Quantifying twelve fungal isolates associated with maize root and crown rot complex in South Africa

Abstract

Maize is South Africa’s most important crop contributing to dietary staple, livestock feed and to the gross domestic product of the country as an export crop. Root and crown rot on maize in South Africa threaten the optimal production of this staple food. A complex of fungal pathogens is responsible for causing these diseases and the best management strategies need to be applied to prevent yield loss. These pathogens have certain environmental preferences and conditions in which they thrive. Altering these conditions through applying cultivation practices in different climatic regions in the country together with other management strategies can limit root and crown rot. For these practices to be efficient the different pathogens need to be known and evaluated separately. How these pathogens co-exist in the different environments, as well as the mechanism by which the inoculum of each pathogen change over and between seasons, should be known. The overall aim is to understand the disease complex causing root and crown rot and its succession over time, to quantify the disease incidence and severity and to formulate management strategies accordingly. Limited research like this has been done for disease complexes and through this study many shortages will be identified and opportunities will arise for better and more research to optimize management of root and crown rot on maize. In this study, the influence of tillage and no-till, mono-cropping and crop rotation, dryland and irrigation, different localities (provinces) and tissue specificity on the presence and abundance of twelve commonly known fungal pathogens of root and crown rot in South Africa (Curvularia eragostidis, Exserohilum pedicellatum, Fusarium chlamydosporum, F. equiseti, F. graminearum, F. oxysporum, F. verticillioides, Macrophomina phaseolina, Pythium species, Phoma species, Rhizoctonia solani and Trichoderma species) were studied. Visual evaluations and disease ratings, DNA extractions and qPCR (quantitative Polymerase Chain Reaction) technology, being effective, quick and precise, were used to separately analyse each pathogen with these above-mentioned variables. Overall the complex showed significant root preference compared to crowns. In conventional cultivation practices the qPCR results showed that Phoma spp., Pythium spp., F. oxysporum and F. chlamydosporum were the most prominent and Phoma spp., F. chlamydosporum, Pythium spp. and F. oxysporum were prevalent in conservation agricultural practices. There was a significant tillage x province interaction for F. oxysporum (P=0.00), irrigation x province interaction for E. pedicellatum (P=0.02) and R. solani (P=0.04). F. verticillioides showed significant differences between different rotated crops (P=0.01). R. solani was found significantly more in no-till fields compared to tilled fields, and between rotations with different crops (P<0.0001). From three cultivars (BG 3292, IMP 50-10 B and DKC 61-94 BR), BG 3292 had the lowest root and crown rot severity ratings and the highest root and crown plant-biomass. For C. eragostidis (P=0.00) and E. pedicellatum (P=0.03) a significant locality x sampling date interaction was indicated, while F. oxysporum had significant cultivar x plant part x locality interactions (P=0.04). Phoma spp. were significantly affected by the sampling date and plant part interaction (pathogen presence increased in the roots with time and decreased in the crowns) (P=0.00) and Pythium spp. with the sampling date x plant part x locality interaction (pathogen presence increased in the roots of maize plants at Vaalharts with time and decreased in the crowns of maize plants at Vaalharts and Potchefstroom) (P=0.00). Trichoderma spp. showed the highest order interaction that contributed to the infection: sampling date x plant part x cultivar x locality (P=0.01). This study revealed the value of using molecular technology in studying the different variables contributing to the occurance and severity of these diseases (the fungi present and to which degree it contributes to the root and crown rot disease complex).