Utilizing a Preplant Soil Test for Predicting and Estimating Root Rot Severityin Sugar Beet in the Central High Plains of the United States

Harveson, R. M., Nielsen, K. A., and Eskridge, K. M. 2014. Utilizing a preplant soil test for predicting and estimating root rot severity in sugar beet in the Central High Plains of the United States. Plant Dis. 98:1248-1252. Aphanomyces cochlioides and Rhizoctonia solani are important soilborne pathogens causing root diseases that are primary constraints to sugar beet production in Nebraska, Colorado, and Wyoming. These types of diseases are difficult to control because they are often not noticed until substantial damage has already occurred. Efforts to manage them would be more effective if techniques were available that were more predictive than reactive. Therefore, a preplant soil test was developed to estimate the relative pathogen populations in the soil and to predict potential root disease problems later in the growing season. Preplant soil samples collected from fields to be sown with sugar beet were planted with a susceptible cultivar and tests were conducted for 1 month in the greenhouse. A preplant disease index was developed based on the time period during the test that seedlings became infected and was calculated on a 0-to-100 scale. Disease index values were compared with yields obtained from the same fields after harvest. Analysis of data collected for 5 years (2003 to 2007) with analysis of covariance revealed a strong relationship between the preplant disease index values and recoverable sucrose and root yields but not sucrose concentration. Results indicated that, for each unit increase in the preplant disease index, root yield decreased by 0.27 metric tons (270 kg) per hectare (P < 0.05, R2 = 0.44) and recoverable sucrose decreased by 49 kg/ha (P < 0.05, R2 = 0.45). We concluded that this preplant soil test can accurately predict root disease potential due to R. solani and A. cochlioides, and has the potential to help producers make effective management decisions in production fields using the index procedure. This soil assay has additionally provided new information on the biology, incidence, and distribution of root pathogens in production fields throughout the Central High Plains. In western Nebraska, southeastern Wyoming, and northeastern Colorado, root diseases induced by Rhizoctonia solani (Kühn) and Aphanomyces cochlioides (Drechsler) cause significant problems in sugar beet production. Other root pathogens causing rhizomania and Fusarium yellows and root rot are present in this region but presently cause fewer problems, due to a number of factors. Rhizomania has become much less problematic over the last decade due to the genetic resistance now incorporated into all approved cultivars for the South Region of the Western Sugar Cooperative, while the incidence of the two Fusarium diseases (root rot and wilt) has been erratic from year to year over the same period of time. R. solani, the pathogen that causes Rhizoctonia root and crown rot of sugar beet, has been present in this region for many years (10,11,16,19,20). This disease is well known by industry personnel and is currently considered the most widespread and consistently damaging sugar beet disease in this area (6,10). It appears throughout the growing season and has been documented to result in yield losses as high as 50% in Nebraska (11). Aphanomyces root rot, caused by A. cochlioides, has more recently been identified formally from Nebraska and Wyoming sugar beet fields (5), although it has likely been present throughout this region for some time. Finding fields infested with both pathogens infecting crops simultaneously is becoming more the norm than the exception. R. solani and A. cochlioides possess several common characteristics, including the ability to cause both a seedling disease and a chronic root rot later in the growing season. Additionally, both pathogens are soilborne, can survive in the soil for many years, and are favored by generally warm temperatures and moist soils (1,6,14, 15,17,18,21). Yet they also differ substantially. Taxonomically, they are not closely related, and the zoosporic Aphanomyces is much more dependent upon high levels of soil moisture than Rhizoctonia for disease development and spread (22,23). Additionally, Rhizoctonia spp. infect a wide range of host plants, whereas Aphanomyces spp. are limited to causing disease on plants related to sugar beet (6,14). Management options are available for both diseases, including seed treatments with several fungicides, resistant cultivars, and cultural practices such as early planting and irrigation management (1,15). Unfortunately, no one management method is adequate when both pathogens are present. Although both pathogens can cause seedling disease, in this region, severe damage is more commonly incurred as acute root rots. Therefore, a predictive technique for the occurrence of Rhizoctonia and Aphanomyces root rots later in the growing season would be useful in estimating disease potential, thus allowing greater flexibility for growers to make management decisions on a timelier basis. Based on this need, we developed a preplant soil test technique with the purpose of predicting potential root disease problems prior to planting, emphasizing the root rots caused by A. cochlioides and R. solani. This test can also identify and estimate populations of other specific soil pathogens utilizing a seedling disease assay performed in the greenhouse. Several preliminary reports on this technique have been previously published (7,8). The objectives of this study were to describe the preplant disease test and evaluate its capabilities for disease prediction based on comparing results with yield parameters from 109 production fields over the 5-year period of 2003 to 2007. Materials and Methods This project was initially started as an expansion of a sugar beet grower service that tested preplant soil samples for the presence of Beet necrotic yellow vein virus, the causal agent of rhizomania. It was conceived after discussions with growers and learning of their desire to proactively obtain similar information on other potential root pathogens residing in their soils in a similar manner as was Corresponding author: R. M. Harveson, E-mail: [email protected] Accepted for publication 27 March 2014. http://dx.doi.org/10.1094/PDIS-11-13-1186-RE © 2014 The American Phytopathological Society