An improved understanding of Diplodia ear rot in Kentucky corn

Stenocarpella maydis and Stenocarpella macrospora are both causal agents of Diplodia ear rot (DER) of corn in the U.S. The current prevalence and distribution of each is unknown worldwide. Signs and symptoms of DER include white mold on and between kernels, bleached husks, and pycnidia visible in the cob pith. The term hidden Diplodia describes when no mycelia or bleached husk is present, but pycnidia are still visible inside the cob. This phenomenon is reported to be caused by late infections of Stenocarpella spp. New fungicide nozzle technology has been promoted to increase spray coverage in the lower canopy, which may influence fungicide efficacy against DER. The goals of this research are to 1) understand the species distribution in Kentucky of the two pathogens causing DER, 2) the conditions which influence hidden Diplodia, and 3) the potential for in-canopy fungicide applications to improve control of DER. To determine the prevalence of causal agents of DER, Kentucky corn fields were surveyed in 2019 and 2020. Ears with signs or symptoms of DER were collected in a specific sampling pattern and taken to the laboratory for analysis. In 2019, 98 fields were surveyed and in 2020, 88 fields, with 133 and 46 symptomatic ears collected each year, respectively. Causal agents were isolated from ears and identified based on conidia morphology. A subset of isolates was used to inoculate corn ears to measure aggressiveness. S. macrospora was identified as the cause of 33.8% and 36.9% of the total DER observed in 2019 and 2020, respectively. While Stenocarpella sp. isolates differed in aggressiveness (P=0.8863). Although S. maydis has been described as the main pathogen causing DER, this research indicates the importance of both pathogens in relation to DER. From 2019 to 2021, the influence of inoculation site and timing of both Stenocarpella spp. was assessed under irrigated and dryland conditions. Both species were inoculated at one of four growth-stage timings (V8, R1, R2, R3) or one of four sites on the plant (whorl, silk channel, ear shank, foliar spray). DER severity was significantly greater in R1 inoculation timings compared to R2 and R3 timings. Yield was reduced by an average of 700 kg/ha in treatments inoculated at R1 compared to non-inoculated controls. In the 2020 irrigated trial, whorl inoculation resulted in higher incidence of hidden Diplodia compared to other sites (P=0.0055). Results confirm reports that DER can be yield limiting, although there was no evidence to suggest that later infections increase hidden Diplodia. In 2020 and 2021, trials were established to examine the effect of adding in-canopy drop nozzle or 360 Undercover nozzle fungicide application methods on DER severity, yield, and spray coverage of corn leaves and ears. Two fungicides labeled for suppression of DER were used with each application method. Coverage was assessed via spray cards in experimental plots. Yield and DER severity were not affected by application method, however DER severity in control plots was low (0-11%). The addition of 360 Undercover nozzles increased spray coverage on the ear (P=0.0139) and ear leaf (P \u3c 0.0001) compared to other methods. Although using in-canopy-nozzles did not affect DER severity, the increase in coverage may lead to improved control under higher disease pressure situations