By Robert Harveson
Associate Professor
University of Nebraska - Lincoln
Department of Plant Pathology
Panhandle Research and Education Center

Soil Root Disease Index

Root diseases caused by Rhizoctonia solani and Aphanomyces cochlioides are significant problems in Nebraska sugar beet production fields. It is becoming more common to find both pathogens infesting fields and infecting crops simultaneously. The two pathogens possess several common characteristics, including the ability to cause both a seedling disease and a root rot later in the season. Additionally, both pathogens are soilborne, can survive in the soil for many years, and are favored by generally warm soil temperatures. Yet they also differ substantially. Taxonomically, they are not closely related, and Aphanomyces is much more dependent upon high levels of soil moisture than Rhizoctonia. Additionally, Rhizoctonia attacks a wider range of host plants, including dry beans, whereas Aphanomyces is limited to causing disease on plants related to sugar beets.

A number of management options are available for both diseases, including seed treatments with several fungicides, using resistant cultivars, and cultural practices like early planting and irrigation management will all contribute to reducing disease problems from these diseases. Unfortunately, no one method by itself will adequately address those situations where both pathogens are present. Thus, some type of predictive technique would be welcome for estimating potential for root disease problems later in the season, allowing greater flexibility for growers to make management decisions.

We are currently investigating a new technique begun in 2003 that attempts to estimate relative pathogen populations in the soil, and predict potential root disease problems while also identifying specific pathogens present in soil samples. Optimally, samples should be taken before planting so decisions may be made before problems occur in the field. This technique is called a disease index and entails using soil samples collected from fields in a manner like those for testing fertility prior to planting. These samples are planted with a susceptible cultivar and maintained for one month. Seedlings are observed daily, and pathogens identified after disease symptoms appear. The index is based on the time when seedlings become infected, and is calculated on a 0-100 scale. An index value of 40-65 would represent a moderate risk of disease problems later in the season. Anything above 65 would represent a high risk, while below 40 would be considered to be a low risk. We are also interested in comparing the index values from the pre-plant assays with yield information from those same fields in order to better test the predictability of this technique for later root disease infections.

The popularity of this service has increased, and our sample number has additionally increased from 44 in 2003, to approximately 150 prior to 2004. During the 2004 season, we additionally obtained yield information from 38 of these fields to compare with index values. The data from these tests are averaged by the three distinct index values and compared with average root and sugar yields.

We also have tried to quantify different concentrations of the pathogens in the greenhouse in order to better understand the index values and what they mean for potential disease development:

Aphanomyces utilized 4 treatments – control, 500, 1,000, and 2,000 overwintering structures (oospores) per gram of soil.

Rhizoctonia utilized 5 treatments – control, 0.5g, 1.0g, 2.0g, and 4.0g of infested barley seed per gram of soil as inoculum.

To summarize results, the average results from fields with index values representing the 3 major categories are very interesting. The yields obtained from 38 fields indicate a strong relationship between the disease index and resulting root and sugar yields. The average sugar yields form the 21 fields testing as low risk (<40) produced sugar yields in excess of 2500 lbs than those 6 fields testing as high risk (>65). As would be expected, the moderate risk category (40-65) was also intermediate between the 2 extremes. However this still amounted to an improvement of almost 1100 lbs sugar for the low risk compared to the moderate risk category. These data suggest that predicting root disease potential and better yields based on the results of testing soils pre-plant with the disease index method is still a viable possibility.

The index values obtained from Aphanomyces testing showed that it took a level of 2,000 oospores to get into the moderate risk category, while somewhere between 2.0 and 4.0 g of Rhizoctonia were necessary to get into this category. A high level of Aphanomyces found may require a seed treatment of Tachigaren, or perhaps the selection of a tolerant cultivar. It is also very noticeable that the higher level of Rhizoctonia inoculum resulted in much poorer stands. We later demonstrated that these seeds were attacked and killed before emergence. Given this type of information from a grower’s soil sample, we recommend seed treatment be critical, and perhaps use a Rhizoctonia-tolerant cultivar, or consider applications of Quadris later in the season.

We need to continue to test this concept, and collect more data as the technique continues to be refined. We also need to establish standards for both pathogens simultaneously. However, the data obtained to this point is still very encouraging and we are optimistic because this technique continues to show promise for predicting root disease problems. It also allows for the identification of specific pathogens, which would enhance a grower’s disease management decisions.