The impact and management of clubroot
13 Dec 2018
Oilseed rape is still the most popular and profitable break crop in the arable rotation, however, growing this crop is certainly not without its challenges. In this blog Julie Smith, Senior Plant Pathology Researcher at ADAS talks about the increasing problem of clubroot in this crop and the agronomic measures growers can take to alleviate its effects.
“All of the evidence suggests that clubroot is increasing in the UK and indeed across Europe. Recent warmer, wetter autumns have given a wider window where soil temperatures are above 16oC and there is adequate soil moisture, both of which are necessary for clubroot infection to occur.
Clubroot infection can result in complete crop loss or the more characteristic lost patches in the field. Under AHDB funded work, we quantified the negative impacts of disease on yield and showed that for every 10% of plants infected with clubroot, there was 0.3t/ha yield loss in susceptible varieties.
We would always advise people to pull plants from across the field to check for galling of the roots because often there are no above ground symptoms of clubroot until it is quite well established.
Soil testing can be done in the lab to confirm the presence of clubroot, either as a traditional bioassay or as a molecular diagnostic (qPCR).
Spread of Clubroot
The main cause of clubroot spread within and between fields is soil movement on machinery, although it can occur via flooding or can be introduced through infected green manures. If you believe you have clubroot in any field then the sensible course of action is to cultivate that field last to avoid going from a potentially infected field to a clean field.
Agronomic Control Measures
There are no fungicidal or biocontrol options for clubroot control, so it is essential to consider agronomic factors.
The first thing to do if you have, or suspect you have clubroot is to extend the rotation to at least one in four, as a minimum. Clubroot is a very robust pathogen. Inoculum can survive in soil for 15 years and has a half-life of approximately 4 and a half years.
There is a very strong relationship between clubroot and pH. If you can lime to increase the pH to above 7 then you have a better chance of preventing clubroot.
There is evidence that clubroot is sensitive to boron, so any boron deficiencies should be addressed.
Clubroot requires adequate soil moisture to allow its zoospores to swim through the soil water and infect the root hairs of the crop, therefore draining any low lying wet patches in fields should help with disease management.
There is evidence that this fertiliser can have a beneficial effect in reducing clubroot infection, although the cost can make it prohibitive to broad acre arable growers.
If you can delay drilling until the soil temp is below 16oC, then this can help avoid clubroot. However, there is a trade-off in terms of drilling date depending on which other pest and disease challenges are present on the farm.
Early season weed control is essential for managing clubroot.
Clubroot Resistant Varieties
Resistant varieties are the most effective control measure and by and large they do a good job. However, repeated deployment of resistant varieties in high pressure situations has resulted in the resistance being defeated and there are parts of the UK where these varieties just don’t work anymore.
All of the resistant varieties are based on the same resistance; a single dominant gene, and the resistance is not effective against all pathotypes of clubroot. Here in the UK we advise growers to sow a resistant type as part of an integrated strategy and only to grow it on moderately infected land. If significant symptoms are observed on a resistant variety, then it is not advisable to grow any Brassica species in the field for at least 5 years.
Plant Growth Regulators
It is possible that a plant growth regulator, such as those containing metconazole, which enhances rooting at depth, may give the plant an advantage because the roots might be able to escape or tolerate clubroot infection better than a plant that hadn’t received the treatment.”