By Bob Harveson, Extension plant pathologist
Rhizoctonia root and crown rot (RRCR), caused by the soilborne fungal pathogen Rhizoctonia solani, is commonly found worldwide. In Nebraska, it is widespread and the most consistently damaging disease in sugar beets. It has been problematic for Nebraska producers for more than 100 years.
Due to its presence in the soil, once R. solani becomes established, it is difficult, if not impossible to eradicate. Thus, managing it effectively before severe damage occurs is also challenging. One of the methods we have used to manage RRCR is a predictive technique referred to as the soil disease index.
The “disease index” is a pre-plant soil test developed by the plant pathology lab at the University of Nebraska-Lincoln Panhandle Research and Extension Center (PHREC). Using samples of soils where sugar beets are to be planted the following spring, a greenhouse test is conducted to identify and estimate relative pathogen populations. This information is then used to predict the potential for root disease problems during the crop season.
We can monitor for Fusarium, Aphanomyces, and Pythium; however, the test is particularly useful for estimating risk of disease problems caused by Rhizoctonia. We began this concept as a service for the growers in 2003, and to date have analyzed well over 3,600 soil samples, each representing one sugar beet field.
Disease Index Methodology
Soil samples are taken from the upper 4 to 6 inches of soil in multiple locations within a field to give a better representation of the entire field, similar to samples taken for fertility analysis. Collected samples are taken into the Panhandle Research and Extension Center's Plant Pathology Diagnostic Lab in Scottsbluff, where they are planted with a susceptible sugar beet cultivar and maintained for four weeks.
Seedlings are observed daily, and pathogens are identified after symptoms appear. An index based on when the seedlings become infected during the 30-day test was calculated on a 0-100 scale. We have designed a risk-assessment system (high, medium, and low) for each tested field based on the disease index values obtained from the soil assay. We consider an index value of 30-45 to represent a moderate risk of disease problems from these pathogens later in the season. Anything above 45 would represent a high risk, while any values below 30 would be considered to pose a low risk.
Comparing Disease Index Values with Yield Results
To validate our concept, pre-plant index values were compared with yields obtained from the same fields after harvest. Results revealed a strong inverse relationship between the pre-plant disease index values and sucrose and root yields, but not sucrose percentage. This means that the fields with higher disease index values also resulted in lower root yields and total sugar per acre.
As an example, after further analysis with linear regression of the data our test showed that for each single unit increase in the disease index, a corresponding decrease of 0.12 tons (240 pounds) per acre and 44 pounds sucrose per acre, respectively, was observed.
In summary, we feel the disease index can accurately predict root disease potential, particularly of Rhizoctonia root and crown rot. Another benefit is the identification of low-risk fields, where action would likely not be needed, saving the cost of an unnecessary treatment.
More importantly, our studies suggest that information from the tests will help growers make decisions based on the disease index predictions. We will continue this service for as long as there is an interest in using it.
Soil Testing for Sugar Beet Diseases
If you're interested in submitting a soil sample from your sugar beet fields, contact Harveson at firstname.lastname@example.org or 308-632-1239. Cost per sample is $35.