Transmissão e desempenho de sementes de milho portadoras do complexo Stenocarpella sob condições controladas

Abstract

Stenocarpella maydis and Stenocarpella macrospora are species causing stem/ear rot and leaf spots in corn and can be transmitted by seeds of this cereals. The objectives in this study were to evaluate the relationship of S. maydis and S. macrospora with maize seeds with a focus on transmission of these organisms, from seeds to plants, and the resulting effects of this interaction taking into account some important factors that may affect this relation. To determine the rate of transmission and to evaluate the effects of the pathogens on the performance of seeds/seedlings of corn, two isolates of each species of Stenocarpella were used. Inoculation was performed by the osmotic conditioning method using two cultivars of maize, one susceptible (C1-RB9308YG) and the other moderately resistant (C2-RB9108). Different inoculum potentials were obtained by varying the time of contact between seeds and fungal colonies, establishing the periods of 24 (P1), 48 (P2), 72 (P3) and 96 (P4) hours. The experiments were conducted in growth chamber with temperatures of 20 °C and 25 °C, the effects being evaluated daily. The total transmission rate of each pathogen was calculated by the sum of the death pre-emergence rate and transmission rate with symptomatic and asymptomatic infection. Transmission of S. maydis occurred at all inoculum potentials, the total rate ranging from 25% for C2 with P1 potential at 20 °C and 93% for C2 with P3 at 25 °C. The total transmission rates of S. macrospora, from the seed to plant, were directly proportional to the increase of the inoculum potentials, the highest rate 84% occurred in the cultivar C1 at the highest potential inoculum (P4), with cultivation at 20 °C and the lowest rate of 6.75% in the C2 with P1 and cultivation at 20 °C. Regarding the physiological effects the germination and vigor index (Emergency Speed Index- ESI), initial and final stands, height and fresh and dry weights of plants, cold test and electrical conductivity, and in addition to the health test, in inoculated seeds with the same inoculum potential used to assess the rates of transmission of pathogens were applied. It was observed that for both species of Stenocarpella, higher initial inoculum potentials in the seeds provoked bigger damages on the quality of seeds and in the resulting plants. In plants emerged from seeds with the highest potential (P4) it was observed the lowest values of ESI, stands, weights and heights of plants. By the quality seed test, it was also observed that the higher the inoculum potential caused the lower percentages of normal seedlings, both germination and cold test. At these potentials higher percentages of incidences of the pathogens in seeds were observed. In those infected seeds it was verified the occurrence of an enhancement effect of membrane degradation as indicated by the electrical conductivity analyzes. For studies using genetic markers that express green fluorescent protein (GFP) and red fluorescent protein (DsRed), isolates of both S. maydis and S. macrospora have been successfully transformed by the use of their respective protoplasts and selected plasmids. In the artificially infected seeds by the transformed isolates of both species, it was observed different colonization patterns in accordance with the intensities of fluorescence revealed by fluorescence microscopy analyzes. The inoculum potentials P3 and P4, in the infected seeds, were the ones that produced the highest levels of emitted fluorescence in the colonization by the pathogens. By scanning electron microscopy, the analyzes were effective to demonstrate the dynamics of colonization of S. maydis and S. macrospora in the tissues of inoculated maize seeds at all inoculum potentials. In the higher inoculum potentials (P3 and P4) abundant and comprehensive mycelial growth in the structures of seeds were observed.