Rhizoctonia solani attacks the stem and roots of young seedlings of many crops, including oilseed rape. Damage can lead to poor emergence and significant bare patches within the crop.
Thiram-based seed treatments provided some control, but such products have recently been withdrawn and must not be used after 30 January 2020. AHDB has co-funded research into this disease for the last three years. Due to conclude shortly, one of the project’s outputs has been to produce information about the disease and its management: ahdb.org.uk/rhizoctonia
Due later this year, the final report will also include information about alternative seed treatments and varietal tolerance.
So, what are the symptoms? 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. Other soilborne pathogens can cause similar symptoms, making accurate diagnosis a challenge.
R.solani is a species complex that comprises 14 anastomosis groups (AGs). Pathogenic isolates are allocated to a group based on their ability to fuse (i.e. 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 can survive on crop debris for long periods, and on crop volunteers and weeds.
Oilseed rape releases chemicals (exudates) 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 (e.g. 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.
Globally, R. solani is a common soilborne crop pathogen. AGs 2-1, 4 and 8 are most pathogenic to oilseed rape. AG 2-1 isolates are prevalent in soils in England. The pathogen is associated with establishment losses of 17–65%. 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.
The risk factors are:
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 an 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
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
Plough or deep till down to 8–10 cm to disturb hyphal networks and reduce inoculum
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
This information has been produced as part of one of twenty-four innovative agri-tech projects. These were awarded £16 million through the government’s Agri-Tech Catalyst. This project ‘Integrating control strategies against soilborne Rhizoctonia solani in oilseed rape’ was awarded £619,000 and included £80,000 from AHDB. The project was delivered by Syngenta and the University of Nottingham.