Biologicals and Anaerobic Soil Disinfestation

There are two major types of plant diseases that can affect yield and quality of plant products. One group infects foliage – leaves, stems and fruit – and causes necrosis of tissues to compromise plants’ ability to thrive, eventually killing the plant. These disease-causing organisms (pathogens) primarily spread by wind or rain splash from diseased to healthy plants. It is relatively easy to manage foliar diseases if diagnosed early, and measures are taken on time by adding highly effective fungicides or bactericides to the disease management program. 

Another group of pathogens survive in the soil and infect plants through the root system, colonizing the vascular or water-conducting tissues to plug the tissues. As a result, water and nutrients can’t move upward to the foliage. Plants dry up gradually, which is known as wilt. Fungal pathogen Verticillium is one of them. But, there also are other soil pathogens, such as Fusarium, Pythium, Rhizoctonia, Phytopthora, Sclerotinia and tomato wilt causing bacteria, like Ralstonia solanacearum. As these pathogens survive in the soil organic matter or plant debris, applications of fungicides or bactericides are usually not effective. 

The most effective measure growers can take is rotating crops from a different family. In the absence of a rotational option, growers would use fumigation for high value crops, such as strawberries and tomatoes. However, after phasing out of the most effective synthetic fumigant methyl bromide, alternative products such as Basamid (dazomet), Telone C-35/C-17, Pic-clor-60, Vapam (metam sodium), have been used with lower and variable efficacy. In addition, these restricted category products can’t be used without large specialized equipment. Small growers and landowners near schools, churches or dwellings are not allowed to use these products due to health and environmental regulations. Consequently, small and organic growers want alternate options that are environment- and user-friendly. 

Experiments on biologicals and biofumigation for the last few years at the WVU Research, Teaching and Outreach Center to manage soilborne diseases, including Verticillium wilt, provided results that can be used by organic as well as conventional growers. This approach was tested in strawberry, tomato, eggplant and okra with positive results. 

The major principle of biological control is indirect antagonism or competition. If you have beneficial microbes on the root system of plants, harmful ones do not get space to infect. These beneficial microbes also take up nutrients that are mostly available on roots’ surface from fluids emitted through the roots to deprive harmful ones. These microbes are also called probiotics, as they can take nutrients, such as phosphorus and potassium, from the earth’s crust and make them more soluble so they’re available to plants and can produce phytohormones, which reduce environmental stress to plants. 

As part of the experiments, a commercially available biological control product called TerraGrow that has five different beneficial bacterial species was added to Johnny’s organic planting mix, followed by thorough mixing. Seeds also were soaked in the product slurry for five minutes before planting. The planting mix was dispensed in a plastic plug tray, where seedlings were grown in the greenhouse for six weeks. Some seedlings were grown in medium without any beneficial microbes. While seedlings were growing in the greenhouse, we inoculated field plots with Verticillium pathogen grown on oat grains. We had three different treatments, including no treatment of field plots where non-treated seedlings were planted. 

In another treatment, seedlings grown on non-pasteurized media inoculated with beneficial microbes were planted in anaerobically disinfested soil (ASD). In the third treatment, pasteurized media were inoculated with probiotic bacteria to grow seedlings that were planted in ASD soil. 

ASD was done in three different steps. Plots were plowed and mustard meal was added at the rate of 5 tons per hectare, mixed with a walk-behind rototiller, followed by covering with plastic mulch. Plots were than irrigated through dripline up to saturation. So, all the air was out of the soil to make an anaerobic condition. Beds were kept undisturbed for three weeks for anaerobic microbes to multiply and produce volatile organic compounds (VOCs) that may have suppressed Verticillium. Plastic was cut perpendicularly 24 hours before seedlings were planted in field plots by digging holes to ensure toxic gas has dissipated. 

Plants were grown to maturity; fruits were harvested three days a week and cumulative yield was recorded by number and weight. Significantly higher yield was obtained from treated plots compared with non-treated in Verticillium inoculated soil (Table 1). More than 50% of plants wilted and died in non-treated plots, whereas only one plant died in the biological treatment and no plants died in the combination treatment by the end of the growing season (Figure 1). The combination treatment of seedling production in beneficial microbe inoculated planting mix and planting them in ASD field plots seems a promising option for growers for managing soilborne disease those who can’t use synthetic fumigants. Although the work mentioned in this article relates to Verticillium wilt control on okra. Similar methodology may work against other soilborne diseases in other crops, such as eggplant, tomato, strawberry, etc.

Table 1. Fruit yield, plant height and mortality as affected by treatments of biologicals and anaerobic soil disinfestation at seedling stage, field and combination of both. 

Numbers followed by different letters in a column are significantly different from each other according to Fisher’s protected LSD test (P=0.05).

  

Figure 1. Management of Verticillium wilt on okra: a) Non-treated (left); b) Seedlings grown on pasteurized planting mix inoculated with TerraGrow + ASD (center); c) Seedlings grown on regular planting mix inoculated with TerraGrow + ASD (right).

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