Rhizoctonia spp. is a growing concern in Western Canada because of its potential impact on yields but perhaps more importantly, due to its effect on root health and its capacity to destroy a seedling before the plant even has a chance to emerge. Until recently, the relationship between rhizoctonia, root and seedling health and yields was not widely recognized.
Capable of causing 20 to 40 per cent yield reductions, Rhizoctonia is a common soil disease around the world. Western Canadian crops suffer from rhizoctonia because of favourable soil conditions, ideal temperatures and the increasing use of reduced tillage practices on the Prairies. Rhizoctonia thrives in cool, moist early-season soil conditions anywhere between 10 C and 15 C followed by warm, dry periods, and is most prevalent in areas where less than 300 millimetres of rain typically falls in a season.
In wheat, rhizoctonia causes pre-and post-emergent damping off and shows up in outer leaves on newly emerged plants. Surveys have shown the threat is likely under-diagnosed as farmers often misattribute rhizoctonia-related symptoms to other causes. More severe under no-till or direct-seeding methods, it is often mistaken for drought damage, wireworm infestations or nutrient deficiencies. Above ground, the consequence is bare patches scattered through the field. Below, the pathogen attacks roots, producing brown lesions or shortened roots with darkened tips.
ANATOMY OF THE DISEASE
Rhizoctonia survives in the soil as mycelium, when two mycelia come into contact, they fuse, exchanging nuclei and becoming a new rhizoctonia fungus. They will only fuse, however, if they are related if they belong to the same Anastomosis Group (AG).
Anastomosis Groups are categorized into levels 1 through 13. Each strain affects different plant seeds in different ways. Research currently underway is showing that certain AG strains originally thought to adversely affect only one crop type are actually negatively impacting other crops types. This fact heightens the need for accurate disease identification and prevention and control activities.
Rhizoctonia infects the entire root system. The pathogen hides in dead roots. It forms a mycelium that is thick-walled and sits in the soil, surviving and waiting. When the pathogen senses the root growing, and the soil s moisture level and temperature have become favourable, the mycelium or hyphae will contact the root, stimulate it to grow, and then penetrate. Once the pathogen infiltrates, it kills the root tissue, the root tips and the root cortex invading the entire root system.
In addition to damaging the roots, rhizoctonia also attacks young seedlings, impairing their ability to absorb water and nutrients and consequently constricts proper emergence and stunts plant growth. As a result, growers may experience reduced yields, even when stands are not visibly diminished.
The key to rhizoctonia prevention is field monitoring. Because rhizoctonia is often mistaken for other soil-borne diseases due to similar symptoms. The best way to know for sure is to send tissue samples from suspected plants to a university or research clinic for testing.
Seed treatments are important in preventing rhizoctonia and other soil-borne pathogens particularly where field history, environmental conditions or cultural practices indicate a possible threat. And, while there are no rhizoctonia- resistant varieties available to cereal growers, planting a fresh, clean, certified seed variety may be more beneficial than planting an older variety. The older the seed, the more susceptible it may be to certain soil-borne diseases.
Armed with a deeper understanding of the impact invisible threats such as rhizoctonia can have on a crop, farmers can protect seeds and young seedlings more effectively. A stronger defense at the beginning of the season will help generate healthier roots, more vigorous crops and, ultimately, better yields at harvest.