Stenocarpella stalk and seed rot of maize and the role of mating-type genes in aggressiveness of Fusarium graminearum to wheat

Abstract

The fungus Stenocarpella maydis (Berkeley) Sutton [Sin. Diplodia maydis (Berkeley) Saccardo] is an important necrotrophic pathogen causing both stalk and ear rot on maize. Similarly, F. graminearum is also an important maize pathogen causing both Gibberella ear and stalk rot (GER and GSR), but it is also an important pathogen of wheat causing Fusarium head blight (FHB). These diseases cause significant economic losses not only due to reduction in grain yield, but also can reduce crop value due to contamination of grain with mycotoxins. The present dissertation investigated these two important pathogens, Stenocarpella maydis and Fusarium graminearum and has been structured in a series of three studies. In the first study, the main objective was to study the infection process of S. maydis in maize stalks using different microscopy techniques. Plants of two maize hybrids, 30F53 and DKB 390 were grown in a greenhouse and inoculated with conidial suspension of the pathogen in the concentration of 6 x 104 conidia/mL at the V6 growth stage. After inoculation, samples were collected from stalk samples at 0, 12, 18, 24, 36, 48, 72 and 96 hours after inoculation and at 14 and 21 days after inoculation, prepared following standard and then, visualized both in confocal laser microscope and scanning electron microscope. The generated images allow us to visualize the colonization of the stalks by the fungus, which contributes to future pathogen interaction studies. The second study aimed to evaluate the effects of essential oils extracted from Cinnamomum zeylanicum, Copaifera langsdorffii, Origanum vulgare, Cymbopogon martinii and Melaleuca alternifolia at the rates 0.25, 0.50, 0.75 and 1.00% on S. maydis mycelial growth in vitro. Additionally, the best oils and concentrations were used to treat maize seeds to determine their possible effects on seed health and physiology. It was found that the oils from C. zeylanicum and O. vulgare were able to inhibit fungal mycelial growth at concentrations higher than 0.5% at similar levels of the standard treatment with fungicide. Origanum vulgare oil performed better in reducing the S. maydis incidence in inoculated seeds compared to C. zeylanicum and the fungicide treatment. The oil from C. zeylanicum at 0.75% showed seed germination rates of 92.4% which were higher than that from the nontreated and fungicide-treated control. In contrast, O. vulgare oil at both tested concentrations (0.25 e 0.50%) had physiological effects on maize seeds. Lastly, the third study of this dissertation aimed to investigate role of mating-type genes in pathogenicity of F. graminearum to wheat. For such, independent knockout mutants (KOs) each of MAT1, MAT1-1-1, and MAT1-2-1 in the PH-1 strain of F. graminearum were used. As expected, all the KOs lost their ability to produce ascospores. However, when MAT1-1-1 KO were paired with the MAT1-2-1 KO, formed fertile perithecia. Most (but not all) KOs strains of the individual specificity genes resulted in reductions in aggressiveness and, in most cases in DON production. There was no consistent reduction in aggressiveness on MAT mutants. In summary, the three works of this disseration provided new knowledge to understand the life cycle and also manage such important pathogens in Brazil and worldwide.