Sub clover root rot affecting productivity in the South West’s high rainfall zone

If your sub clover isn’t germinating or seems to be dropping out of your pasture sward during winter, there’s a fair chance it’s suffering from root disease.

Colloquially known as root rot, soil-borne fungal pathogens dominate most Mediterranean ecosystems and pose a serious risk to self-seeding pasture legumes such as sub clover.

While the presence of sub clover root rot has been known in the South West for decades, a 2019 survey of root systems provided a reminder of its significance and potential role in unexplained sub clover decline.

The survey, run by South West Catchments Council (SWCC) in partnership with Western Beef Association Inc., sampled about 400 sub clover plants across 21 farms, primarily to assess nodulation. While nodule health was disappointing with 80% of sites having less than adequate nodulation, the degree of root rot was equally disturbing.

One person not surprised by the presence of disease was University or Western Australia Professor Martin Barbetti, a world expert on root rot in pasture legumes. Professor Barbetti reviewed 43 photos of individual plant root systems collected in the survey and characterised two-thirds as having severe root rot.

“Root rot is a complex of several diseases, the main ones being Aphanomyces, Pythium, Rhizoctonia and Phytophthora,” Professor Barbetti said. “Pre-emergent and post-emergent damping off can result in the loss of 90 percent of seedlings.”

While plant mortality is an extreme outcome, root rot also greatly reduces nodulation, plant growth and nutritional value of sub clover.

“Where disease is most apparent, nodulation tends to be poorest. Trials that have successfully treated disease have also invariably improved nodulation.”

There was a slight trend to this effect in the SWCC survey, with the most severely diseased plants having the least amount of nodulation. However, other factors such as soil pH and low molybdenum levels, which were likewise evident in the survey can also affect nodulation.

The greatest reduction in plant growth occurs during winter and is typically associated with tap root rot.

“Disease on the tap root means it is very severe, often resulting in complete collapse of the root system below the lesion. Disease on lateral roots can also be severe, but these roots can be replaced if conditions become warmer in spring.”

 Under good growing conditions later in the season, plants are able to resist disease by growing new lateral roots. As growth improves in spring, less affected roots can grow away from the shallow organic layer where disease is concentrated.

The importance of root growth means that good grazing management and nutrition can help the plant withstand disease. The use of organic amendments that increases microbial activity could also increase competition for some fungal pathogens and supress their effect.

In contrast, when growing conditions for plants are poor in winter, root rot is often most severe. Perhaps apart from Rhizoctonia, common pathogens are all strongly favoured by wet soil conditions in the higher rainfall zone (700-1000 mm). While the pathogens can tolerate a range of soil temperatures, some studies showed root rot was most severe at colder soil temperatures in late autumn and throughout winter.

Monitoring sub clover density and health during winter can help identify the presence of disease. Plants with severely damaged root systems often show signs of stress such as stunted growth with yellowing or red-purple foliage, and stressed plants can occur amongst healthy-looking plants. These signs can be symptomatic of other stresses such as phosphorus deficiency, so digging up and washing plant roots will provide a more reliable indication of disease, commonly seen as brown or stubby laterals or damaged tap roots.

Management of root rot is complicated by the presence of multiple diseases with an unpredictable nature.

“Disease tends to vary from year to year and from site to site, sometimes occurring as patches in a paddock and sometimes in one paddock and not another. And the fact that it is a disease complex of several diseases, fungicides typically don’t work.”

Perhaps the best management approach is to use resistant varieties of sub clover. Varieties with resistance to individual pathogens have been identified, but the challenge is to find and develop varieties with good resistance across the suite of soilborne pathogens. Professor Barbetti said recent studies to identify varieties with field tolerance to the soilborne pathogen complex are very encouraging.

“A resistant or tolerant variety can increase productivity by four to five times due to increased seedling emergence, survival and plant productivity.”

Another treatment that can help is cultivation.

“Pathogens don’t like cultivation. Less compaction (evident in the survey) also leads to faster root growth which beats disease as they grow away.

“Cultivation to 10 cm can see a big response. There may even be a response to shallower cultivation. The more you mix the soil the better. A disc seeder hardly has any effect.”

Professor Barbetti said that cultivation isn’t a long-term solution because disease typically returns within four to ten years. However, farmers with a cropping program have successfully incorporated cultivation as an effective break after losses from disease in the pasture phase.

There is also a risk of soil erosion with cultivation, which will reduce soil fertility and the plants ability to resist disease. However, in some situations, cultivation could be combined with liming to treat soil acidity below the surface, which is another constraint in the South West.

So as winter approaches and we await the emergence of sub clover, keep an eye out to see if you’re getting the expected productivity. Any shortfall might be caused by root disease and can be checked by digging up plants and washing away the soil.

This project is supported by South West Catchments Council and Western Beef Association Inc, through funding from the Australian Government’s National Landcare Program.