Rhizoctonia in oilseed rape

Pathogen

Rhizoctonia solani

Thanatephorus cucumeris (sexual stage)

Hosts

Oilseed rape, broccoli, cabbage, cauliflower, peas, beans, radish, wheat, barley, potatoes and many other crops and weeds.

This web page focuses on oilseed rape but many of the points raised are valid in other host plants.

Symptoms

Rhizoctonia solani is an aggressive, ubiquitous soil-borne pathogen of crops worldwide. Crops are most susceptible to the pathogen during the seedling stage. Infection can result in seed decay, as well as pre- and post-emergence damping off. Early symptoms include thinning and elongation of stems, known as ‘wirestem’, and ‘spear heading’, where roots are severed at the point of infection. The pathogen can also cause root and stem rots on mature plants. Symptoms on mature plants include brown, necrotic lesions, which may girdle the root or stem circumference. Other soil-borne fungi can also cause these or similar symptoms, making accurate diagnosis a challenge.

Life cycle

R. solani is a species complex that comprises 13 anastomosis groups (AGs). Pathogen isolates are allocated to a group based on their ability to fuse (i.e. anastomose) with each other. The groups indicate how genetically related the isolates are. Subgroups also exist, based on their host specificity, biology and genetics. Isolates belonging to each anastomosis group and subgroup are able to infect a diverse range of crops.

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

As oilseed rape grows, it releases substances (exudates) that stimulate the production of runner hyphae, from germinated sclerotia or overwintering mycelia. Soil characteristics, such as bulk density and soil porosity, play a role in the ability of the fungus to spread and reach the host. Once the host is reached, the pathogen forms an infection peg and then produces digestive enyzmes to degrade plant cell walls, so that primary infection can occur.

Plants become more tolerant to infections as they mature. Older mycelia form sclerotia on infected tissue. Where tissue is left in/on the soil (i.e. crop debris), it can become a source of infection in following crops.

Importance

R. solani is an important soil-borne pathogen globally. In addition to establishment losses, the pathogen also affects crop development and can result in delayed maturation and uneven flowering across the field.

AGs 2-1, 4 and 8 are the most pathogenic to oilseed rape. In particular, AG2-1 isolates are highly aggressive, and cause infections on the hypocotyl and tap root. Isolates can also reduce the number of primary roots, root volume and root surface area.

AG 2-1 isolates are the most commonly found in English soils. They increase in concentration when oilseed rape is grown in close rotations. Establishment losses of 17–65% have been reported. Under high inoculum density, AG2-1 isolates can reduce yield by up to 50%.

AG 4 isolates are the primary cause of stem and root rots in mature plants.

Risk factors

  • High inoculum density of the pathogen in soil or crop residues
  • Frequent (short) rotations of host crops
  • Fields with a history of damping off or emergence problems
  • Direct drilling or reduced cultivation systems
  • Deep sowing (below 6cm)
  • Presence of weeds or volunteers that harbour the pathogen
  • Low seed viability or crop vigour (as crop remains in susceptible early growth stages for longer)
  • Sandy loam soils, low-to-medium soil moisture levels and warm soils increase risk*

*AHDB-funded research shows that the pathogen prefers high soil porosity and low-to-medium soil moisture. Under very dry or wet conditions the pathogen stops colonising the soil. Under these conditions, plants suffer but this is not because of the disease. Conditions that favour plant growth, favour the pathogen too.

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 1 in 4 or 1 in 5 years. Remember: wheat, peas/beans and potatoes can also harbour AG2-1, 4 and 8
  • Cultivate to disturb hyphal networks – plough or deep till down to 8–10 cm
  • Remove or destroy volunteer plants and weeds – to reduce source and survival of inoculum
  • Select more tolerant varieties – no oilseed rape varieties carry resistance against R. solani AG2-1. However, varieties with high seed viability and vigour are more able to survive attack
  • Increase seed rates – especially for later sowings in higher-risk situations
  • Consider seed treatment – Thiram-based seed treatments provided some control. However, sale of thiram seed treatment is no longer permitted and treated seeds must be used up by 30 January 2020. Research trials have established the potential of other active ingredients but these are not currently authorised for use in oilseed rape

Inoculated with Rhizoctonia solani AG2–1, this untreated winter oilseed rape trial plot shows areas of crop loss caused by the pathogen

The secret world of rhizoctonia

New research is shedding light on the rhizoctonia pathogen which will lead to new guidelines to help growers manage the disease in OSR. CPM finds out more.

The secret world of rhizoctonia (CPM article, April 2019)