Managing rhizoctonia solani in oilseed rape

The ICAROS project is expected to lead to improved oilseed rape yield and cost savings for growers over the longer-term.

R solani is an aggressive soil-borne fungal pathogen that effects oilseed rape worldwide. Crops are most susceptible to the pathogen during the seedling stage and become more tolerant to the disease as they mature.

Infection can result in pre- and post-germination damping off, causing seed decay and seedling death. Damping off is the most obvious symptom, as it results in poor emergence, plant survival and establishment (Figure 1).

The pathogen causes reductions in taproot length, lateral roots and root surface area. Early symptoms include thinning and elongation of stems, known as ‘wirestem’, and ‘spear heading/tipping’, where roots are severed at the point of infection. Such symptoms are not easily diagnosed in the field, as young seedlings are fragile and can break at the point of infection during sample collection.

Symptoms on mature plants include brown, necrotic lesions, which may girdle the root or stem circumference. Infected plants may also exhibit delayed flowering (Figure 2).

Yield losses of up to 30% have been estimated due to root rot caused in mature plants. However, other soil-borne pathogens can cause similar symptoms, making accurate diagnosis a challenge.

The aim of ICAROS, which ran from July, 2016, to October 2019, was to investigate genetic resistance/varietal resistance (Challenge one) and develop integrated strategies, inclusive of novel seed treatments (Challenge two), for the control of soil-borne R solani in OSR. An additional challenge was to investigate yield loss to it and develop new guidelines for disease management.

Life cycle

This is a species complex that comprises 14 anastomosis groups (AGs). Pathogenic isolates are allocated to a group based on their ability to fuse (ie anastomose) with each other.

In the soil, the pathogen survives in the form of resting structures called sclerotia (made of compacted mycelia) or as vegetative hyphae on crop debris or seeds. In the absence of a host, R solani survives on crop debris for long periods, volunteers and weeds.

OSR releases chemicals that stimulate the production of runner hyphae from germinated sclerotia or overwintering mycelia. Once the host is reached, the pathogen forms an infection peg and produces digestive enzymes to degrade plant cell walls.

Older mycelia form sclerotia on infected tissue. Where tissue is left in/on the soil on crop debris, it can become a source of infection in following crops. Pathogen inoculum in soil increases when susceptible crops are grown in close rotations.

Importance

Globally, it is a common soil-borne crop pathogen. AGs 2-1, 4 and 8 are most pathogenic to oilseed rape. The pathogen is associated with establishment losses of 17–65% and where plant numbers are reduced to below the optimum (25-35 plants/m2), yield loss can occur. However, yield reduction can also occur if mature plants suffer significant root rotting.

Risk factors:

Frequent (short) rotations of host crops

High inoculum density of the pathogen in soil

Weeds or volunteers that harbour the pathogen

Soil type and conditions during sowing and emergence (soils with good porosity and warm conditions increase disease severity)*

Deep sowing (below 6cm), which slows the time to emergence

Plant susceptibility – hosts are most susceptible during emergence and seedling development. Low seed viability/vigour increases risk

AHDB-funded research shows the pathogen prefers dry soils with high porosity.

Management

A strategy based on integrated control methods is likely to be the most effective:

Diversify the rotation. Include non-host crops or extend the break between the most susceptible hosts. Grow oilseed rape no more than 1 in 4

Remove or destroy volunteer plants and weeds to reduce source and survival of inoculum

Select varieties with high seed viability and vigour as these are at lower risk of damping off. No oilseed rape varieties carry resistance to R. solani AG2-1

Increase seed rates, especially for later sowings in higher-risk situations.

Seed treatment

Thiram-based seed treatments provided some control. However, sales of products based on this active ingredient are no longer permitted and products had to be used up by January 30, 2020.

The ICAROS research project aimed to provide new effective seed treatments that can be made available to growers to protect against R solani. A key achievement has been the identification and validation of the effectiveness of novel chemistry based on succinate dehydrogenase inhibitors (SDHIs), in field, for the benefit of the UK oil seed rape production industry.

Project outcomes

1) The project has led to a wide range of discoveries including the complex relationship between R solani and the soil environment, and the effect on OSR including, for example, delay in flowering and synchrony, with evidence that the new treatment will promote synchronous flowering which will provide a further benefit for producers that adopt it.

2) Candidate genes for improved resistance varietal improvement of oilseed rape for crop production to be pursued with breeding companies.

3) A new seed treatment product that is expected to lead to improved oilseed rape yield and cost savings over the longer-term.

AHDB guidelines for the management of disease in oilseed rape are available online https://ahdb.org.uk/rhizoctonia

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