Growers will soon be planning for manure or compost applications to new gardens. Over the past few months, we have been conducting a literature review into alternative disease management options. Part of the literature review involves an examination of research on biocontrols and amendments. The research has shown that composts can have a significant impact on soil-borne pathogens and the success of biocontrols.

Compost ready for application to a field.

Biocontrol is the addition of organisms to control or suppress a pest. For ginseng, the main target of biocontrol is soil-borne fungi and usually involves the addition of a specific organism such as Trichoderma. Composts often contain similar beneficial organisms to those contained in commercial biocontrol products and these can also control or suppress soil-borne pathogens. Biocontrols and beneficial organisms in the soil work to reduce disease in four main ways: 1) compete with pathogens for resources, 2) directly attack pathogens, 3) release chemicals that suppress pathogens, and 4) induce resistance in the crop. While the main purpose of application of manure or compost in ginseng is to increase soil organic matter content and fertility, the addition of these can have a significant impact on the microbial community around the roots and the pathogens in the soil. 

Research has shown that the addition of raw manure or materials that are not well composted can actually increase pathogens in the soil. Many of the soil-borne pathogens of ginseng such as Fusarium, Pythium, Rhizoctonia and Cylindrocarpon can live on dead and decaying materials in the soil when not attacking ginseng roots. Manure can act as a food source for these pathogens, and populations can increase, depending on the pathogen and the type of manure applied. Raw manures do not contain many beneficial organisms and can contain pathogen spores, so pathogens can have a head start in colonizing the soil. It is common in ginseng to apply manure several weeks before fumigation. Fumigation will reduce pathogen populations again, but rarely provides 100% control.

Once manure has a chance to break down sufficiently it will support more biological activity in the soil and begin to suppress pathogens. This is dependent on having very good fumigation practices to control pathogens that are promoted by the manure application. Otherwise their populations will rebound right after fumigation. Application of a poorly composted or raw product after fumigation should be avoided because of the potential to increase pathogen populations.

During the composting process, the heating caused by microbial activity kills most of the organisms in the compost, including many of the beneficial ones. After the heating period, any organisms in the cooler exterior of the compost pile start to re-colonize the compost. The compost will have peak activity of beneficial organisms within a month or two of the heating period. This is the ideal time to apply the compost to ginseng. Compost application is best done after fumigation to avoid killing these beneficial organisms. For many crops, application of compost at this stage has resulted in reduced root disease, especially Phytophthora and Pythium. On the other hand, research from Korea showed an increase in Fusarium and Cylindrocarpon following composted manure application to Asian ginseng, but it is unknown if the material was properly composted.

The research has also shown that the type of compost applied can have a major effect on which beneficial organisms predominate in the compost and which pathogens are controlled by those organisms. For example, pine-bark composts get colonized by Trichoderma species which suppress Rhizoctonia in the soil. There is insufficient research on composts on ginseng to know what effect different composts will have in the soil. Growers should keep in mind that switching to a new compost could change the balance of pathogens in the soil so that a disease that was suppressed by one compost may not be suppressed by the next. This may result in more disease if the grower was not accustomed to controlling that disease through other practices.

Once a compost pile has been re-colonized by the microbial community after the heating stage, the organic matter in the compost is continually broken down. Eventually there is very little food source left and microbial populations decline. This compost still has value for increasing stable soil organic matter content, but no longer has much value in suppressing pathogens in the soil.

Biocontrol products (biopesticides) such as RootShield and Serenade Soil contain specific strains of fungi or bacteria that compete with or attack pathogens in the soil or on the roots. Research has shown that the activity of some of these types of products can be improved through the addition of compost. The compost acts a food source to build and maintain populations of the biocontrol agent. It is unknown if compost application will improve the activity of the biocontrols registered on ginseng.

Here are some take home messages:

1. If applying compost after fumigation, ensure it that it is fully and properly composted, but not so old that it has little value left.

2. When switching from manure to compost or between two different composts, there can be many unintended consequences on the balance of pathogens in the soil. It is always best to research the new practice on small area first before converting your whole operation.

3. Avoid application of raw manure after fumigation.

4. Compost application may improve the activity of biocontrols.