Use of infested sorghum for Cercospora beticola inoculation.
Cercospora leaf spot (CLS), caused by Cercospora beticola Sacc., is responsible for significant economic loss in sugarbeet growing regions worldwide. Establishing uniform CLS disease pressure across a field trial is necessary to conduct reproducible field research for screening varieties as well as evaluating fungicide management programs. In 2022, a field trial was conducted to evaluate the use of grain sorghum for preparing C. beticola inoculum. A randomized complete block design with four replications was used, and a moderately susceptible sugarbeet variety with a CLS rating of 4.9 was sown on May 24. Four isolates of C. beticola were grown on autoclaved sorghum grains in breathable polypropylene bags with a filter. On July 13, treatments were applied with a modified duster over each row and included C. beticola-infested sorghum grains milled into a powder and mixed with fine talc (2:1 w/w) applied at 4.5 lbs/A and 9 lbs/A, and without fine talc applied at 4.5 lbs/A. In addition, C. beticola-infested whole sorghum grains were broadcast at 11 lbs/A. Treatments were compared to a non-inoculated control and a standard inoculation method of dried ground CLS-infected sugarbeet leaves mixed with fine talc (2:1 w/w) applied at a rate of 4.5 lbs/A. There were significant differences for disease severity, yield, and recoverable sucrose per acre. All treatments with C. beticola-infested sorghum grains resulted in higher disease levels than the CLS-infected sugarbeet leaves. The non-inoculated control resulted in the lowest level of disease. The use of sorghum grain allowed for greater control of inoculum quality and resulted in higher disease pressure than the CLS-infected leaf inoculum. Additionally, the broadcast application does not require mixing with talc and significantly reduces the time needed for inoculating large areas. This trial will be repeated in 2023.