Source: Syntenta Seed Care, Seeds of Progress Newsletter, Fall 2011
Above ground, the consequence is bare patches scattered through the field. Underground, this pathogen attacks roots, producing brown lesions along roots or shortened roots 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).
The PNW suffers from soil-borne diseases, such as Rhizoctonia, more than any other region in the country because of favorable soil conditions, ideal temperatures and increased use of reduced tillage practices. Due to soil erosion and the need for improved soil structure and organic matter, growers in the PNW often implement direct-seed and no-tillage practices. Unfortunately, no-tillage often creates a soil environment where Rhizoctonia thrives.
Another driver of Rhizoctonia incidence is temperature. The fungal pathogen thrives in cool, moist conditions, anywhere between 10 and 15 degrees (C), according to Dr. Tim Paulitz, plant pathologist at Washington State University.
The symptoms of this chronic root disease include brown lesions along the roots and shortened roots with darkened tips. The fungal pathogen is often recognized by distinct bare patches in the fields where wheat or barely was stunted or killed off. Often, Rhizoctonia is mistaken for other soil-borne diseases due to similar symptoms, such as darkened lesions along the roots. The best way to know for sure, according to Dr. Paulitz, is to send a sample to a university or research clinic for testing.
"Rhizoctonia infects the entire root system," Dr. Paulitz explained. "The pathogen hides out in dead roots. It forms a mycelium that's thick-walled, 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 the root system, 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, producers may experience reduced yields.
Anastomosis Groups (AG-groups) Explained
To better understand this commonly mistaken disease, Syngenta and Dr. Paulitz are collaborating to map the occurrence of Rhizoctonia across the United States, as well as identify the different AG-groups, or anastomosis groups, present in soils. "In our studies, we're finding certain groups are more prevalent in certain soil types or geographies," Dr. Paulitz explained.
AG-groups are categorized on a level of 1-13 (and that number continues to rise as more groups are discovered) to differentiate the various strains of Rhizoctonia. The AG-groups allow scientists to understand the interactions of these strains and the potential impact on the crop. Essentially, an AG-group is a tool to divide all different types of Rhizoctonia.
Dr. Paulitz noted that Rhizoctonia solani AG-8 is the primary cause of root rot, bare patches and yield loss in direct-seeded wheat. "Different groups cause different diseases and have different host ranges; they're almost like distinct species," Dr. Paulitz said.
How two sample strains of the fungal pathogen interact with one another is indicative of their compatibility, or their ability to exchange genetic information and reproduce. For example, Dr. Paulitz reported that AG-8 would be compatible with other AG-8 groups but may not be compatible with an AG2-1 group.
Regardless of compatibility, it is critical to monitor and observe fields for symptoms of Rhizoctonia to prevent it from invading the following growing season. Dr. Paulitz stressed the importance of 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," Dr. Paulitz noted. "Seed treatments are important in preventing Rhizoctonia and other soil-borne pathogens, like smuts and bunts, in wheat. It's a relatively small cost compared to other inputs."
While there are no Rhizoctonia-resistant varieties available to cereal growers, Dr. Paulitz said planting a fresh, clean, certified seed variety may be more beneficial than planting an older variety. The older the seed, he said, the more susceptible it may be to certain soil-borne diseases.
And, while direct-seed and no-till systems may cause the pathogen to activate in the short-term, it may be worthwhile to be patient and let nature run its course. According to Dr. Paulitz, growers who no-till and direct-seed their cereal crop may notice successful results in the long run.
"There appears to be a natural suppression of Rhizoctonia that happens after a number of years of direct-seeding. The Rhizoctonia pathogen appears to happen early on in the conversion from conventional to direct-seeding, but then after that, the disease declines. We've documented that with a number of direct-seed growers," he explained.
Syngenta also is conducting studies to develop effective, yet sustainable ways to protect cereal crops from such pathogens. Currently, Syngenta is researching and testing various fungicide seed treatment chemistries to help growers better manage soil-borne fungal pathogens like Rhizoctonia.
"We're excited to partner with Dr. Paulitz on his Rhizoctonia mapping project. The results of this important research will help build awareness around this relatively unaddressed challenge in cereal crops," said Chad Shelton, asset lead, Syngenta.
"Syngenta is committed to remaining at the forefront of seed treatment chemistry research by providing the most valuable pest management solutions for growers," Shelton said. "In wheat, barley and other crops, we're currently testing a new seed treatment fungicide to enhance protection already provided by products such as Dividend Extreme® seed treatment fungicide, particularly protection against Rhizoctonia and true loose smut."
Armed with a deeper understanding of the impact unseen diseases like Rhizoctonia can have on a crop, growers can more effectively protect seeds and young seedlings. A stronger defense at the beginning will help generate healthier roots, more vigorous crops and, ultimately, better yield stability in the field.
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