Researchers are taking a fresh approach to biofumigation that could provide suppression of potato cyst nematode with much lower input than previously thought.

Harper Adams University has been involved in a number of projects looking into the benefits of incorporating brassica cover crops into the soil to control PCN.

Growing species such as Indian mustard over the summer months, then macerating and incorporating them into the soil was thought to be the optimum means of achieving a biofumigation effect.

The same catch crops also provide the added benefit of nutrients in the form of “green manure” and can contribute to “greening” requirements of new Basic Payment rules.

See also: Cover crops to control potato cyst nematodes

However, a recently completed PhD project by Bruno Ngala at the Shropshire-based university has explored the concept that interactions between mustard or oilseed radish roots and soil microbes provides a “partial” biofumigation effect.

Project leader and nematology expert Matt Back tells Farmers Weekly that the most exciting aspect of its findings is that growers could reduce PCN populations with very little work.

“Growers like the concept of biofumigation, but not the idea of having several bits of kit in the field at the same time [to chop and incorporate] and fitting it all into their system.

“In the future it could be possible to focus on just growing the crop, spraying it off with a broad spectrum herbicide and still getting some biofumigation effect from the roots alone.”

Dr Back adds this would also enable those using reduced tillage system to get the benefit and also reducing machinery, fuel and labour costs.

Soil biology

Anecdotal evidence while conducting previous biofumigation field experiments hinted that PCN populations were being reduced before biomass was incorporated.

It is thought that soil microbes – bacteria and fungi – are using glucosinalates exuded by the brassica roots as a food source and to break it down by producing an enzyme called myrosinase.

When glucosinalates combine with myrosinase and water, isothiocyanates (ITCs) are produced, which is the substance that provides the biofumigation effect that reduces the viability of PCN eggs.

Dr Back says that in recent pot experiments using unsterilized and sterilised soil free of microbes, the biofumigation effect was much higher where microbial activity remained.

“Both mustard and oilseed radish have potential, but oilseed radish looks to have much more chemistry in its root tissue compared with Indian mustard.”

The next step is to investigate which specific bacteria or fungi are creating the effect, how widespread they are and whether they influenced by soil type.

“That is the missing link,” adds Dr Back.

While this work is in development, Dr Back urges growers to follow guidelines on conventional biofumigation and ensure that the biomass if finely chopped and incorporated.

Potential problems

Problems with brassica biofumigation crops during the extensive work at Harper Adams University, including pest and disease build-up that could have implications elsewhere in the rotation.

Mustards are particularly susceptible to clubroot, so will perform poorly where the soil borne pathogen is present, while oilseed radish is not susceptible, but it will allow the disease to propagate.

In terms of pests, cabbage root fly and turnip sawfly can thrive in such crops, which may have consequences for other brassica species in the rotation, such as oilseed rape.

“There are also anecdotal reports to suggest that free-living nematodes are increased by certain biofumigation crops, which needs to be fully investigated.

“There are many cover crop blends coming on to the market commercially claiming various effects and I would urge growers to use independent advice to ensure they are using them in the right place,” says Dr Back.