Above ground, bare patches are scattered through the field. Underground, roots have brown lesions or are shortened with darkened tips. Capable of causing 20 to 40 percent yield loss, this antagonist is called Rhizoctonia and is a common soil disease in cereal fields around the world, throughout the United States and particularly in the Pacific Northwest (PNW). As you meet with growers this fall, remind them of root diseases like Rhizoctonia that lurk below the surface and help to provide viable solutions to control the pathogens in time.
Favorable soil conditions, ideal temperatures and increased use of reduced tillage practices make the PNW a hotspot for soilborne diseases like Rhizoctonia. Due to soil erosion and the need for improved soil structure and organic matter, direct-seed and no-till practices are common. Unfortunately, no-till often creates a soil environment where Rhizoctonia thrives.
According to Tim Paulitz,Ph.D.,research plant pathologist with the USDA-Agricultural Research Service, the fungal pathogen thrives in cool, moist conditions, between 50 and 60 degrees Fahrenheit. Symptoms include brown lesions along roots and shortened roots with darkened tips. It is often recognized by distinct bare patches in fields where wheat or barley was stunted or killed off. To complicate diagnosis, Rhizoctonia is commonly mistaken for other soilborne diseases with similar symptoms. Paulitz recommends sending a sample to a university or research clinic for an accurate diagnosis.
“Rhizoctonia infects the entire root system," Paulitz explains. “The pathogen hides out in dead roots. It forms a thick-walled mycelium, so it sits in the soil, surviving and waiting. When the pathogen senses the root growing, and the soil’s moisture level and temperature have become favorable, 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, the root cortex – the pathogen invades the entire root system."
In addition to damaging roots, Rhizoctonia attacks young seedlings, impairing their ability to absorb water and nutrients, which reduces emergence and stunts growth.
Anastomosis Groups (AG-groups) Explained
To better understand this commonly mistaken disease, Syngenta and Paulitz are collaborating to map the occurrence of Rhizoctonia across the United States and identify the different anastomosis groups (AG-groups) present in soils.
AG-groups are classifications that categorize various strains of Rhizoctonia. These groups help scientists understand interactions between strains and their potential impact on the crop. How two strains of Rhizoctonia interact indicates their compatibility, or ability to exchange genetic information and reproduce.
“Different groups cause different diseases and have different host ranges; they’re almost like distinct species," Paulitz says. “We know that Rhizoctonia solani AG-8is the primary cause of root rot, bare patches and yield loss in direct-seeded wheat. It would be compatible with other AG-8 groups but may not be compatible with an AG2-1 group."
It is critical to monitor and observe fields for symptoms of Rhizoctonia to manage it the following growing season. Paulitz stresses understanding the history of the field, maintaining awareness of the disease and implementing both cultural and chemical management tactics.
“Apply some starter fertilizer in the furrow at the time of planting to help the seedling pick up nutrients, even if some of the roots are nibbled away by Rhizoctonia," he says. “Seed treatments are important in preventing Rhizoctonia and other soilborne pathogens in wheat. It’s a relatively small cost compared to other inputs."
While there are no Rhizoctonia-resistant cereals varieties available, Paulitz says planting a fresh, clean, certified seed variety may be better than planting an older variety. The older the seed, the more susceptible it may be to soilborne diseases.
And, while direct-seed and no-till systems may activate the pathogen in the short-term, it may not be an issue forever. “There appears to be a natural suppression of Rhizoctonia after a number of years of direct-seeding. We’ve documented that it appears early in the conversion from conventional to direct-seeding, but then the disease declines," he explains.
Syngenta is also conducting studies to develop effective ways to protect cereal crops from such pathogens. Currently, Syngenta is researching and testing fungicide seed treatment chemistries to help growers better manage soilborne fungal pathogens like Rhizoctonia.
“We’re excited to partner with Paulitz on his Rhizoctonia mapping project. The results of this important research will help build awareness around this relatively unaddressed challenge in cereals," says Kiran Shetty, crop expert, Syngenta.
“Syngenta is committed to remaining at the forefront of seed treatment chemistry research by providing the most valuable pest management solutions for growers," Shetty says. “In wheat, barley and other crops, we’re currently testing a new seed treatment fungicide to enhance protection already provided by products such as