Rhizoctonia oryzae and Rhizoctonia solani AG-2-1 have been identified as the most common groups of Rhizoctonia in U.S. wheat fields, while Rhizoctonia solani AG-8 appears isolated to the Pacific Northwest (PNW), according to a Syngenta-sponsored study.
"We were surprised to find that AG-8 is not found outside the Pacific Northwest," commented Tim Paulitz, Ph.D., research plant pathologist, USDA-ARS, Washington State University. "We were also surprised to find that Rhizoctonia oryzae is so widespread and can be quite virulent."
Syngenta partnered with Paulitz and his research associate, Kurt Schroeder, Ph.D., on a two-year project that involved collecting and analyzing soil samples from across the country to categorize various species of Rhizoctonia and their pathogenicity on wheat and barley.
Over the years, Rhizoctonia research primarily focused on the PNW and the most damaging group in cereals, AG-8, which causes classic bare patches and up to 50 percent stunting in wheat. Through this new study, Paulitz and Schroeder learned that AG-8 is only found in the PNW and typically associated with sandier, coarser soil types.
In cereal crops, Rhizoctonia destroys root cells, providing a habitat in which the fungus can thrive. Later, the disease can result in crop stunting, distinct bare patches in fields and uneven stands, which also mean yield loss. As with many soilborne pathogens, Rhizoctonia nibbles away at entire root systems and impedes the crop's ability to utilize water and nutrient resources.
Its strains are classified into anastomosis groups, or AGs, based on the ability of the pathogen's hyphae to fuse with tester strains in the lab, exchange genetic information and reproduce. Hyphae are microscopic threads that make up the body of the fungus.
"There is a diversity of AGs; there can be as many as four AGs in any given soil. Part of our mapping project was to tease out which AGs are present and how much of the fungus it takes to have an impact," Paulitz explained.
To date, 79 total different isolates from 51 locations have been sequenced, with identification pending on six. Results through October 2011 reveal the following geographical breakouts of the different Rhizoctonia groups:
· Rhizoctonia oryzae: 28 isolates found in Arizona, Colorado, Idaho, Kansas, Kentucky, Michigan, Minnesota, Montana, North Carolina, North Dakota, South Dakota and Washington.
· Rhizoctonia solani AG-2-1: 13 isolates found in California, Idaho, Kansas, Montana, North Dakota and Washington. It previously was thought to be more restricted to Idaho, Montana, North Dakota and Washington.
· Rhizoctonia solani AG-2-2: 1 isolate found in South Carolina.
· Rhizoctonia solani AG-3: 3 isolates found in Oklahoma and Washington.
· Rhizoctonia solani AG-4: 13 isolates found in Colorado, North Carolina and South Carolina.
· Rhizoctonia solani AG-8: 2 isolates found in Washington.
· Rhizoctonia solani AG-10: 2 isolates found in Washington.
· Rhizoctonia solani AG-11: 3 isolates found in Arizona and Montana.
· Rhizoctonia solani AG-13: 1 isolate found in North Carolina.
· Ceratobasidium spp. AG-A: 1 isolate found in Montana.
· Ceratobasidium spp. AG-E: 1 isolate found in South Carolina.
· Ceratobasidium sp. AG-F: 2 isolates found in North Carolina.
· Ceratobasidium spp. AG-I: 6 isolates found in Kentucky, North Dakota and Oklahoma.
· Waitea circinata var. zeae: 3 isolates found in North Carolina.
Because many AGs have a wide host range, crop rotations appear to have minimal impact on prevalence or control. However, according to Paulitz, tillage practices may affect the pathogen's prevalence but primarily when transitioning from tillage to direct-seeding systems. Research shows that the transitional effect, if there is any at all, will increase the disease early on, but populations will drop back as the ground acclimates to no-till.
Translating research into management recommendations
According to Paulitz and Schroeder, mapping Rhizoctonia strains across the U.S. will ultimately help establish tools for cereal growers to assess and determine the risk of disease before planting.
"The idea is to develop risk levels based on how much Rhizoctonia is present. Growers can then gauge that before planting and make decisions on how they plant or how they plan to manage that problem. Based on prior knowledge of how much Rhizoctonia is in the field, growers can make educated decisions going forward," Schroeder explained.
The best way to confirm Rhizoctonia presence, according to Paulitz, is to send a soil sample to a university or research clinic for testing. And because Rhizoctonia-resistant varieties are unavailable to cereal growers, Paulitz and Schroeder recommend both cultural and chemical control strategies, such as reducing the chance of green bridge from overwintering weeds by increasing the interval between burndown herbicide spraying and planting.
"If you spray out shortly before planting the new crop, the pathogen 'green bridges' or jumps from the dying plant onto the new planted crop. With an increased interval from a few days or a week to two to three weeks or more, populations of Rhizoctonia will have time to die out or decline before seeding the new crop," Schroeder explained.
Seed treatments are also effective in helping to prevent disease problems. "We've done a lot of work with seed treatments, and they certainly benefit seedlings," Paulitz concluded.
"To help reduce Rhizoctonia damage, Syngenta currently offers cereal growers Dividend Extreme seed treatment fungicide," said Chad Shelton, Seedcare asset lead, Syngenta. "It protects against more than 16 seedborne and soilborne diseases, including Fusarium and Pythium, as well as Rhizoctonia. For the on-farm application market, Syngenta features CruiserMaxx Cereals insecticide/fungicide seed treatment to protect against wireworms and diseases at the same time.
"Syngenta is also testing a new, experimental seed treatment fungicide anticipating registration in 2012 that will offer a novel mode of action to enhance protection against Rhizoctonia," Shelton noted.
Methodology of mapping
To execute the project, Paulitz and Schroeder analyzed soil samples collected from 143 wheat and barley fields in 19 states, including Washington, California, Montana, North Dakota, Kansas and the Carolinas, among others. The samples represented the different classes of wheat: soft white, durum, hard red spring, hard red winter and soft red winter.
Using a toothpick baiting method, they obtained Rhizoctonia isolates and identified their species by sequencing the internal transcribed spacer (ITS) region of the rDNA and comparing the new findings with previously identified strains. This method enabled Paulitz and Schroeder to identify and calculate which species of Rhizoctonia live in the soils they sampled.
"We copy out that section [unique to each group] and amplify it. The next step is to determine the sequence of that strand. That's what gives us the information about what species we have. Each group has a unique sequence, or fingerprint, to it," Paulitz said.
Paulitz and Schroeder will likely publish the study results in 2012.