Occurrence and significance of Fusarium and Trichoderma ear rot in maize

Ear rots, caused by fungi, are among the most important maize diseases worldwide causing severe yield losses and a reduction of grain quality. Several toxigenic Fusarium species are known to cause yield losses and reduce grain quality, thus endangering the safety of both animal feed and human food products. For this purpose, we investigated the occurrence and significance of Fusarium and Trichoderma ear rot on maize in Germany. Within this framework, we evaluated the impact of environmental conditions and agronomic practices on the prevalence of Fusarium species associated with ear and stalk rot. The most frequent Fusarium species detected in maize ears were Fusarium graminearum, F. verticillioides and F. temperatum, while F. graminearum, F. equiseti, F. culmorum and F. temperatum were the species prevailing on maize stalks. The results indicate that mean temperature and precipitation in July, during flowering, has the strongest impact on the local range of Fusarium species on ears, whereas the incidence of Fusarium species on stalks is mostly affected by weather conditions during September. Ploughing significantly reduced the infection with F. graminearum and F. temperatum, while crop rotation exerted only minor effects. Another aim of the present study was to determine the occurrence, mycotoxin production and pathogenicity of Fusarium temperatum from maize in Germany. For this purpose, a Germany-wide monitoring of maize ears and stalks was carried out in 2017 and 2018. Within this monitoring, 79 isolates of F. temperatum and seven isolates of F. subglutinans were obtained. Inoculation of maize ears revealed the highest aggressiveness of F. temperatum, followed by F. graminearum, and F. verticillioides and F. subglutinans. The temperature optima for infection of maize ears with F. temperatum and F. subglutinans were 24 °C and 21 °C, respectively. Artificially induced infection of wheat ears with all strains of F. temperatum and F. subglutinans caused head blight symptoms, thus indicating wheat as an alternative host. The results demonstrate the increasing importance of F. temperatum in German maize cultivation areas. Thirdly, we investigated the aggressiveness of several Fusarium species in maize in relation to inoculation method, maize variety and location. Therefore, in 2018 and 2019, maize hybrids were tested in four locations (Bernburg, Rustenhart, Kuenzing and Liesborn) in Germany and France. Our results showed that F. temperatum was the most aggressive Fusarium species in both years followed by F. graminearum and F. verticillioides, however, the prevalence differed between locations. Significant differences in genotypic resistance depending on the inoculation method and Fusarium species were found in all locations. In 2018, massive infections with Trichoderma on maize ears were recorded for the first time in a field in Southern Germany. Within this study, first investigations were conducted to identify and verify Trichoderma as a new pathogen causing ear rot disease on maize in Europe. In 2018 and 2019, a total of 13 T. harzianum isolates from maize cobs and maize stalks were isolated and tested, compared to several reference isolates. Four isolates proved to be highly aggressive, two biocontrol isolates, Trichodex (T39) and strain T12, induced slight infection and eleven isolates were non-pathogenic. This, to our knowledge, is the first report on Trichoderma sp. as a pathogen causing ear rot disease in maize in Europe with the potential to incite significant yield losses. We therefore propose to name this disease as ‚Trichoderma ear rot on maize‘.