A soil-borne pathogen found in wheat and oilseed rape crops can cut rapeseed yields by up to 50%, and experts suggest growers need to look at longer rotations and cultivation techniques to control its spread.

The pathogenic fungus Rhizoctonia solani results in pre- and post-germination damping off, causing seed decay and seedling death. 

Crop pathology expert Rumina Ray, from the University of Nottingham, explains that the pathogen has been underestimated and is now a clear problem for UK oilseed rape growers.

“Rhizoctonia solani is an overlooked soil-borne disease and its effect on oilseed rape crops has been ignored. Crop losses attributed to establishment failure due to pests or other soil-borne pathogens could, in fact, be the effects of rhizoctonia,” she warns.

See also: Rain and mild conditions heighten oilseed rape phoma threat

Disease symptoms

The pathogen damages the developing root system in oilseed rape crops, reducing taproot length, lateral roots and root surface area. Seedlings are most susceptible to the disease, and as the crop matures, plants become more tolerant. 

“Oilseed rape has one main taproot, meaning infection by the disease will significantly reduce crop growth,” explains Dr Ray.

Damping off is the most obvious symptom due to poor emergence, plant survival and establishment. Other early symptoms include thinning and stem elongation, known as “wirestem”, where roots become infected and stems appear brown in the middle.

Where there is high disease pressure, the pathogen can cut establishment by more than 80%, and also reduces plant  vigour and delays development, resulting in uneven and late flowering.

Risk factors

The pathogen containing the AG 2-1 isolates that infect rapeseed crops is commonly found in soils across England, with higher pathogen concentrations found in the North than in the South.

 “AG 2-1 isolates are prevalent in 63% of wheat fields across England, and concentrations remain consistent throughout the season,” explains Dr Ray.

Soil moisture, texture and structure can also have an effect on the pathogen’s development.

“Dry, healthy soils with good porosity are most at risk of infection, as the pathogen spreads faster under these conditions,” she adds.

Crop trials

A project run by the University of Nottingham and agrochemical firm Syngenta has investigated in-field effects of the pathogen, while also examining potential oilseed rape genetics and new seed treatments that would mitigate crop yield losses.

“We want to understand how the pathogen affects the host, while determining the effect of different soils and seed rates on disease development and seed treatment,” says Dr Ray.

The varieties Skye, Anastasia, Saveo and Campus were trialled using high (80 plants/sq m) and low (40 plants/sq m) seed rates.

Variety screening demonstrated how Campus and Skye yielded higher under inoculation, particularly when using the optimum seed rate of 80 plants/sq m – potentially allowing resistance genes to be identified.

Untreated crop (left) v treated crop (right)

Variety selection

“Variety choice is therefore extremely important. However, under dry conditions, variety choice has less effect on yield due to greater pathogen risks,” says Dr Ray.

The agrochemical group’s technical support manager, Michael Tait, says soil-borne diseases are more challenging to research than foliar pathogens, but despite this, a clear link between wheat and  oilseed rape crops and soil-borne disease has been found.

A further issue identified by Mr Tait is that rapeseed varieties are available on the market for relatively short time periods, which makes integrating crop protection to a range of cultivars difficult.

In addition, cabbage stem flea beetle damage is found to be greater in crops grown in infected soils, as they show reduced emergence and vigour, allowing time for flea beetles to take their toll. The risk from both the pathogen and flea beetles is increased under dry conditions.

Control strategies

With limited cultural control and no registered seed treatments currently available, AHDB resource management scientist Amanda Bennett explains how a strategy based on integrated control methods will be most effective at controlling the disease.

“It’s important to extend and diversify the rotation to include non-host crops, while being particularly careful with brassica cover crops such as mustard, which can act as potential hosts,” says Mrs Bennett.

Rapeseed should not be grown more than once in every four years. Ploughing or deep tillage to 8-10 cm should be used once within the rotation to disturb hyphal networks and reduce inoculum.

“Cultivation practices must be balanced within the system, as min-till oilseed rape has greater establishment rates,” she says.

Disease awareness

Although there are no oilseed rape varieties that carry resistance to the pathogen, selecting varieties with high seed viability and vigour will reduce the risk of the disease.

“There is no soil test available to identify the R solani pathogen, but raising awareness of the disease, allowing farmers to address potential problems and think ahead is essential,” she says.

Seed rates can also be increased to reduce the effects of the pathogen, especially for later sowings in higher-risk situations.