- Healthy soil supports all sorts of life including invertebrates (worms, insects, etc) plus nematodes, bacteria and fungi (collectively ‘microbes’). Cultivating the right balance of these can help with factors including nutrient cycling, water/nutrient uptake, soil structure, water flow through soil, disease control and biostimulation of plants.
- Some microbes can be pathogenic and need managing, and soil-borne pathogens can be particularly challenging, due to factors including:
- localised communities may mean small patches are initially hard to spot;
- below-ground infection may not be evident until the problem becomes serious;
- pathogens may be transported by water flowing off land, or in soil attached to machinery and hence infections can spread to unexpected locations.
Microbial disease control methods
Microbes in soil
- Microbial communities can be very diverse and complicated, and may be envisaged in similar terms to an above-ground food web such as the example in Figure 1.
- Different microbes perform different functions within the soil, the details of which are often not well known, however impacting on one is likely to have knock-on effects for others.
- For example if there is thought to be a rodent problem in the scenario shown in Figure 1 so mice and rabbits are culled, there is likely to be much more plantain flourishing, whereas the foxes’ food supply is diminished.
- In terms of microbes, removal of a single species, or a small group of microbes, may have unintended consequences such as outbreak of another pathogen which was previously being kept in check.
Figure 1. Food web. See Microbes for potatoes 2016 for reference.
Microbes in bottles
- Bottled mixtures of microbes can be bought to ‘improve soil health’ or ‘manage’ a certain situation. However these may well be expensive and results unpredictable.
- For example the newly applied microbes may out-compete the native species and upset the natural balance so that potentially another harmful species/group can flourish; or the applied mixture may not survive in the new environment and therefore have no effect whatsoever.
- Due to the complexity of microbial communities and interactions between them it is very difficult to predict the effect in a given situation so even favourable evidence from one scenario does not guarantee a benefit in another.
- One organism which has been researched is a strain of the naturally occurring soil bacterium Bacillus subtilis but the results cannot necessarily be applied to other strains or species. More …
Managing microbes in soil
- Rather than trying to manage the details of microbial communities, it is better to cultivate an overall healthy soil, for example using cover crops, consistently good crop rotations and applying plenty of compost / organic fertilisers.
- Well-aged composts are likely to be most beneficial in this regard, and although in general organic materials will help build a diverse, resilient microbial community, care should be taken if considering use of, e.g. sewage sludge, that concentrations of pathogens are not high. This can be tested via commercial laboratories.
- Importantly, beneficial microbial management of this type is only effective for as long as it continues. Therefore it is vital to continue good management practices and consistently care for the biological health of your soil.
Crop rotations and cover crops
- Crop rotations including use of cover crops can reduce potato disease by various mechanisms:
- biofumigation: plants release chemicals
- non-host plant species: pathogens which are plant-specific may not survive if sufficient non-host crop is grown
- modify bacterial communities: competition for resources; direct antagonism between species
- Furthermore, other benefits of crop rotations and cover crops (in terms of microbes) include:
- adding a cover crop between existing rotations can result in overall increases in bacterial populations (and is certainly more beneficial than bare soil)
- more diverse cropping systems result in greater soil microbial biomass and greater microbial diversity
- crop rotation effects on soil microbial communities can be greater than those due to additions of biocontrol organisms
- All of these can create a more robust system, which is able to better cope with any changes (environmental, microbial, plant-based) and maintain ‘healthy’ state with better soil structure and less disease.
- Biofumigation is the idea of using plant chemistry, particularly emission of volatile compounds, to control problematic microbes. In particular, brassicas are known to release glucosinolates (GSL) which break down into a range of toxic compounds including isothiocyanates (ITC).
- Biofumigant compounds have been shown to effectively damage PCN eggs and juveniles, however published trials are often not very representative of field conditions (e.g. using bare soil, high concentrations of active ingredients, in vitro egg hatching). Actual efficacy of biofumigants in the field will depend on:
- specific green manure crop: species, yield, GSL concentration;
- resulting concentration of GSL in soil;
- efficiency of transformation to ITC;
- loss of ITC to air (volatilisation);
- timing of ITC release (e.g. presence of egg vs. juvenile).
- Growing cover crops such as canola (Brassica napus, Brassica rapa or Brassica juncea), rapeseed, mustard, turnip, radish, and then incorporating them into the soil (before planting potatoes) has been shown to significantly reduce the prevalence of diseases in potato crops including black scurf, common scab, powdery scab, silver scurf and Verticillium wilt, and resultantly increasing tuber yields. White mould was not affected, and only a few trials were available for silver scurf and Verticillium wilt, thus the experimental results are likely to be less robust than for the other pathogens.
- As with all biological controls, efficacy may not be consistent between years or sites, however using a biofumigation cover crop/green manure directly before potatoes is likely to have a positive effect on disease prevalence, and if incorporated, will have the added benefit of providing additional N and C for soil health.
- Use this link for for further information on growth of biofumigant crops and use in PCN control.
Notes on microbes
Microbes for potatoes 2016 (Manipulating microbes in the soil environment)
Bacillus subtilis 2012 (Physiological effects of B. subtilis)
Bacillus subtilis 2010 (Background to use of Bacillus spp.)