Multiple organisms

An alternative recent approach is to use populations of microbes manipulating the husbandry environment to their advantage. This is substantially a means of developing artificially suppressive soils that mimic natural processes in the field. The beneficial impact of benign microbes on pests and pathogens can substantially reduce their invasion, colonization, disease, and damage initiation and progress. Slight environmental changes resulting from changed husbandry practices will alter the biological balance between benign antagonists and pests and pathogens. Many microbes are natural antagonists to parasitic organisms; only small proportions of the natural soil flora attack crop plants while the majority are beneficial to crops. The relative numbers of microbes in the top 15 cm are 109 bacteria, 108 actinomycetes, 106 fungi and 105 algae/g of soil. Their roles in soil include decomposition of organic matter, mineralization and immobilization of nutrients for plant growth, nitrogen fixation, environmental remediation, plant growth promotion, biological control for crop protection and only to a small extent pathogenic activities leading to crop disease.

Substantial efforts are being made to understand how cropping practices may be designed to encourage benign organisms and discourage pests and pathogens. The most basic husbandry practice is primary cultivation by ploughing. It has disadvantages, however, since it can increase water loss due to runoff and evaporation by leaving the soil exposed to heat and wind, and increases the loss of organic matter which is required for an active benign microbial population. Reducing tillage (see Chapter 6) under American conditions enhances microbial species diversity, with larger populations in the upper soil horizons, whereas tillage redistributes microbes throughout the upper and lower soil horizons.

Several higher fungi produce ergosterol that can be utilized as a biological marker to measure the activity of soil microbial populations. The efficiencies of extraction of ergosterol are relatively high, and it is an exactly defined compound that can be determined using chromatographic methods. Few other simple methods are available and reliable apart from counting microbial colonies grown in asceptic culture, which is laborious, expensive and long winded. Ergosterol content was greater in non-tillage conditions (Monreal et al., 2000; Krupinsky et al., 2002), indicating increased microbial activity. Conversely, the advantages of tillage lay in the burial of crop residues, levelling, consolidation and warming of seedbeds in spring, reducing surface compaction, breaking up soil pans, incorporation of pesticides and fertilizers, and reducing weed competition.

In those geographical regions with high ambient temperatures, use can be made of solarization to diminish soil-borne pest and pathogen populations. Solarization exploits the heating effects gained by placing plastic sheeting over soil. Population counts of F. oxysporum f.sp. conglutinans were greatly reduced, and cabbage yellows disease was undetected in plots treated with solar heating and cabbage amendments (1% w/w). Dried cabbage was mixed with soil and covered with translucent polyethylene sheeting (tarp) for 4-6 weeks; the use of solar heating or cabbage residues alone was ineffective. The sheeting traps fungitoxic gases derived from the cabbage residues that are concentrated; these together with high temperatures are responsible for the diminution in the viability of pathogen propagules. Ramirez-Villapudua and Munnecke (1987) postulated that these effects result from changes to the biodiversity of soil microflora, diminished soil fungistasis and release of nutrients that may induce chlamydospore germination leading to biological control of F oxysporum f.sp. conglutinans. There may also be direct toxicity from the sulphurous compounds present in cabbage residues as a similar fungistatic effect of brassica residues was reported by Kocks et al. (1998) for black rot (X. campestris pv. campestris). Epidemics of this pathogen build up over years (polyetic) with a carry-over of inoculum from one vegetative period to the next. However, where black rot-infested plants were ploughed back in after harvest and the plants broken up before incorporation, presumably releasing sulphurous compounds, then the risk of infection in the following season was diminished.

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