Rhizoctonia root and crown rot (RRCR), caused by the soilborne fungal pathogen Rhizoctonia solani, is commonly found worldwide. In Nebraska, it is the most widespread, consistently damaging sugar beet disease, and has been problematic for producers for more than 100 years.

Due to its presence in the soil, once R. solani becomes established, it is very difficult, if not impossible to eradicate. Thus managing it effectively before severe damage occurs is also very challenging. One of the methods we have used for managing RRCR is a predictive technique, referred to as the soil disease index.

The “disease index” is a pre-plant soil test that was developed by the plant pathology lab at the University of Nebraska-Lincoln Panhandle Research and Extension Center. This test is conducted in the greenhouse utilizing soils to be planted to sugar beets the following spring, with the purpose of identifying and estimating relative pathogen populations in the soil and predicting the potential for root disease problems caused by these same soilborne pathogens later in the season.

We have the ability to 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.

The soil samples should be taken from the upper 4- to 6-inch depth from multiple locations within a field to give a better representation of the entire field, similar to samples taken for fertility analysis. The collected samples are brought to the PHREC plant pathology diagnostic lab in Scottsbluff, and are planted with a susceptible cultivar and maintained for four weeks.

Seedlings are observed daily, and pathogens identified after symptoms appear. 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.

DISEASE INDEX VALUES

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.

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.

CONCLUSIONS

In summary, 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 those low-risk fields. Based on our results, we would anticipate that no action would be required for low-risk fields, thereby saving the cost of any unnecessary treatment.

More importantly, our studies suggest that the information obtained from the tests will assist growers with making management decisions based the disease index predictions. Therefore we will continue this service for as long as there is an interest in using it.