Soil Building Methods to Reduce Disease
The primary method to maintain healthy soil is to increase organic matter content; this
will improve soil structure and diversify the microbial community. Disease suppression
may occur when soil-building tools such as mulches, cover crops, and compost are
utilized.
Plant residue mulch (straw or no-till cover crop residue) improves soil structure and acts
as a soil conditioner that prevents soil compaction. Reduced compaction provides better
water infiltration during rain or irrigation, and reduces soil splash which can transport
soil-borne pathogens to plants. In Maryland, for example, the prevalence of Septoria
leaf spot of tomatoes was significantly reduced when crops were grown on a no-till hairy
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vetch cover crop (Mills et al. 2002). This cover crop reduced soil splash and pathogen
transmission to the lower canopy. Some organic mulch, such as shredded bark, serves
as physical barriers to foliar pathogens such as the foliar nematode. In addition, both
cover crop mulch and plastic mulch suppresses weeds that potentially harbor
pathogens of the cash crop. Some colored or reflective plastic mulches are used to
repel insects, which are vectors of virus diseases. However, light colored mulch will not
block enough light to inhibit weed growth. There are many benefits of mulch; however,
incorrectly applied mulches may inhibit water penetration or block airflow which could
decrease beneficial microbes and exacerbate disease. The proper selection and
accurate application of mulch should eliminate or alleviate poor drainage reduce, soil
splash and minimize weeds and insect virus vectors.
Compost application also increases the organic matter content of soil, improves
drainage, soil texture, and provides nutrients and habitats for beneficial organisms.
Compost may contain natural antibiotics produced by beneficial bacteria, which inhibit
the growth of pathogens. In certain instances, compost induces systematic resistance in
plants, triggering a plant's defense system prior to the onset of disease. When
pathogens contact the plant, these “primed” plants are conditioned to resist the disease.
Research conducted in Maryland, Ohio, New Jersey and elsewhere confirms that
selected compost application leads to disease reductions in many crops.
Diseased plant material should not be composted because pathogens may not be
eliminated if there is uneven heating of the compost pile. It is advisable to use compost
that has been handled and cured in the appropriate manner to ensure that beneficial
organisms, rather than harmful ones, are present. Compost is most effective as a
disease suppressant if it (1) has been cured for at least four months, (2) is incorporated
into the field a few months prior to planting, and (3) has been inoculated with biocontrol
agents such as Flavobacterium. Adding biocontrol agents to compost is a form of
specific suppression. Specific suppression occurs when one organism directly affects a
pathogen. Various types of organisms, which have the ability to suppress disease, may
be added to compost. For example, the beneficial fungus Trichoderma hamatum, which
produces antifungal exudates that suppress the pathogen Rhizoctonia solani, can be
added.
Green manure cover crops provide many of the same benefits as mulches and
compost. Benefits include building soil structure, fertility, soil tilth, and favoring microbes
that compete with potential pathogens. Some cover crops, particularly when
incorporated as a green manure or soil amendment, can directly or indirectly suppress
pathogens of the cash crop (Table 1). For example, Brassica cover crops successfully
suppress the occurrence of Rhizoctonia on potatoes through the release of
isothiocyanates into the soil. A green manure of Vicia villosa (hairy vetch) suppressed
Fusarium wilt on watermelon in Maryland. Numerous cover crops have proved effective
for suppressing disease in the mid-Atlantic and other regions. See Table 1 for selected
research results from the mid-Atlantic.