Direct Polymerase Chain Reaction-Based Detection of Cercospora beticola in Field Soils

Cercospora beticola Sacc. causes leaf spot diseases of sugar beet (Beta vulgaris L.) (2,19,21), related species in the family Chenopodiaceae (6), and, in recent reports, on safflower (Carthamus tinctorius L.) (11,12) and German statice (Goniolimon tataricum) (3), belonging to plant families outside of the Chenopodiaceae family. Cercospora leaf spot (CLS) is the most important foliar disease of sugar beet (17). Bleiholder and Welzien (1) indicated that the disease occurs wherever sugar beet is grown. According to Weiland and Koch (19), beet crops in warm, humid growing regions are most acutely affected by CLS and these constitute greater than 30% of the area under sugar beet cultivation. Cercospora beticola can survive in soil for at least 20 months (16) and recent evidence by Khan et al. (10) indicates survival of up to at least 22 months. They (10) demonstrated further that C. beticola survives for over 22 months at a depth of 10 cm from the soil surface but for only about 10 months at a depth of 20 cm from the soil surface. Wherever the crop is grown, prevailing weather appears to play a major role in the incidence of CLS. Indeed, according to Windels et al. (21), severity of CLS varies from year to year, depending upon weather conditions and the effectiveness of disease management practices. The primary inoculum of C. beticola in a sugar beet field begins as conidia and pseudostromata in infested plant debris where the pathogen overwinters (9,21). Under favorable conditions, which are characterized by temperature between 20 and 26°C and heavy dew or relatively humidity above 98%, conidiophores and conidia are produced on the pseudostromata (9,18). The current CLS infection model (15,17,21) indicates that conidiophores and conidia that serve as primary inoculum are dispersed by wind, irrigation, rain water, and insects to sugar beet to initiate primary infection. In this manner, the pathogen can be transmitted from the surviving inoculum reservoir to the subsequent crop during the following growing season (9). Among the cultural practices tested to manage CLS, McKay and Pool (15), early in the 20th century, recommended sugar beet fields be sown at least 100 m from fields planted to sugar beet the previous year to avoid spread of conidia from infested debris, a practice that is still recommended (21). It also has been suggested, among other measures, to include deep tillage to turn under residues and to use rotation with nonhost crops, with a minimum of 3 years between beet crops, because of survival of C. beticola in infested beet leaf residues in the soil (21). In addition to cultural practices, applications of fungicides have played a major role in the control of CLS. However, an issue in the management of CLS is the well-documented occurrence of fungicide tolerance in C. beticola populations (7). As a consequence, the control of leaf spot disease necessitates the judicious rotation of fungicide with diverse modes of action as a means of preventing or delaying the development of resistant strains or reducing their prevalence in populations (19). In addition, forecasting models that predict the likelihood of Cercospora spp. infection and severe crop loss in conjunction with timely application of fungicides to manage outbreaks of the disease have become an important complement to genetic resistance in CLS management (5,20,22). Prediction models have contributed significantly toward improved management of CLS in recent years. However, these models lack information about the actual presence of the pathogen in the target environment even under weather conditions favoring infection, thus leading to a recommendation for application of a fungicide. Currently, a polymerase chain reaction (PCR) technique is available for direct detection and identification C. beticola in infected plant tissues (13) and, consequently, can enhance detection and identification of various hosts that may serve as potential sources of inoculum. In addition, an enzyme-linked immunosorbent assay (ELISA)-based protocol that targets C. beticola protein-antigen has been developed to enable direct detection of C. beticola in the soil (4). These two protocols can improve prediction of the incidence of CLS under weather conditions favorable to the disease. Together with scouting for lesions (8), their use could further complement current weather information to improve prediction models, thus enhancing ABSTRACT Lartey, R. T., Caesar-TonThat, T. C., Lenssen, A. W., Eckhoff, J., Hanson, S. L., and Evans, R. G. 2010. Direct polymerase chain reaction-based detection of Cercospora beticola in field soils. Plant Dis. 94:1100-1104.