By ROBERT M. HARVESON
Extension Plant Pathologist
University of Nebraska, Panhandle REC – Scottsbluff
For more than 20 years the UNL Panhandle Research and Extension Center’s Plant Pathology Program has been using a soil index test as a forecasting tool for sugarbeet root rot diseases in Nebraska. A recent conversation revealed that some sugarbeet producers are not aware of this service, so for veteran and newer growers, as well as the public, here is a description of the program.
Disease Index
The disease index is a pre-plant soil test that was developed and utilized by the plant pathology program at the Panhandle Center. This test is conducted in the greenhouse and designed to test soils that will be planted to sugarbeets the following spring.
Since several common soilborne sugarbeet pathogens can cause both seedling and root rot disease, the primary purpose of the test is to identify and estimate relative populations in the soil and then predict the potential for root disease problems caused by these same pathogens later in the 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 as of December 2021 have analyzed well over 4,000 (4,266) soil samples, each representing one sugarbeet field.
Disease Index Methodology
The soil samples should be taken from the upper 4-6 inches of depth from multiple locations within a field and combined into one sample. This gives a better representation of the entire field, like samples taken for fertility analysis.
The collected samples are brought to the Panhandle REC plant pathology diagnostic lab, mixed thoroughly, planted with a susceptible cultivar, and maintained for four weeks. Seedlings are observed daily, and pathogens are identified as symptoms appear and seedlings begin to die. An index was developed based on the time period during the 30-day test that seedlings became infected and was calculated on a 0-100 scale.
We have also empirically 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 a low risk.
Comparing Disease Index Values with Yield Results
To validate our concept, pre-plant index values from 108 fields over a five-year period were compared with yields obtained from those 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. For instance, after further analysis with linear regression of the data, our test showed that for each single numerical 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 revealed.
Bottom line: helping growers make decisions
The take-home message for this work is that we feel that the disease index can accurately predict root disease potential, particularly Rhizoctonia root and crown rot. Another example of the benefit for this test to consider is low-risk fields. Based on test results, no action would be recommended for low-risk fields, thereby saving the cost of any unnecessary treatment.
More importantly, studies suggest that the information obtained from the tests will assist growers with making management decisions based on the disease index predictions. Therefore, we will continue this service for as long as there is an interest in using it.
The cost for this service is $35 per sample. For any questions please contact research technicians Allison Rickey or Tyler Patrick (308) 632-1230 or Bob Harveson (308) 631-5953.