Rhizoctonia Root Rot: What is it and 5 Tips on How to deal with it.

It has been another excellent season for Rhizoctonia patches to show up and hold up our crops. If you haven’t seen this before, it shows up as patches in the paddock where plants are visibly struggling and far behind the rest of the crop. Below we’ll explain how the fungus affects our crops, and what the best management practices are.

What is it?

Rhizoctonia root rot is a disease of cereals caused by the Rhizoctonia solani AG8. Basically, it is a fungus that grows on crop residues and soil organic matter and is adapted to dry conditions and lower fertility soils. It generally occurs in the top 0-5 cm of soil on decaying crop residue. The fungus causes damage to cereal crops by pruning newly emerged roots. This can happen anytime from emergence to crop maturity and can cause yield losses over 50% in the bare patches. The Rhizoctonia infection in the soil also results in water and nutrient stress. The shortened roots are compromised in their ability to translocate both moisture and nutrients. Rhizoctonia inoculum levels will increase on the roots of living host plants and will survive on stubble and crop residues. Their numbers may reduce as moisture assists with stubble breakdown and promotes the growth of other soil organisms that suppress Rhizoctonia. With no resistant cereal varieties and a wide range of hosts, Rhizoctonia can build up quickly and become difficult to control.

How do we deal with it?

There are a few options to manage your Rhizoctonia patches. The most effective strategy to help rid your paddocks of the disease is to combine the following tips.

Tip 1: Reduce summer weed hosts.

Firstly, summer weed control is essential to reducing inoculum levels and slowing down the disease for the following winter. By being on top of summer weeds, you decrease the living host plants and stop further numbers from increasing. This also works in well with a proper summer weed management strategy and will help conserve moisture and nutrients in the soil.

Tip 2: Have a crop rotation plan.

Secondly, it is crucial to choose the right crop variety and rotate your cropping system. As we said before, cereals and grassy pastures promote the build-up of Rhizoctonia inoculum, barley being the worst of the cereals. Therefore, a rotation to a non-cereal crop that is grass-free, such as canola or legumes, is one of the best ways to lower levels. It’s important to note that non-cereal crops can also be affected by high levels of Rhizoctonia, but they will prevent further build-up of the inoculum. The benefits of a proper break crop rotation generally last for one cereal crop season.

Tip 3: Look at fungicide seed treatments.

Our next option is fungicide applied as a seed treatment. These would be used as part of an integrated management strategy as fungicides will only suppress the damage. Responses to seed treatment seem to be higher in barley than in wheat, and these yield responses will vary between seasons. The greatest responses appear to occur when spring rainfall is above average. To find out what fungicide will work best in your situation, get in contact with our agronomists.

Tip 4: Look into tillage.

Tillage and cultivation are good ways to break up the Rhizoctonia fungus network in the top 10cm of soil. By disturbing the soil below the seeding depth, it helps plant roots escape infection and reduces the impact of the disease. The best tillage practices involve deep cultivation and shallow sowing, with minimal time between each event. It has been shown that the disease is higher in disc seeder operations than when knife points are used. In paddocks where the disease is known to be, or is suspected of being present, it is recommended that knife points are cleaned once seeding is complete, in order to help eliminate the spread of the fungus.

Tip 5: Give the crop a good boost of N.

Finally, it may help to feed the affected paddocks a good amount of nitrogen. The added help from nitrogen and other nutrients will help plants get their roots through and past the Rhizoctonia zones. Crops with adequate nitrogen levels will be less affected by the disease.

For more information, please give one of our agronomists a call.

Check out the table below from GRDC on their recommendations, and check out their fact sheet: www.grdc.com.au/TT-Rhizoctonia

Picture 1: Example of Rhizoctonia patches in a cereal crop.