The cellulase levels were consistent with all the different mulch materials. Using decomposition rate alone is not a complete indicator of cellulase production since, poplar, willow and avocado had high rates of decomposition, but their cellulase rates were half those of manuka and eucalyptus
It is clear that the cellulase effect is limited to the layer of mulch and not to depth within the soil. There is some effect at the soil surface, but at 5 cm. cellulase activity drops to background levels (Graph 2). There is earthworm activity at the test sites and one idea was that earthworm incorporation of organic matter would move the cellulase production into the soil. Maybe with further time this would occur. As it is, when mulches are applied to avocado, the roots tend to proliferate in the mulch, out of the soil where the cellulase activity is the least.
Something to keep in mind is that we do not know what levels of cellulase are necessary to control the root rot fungus. It may be that levels seen with pine bark are more than adequate. Also we have measured cellulase production at only one time in a two-year period and it is quite likely that this is not the best snapshot of what is happening before and after. A further reminder is that cellulase is only one of the many by-products associated with decomposition and many of the antagonistic properties that are associated with the microbial biomass are not being measured in this trial. Having developed this screening procedure what needs to be done next is to take high, medium and low cellulase producing mulches and challenge the fungus to verify that this is a good way to evaluate mulches.
Because of potential contamination of mulches from pesticides, mulch use is being called into question and testing may be required for mulches applied to organic orchards. This is a recent ruling. As always, anything applied to a certified organic operation should be cleared with the certifier first.
A recent article in California Agriculture highlights something that we have found in raw organic materials that have been used in mulches for controlling Avocado Root Rot – Phytophthora cinnamomi. The most active stage of a material in the process of composting, is the initial one where all the bacteria and fungi are fighting to consume the easily decomposable sugars and light fraction organics. That's when the material heats up and various gases, such as ethylene, ammonia and others are released. This is the most competitive time in decomposition and Phytophthora just can't compete. In fact, it becomes a food source for the antagonistic fungi, since its cell wall is made of cellulose the material that many leaves are made of. In releasing exudates to decompose cellulose, the Phytophthora is decomposed, as well. As the cellulose is decomposed and disappears more resistant materials remain. A mature compost, is therefore a much more stable material than a raw one. It still has nutritional benefits, as well as physical effects on soil when it is incorporated, but has a lesser impact on Phytophthora.
Compost is used as a soil amendment, largely because it is relatively stable biologically and has nutritional/physical effects. Incorporating raw organic matter into the soil and then planting has its restrictions and should be done cautiously. But as a mulch (material applied to the soil surface) there are fewer problems, as long as some common sense is used. Like don't pile it up against plant stems which keeps moisture and conditions for disease around the plant crown. And there are several other qualifiers, such as don't apply it so thickly that the soil never dries out and becomes a problem in walking across it.
So, read this interesting story of what happens to Phytophthora when introduced into a mature compost:
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Ground urban organic material that shows up in great trailers for use in agriculture has simple rules for its use. 1) It should be a material low in contaminants. If you can clearly see golf balls and plastic, send it back. Over time with decomposition it is only going to become clearer because all the organics go away and all you are left with is a mess to clean up. 2) If used as a mulch (applied to the surface of the soil), it should be as woody material as possible, because it is more persistent and lasts longer and the roots which adjust to the mulch layer are less likely to be disturbed. 3) If it is used as a material to be incorporated into the soil IT MUST BE COMPOSTED. This cannot be emphasized enough. With decomposition, gasses such as methane, ethylene and ammonia are released which are all toxic to roots and it heats up, also which is hard on roots- like it burns the heck out of them. This goes for the material used as a mulch, it should be kept at least 6 inches away from tree trunks so that gasses and heat don't burn the trunks. And 4) no organic material should be incorporated into the planting hole, composted or non. This is one of those myths (planting mix in planting holes) that have been foist on gardeners and farmers alike. If it is organic it is good? Any organic material incorporated into a planting hole is going to decompose over time, and then the soil settles, causing voids. Much better to plant into the mineral soil and mulch and let the worms move any nutrients from the organic surface material into the root zone.
And remember that fresh citrus leaves and stems can harbor Asian citrus psyllid. Dry that material out before moving it around so you don't spread the psyllid.
With the proliferation of pests and disease that might appear in mulch used in orchards, it might appear that the practice will become more limited unless the material can be guaranteed to be safe. Polyphagous Shot Hole Borer and Asian citrus Psyllid are just two examples of pests that might appear in a mulch from an unknown origin. There are many other examples, though, and new threats keep coming. Composting is a process that can insure a quality product, but composts tend to be too expensive to be used as a mulch. There is a new solar processing technique that is being evaluated in the Central Valley that might produce a product that could be used as a mulch.
The proliferation of low-cost, high-efficiency and durable solar panels makes it possible to set up solar-powered composting systems that reduce reliance on alternative forms of power and avoid very expensive grid hook-up costs.
This type of technology lends itself to a more stable site layout, which can then utilise conveyors for material movement, further reducing reliance on diesel-powered equipment. A potato piler, a standard piece of equipment in the potato storage industry, is an ideal prototype for compost conveyors.
Static piles can easily be watered using simple irrigation systems that save water and further reduce diesel reliance. When combined with the already proven technology of a compost cap, reductions of VOCs and greenhouse gases can be substantial.