Reação de genótipos de milho à mancha ocular e caracterização do agente causal

Abstract

This investigation on corn eyespot was based on its causal agent and on the preparation of a diagrammatic scale that helped evaluate the reaction of corn genotypes, in correlation with the response obtained from the plant’s biochemical defenses. In the first step of the study, 35 Kabatiella zeae isolates from different corn genetic materials had their colonies evaluated for color and aspect, and their spores measured in PSA medium. The following culture media were used: PSA (potato, sucrose, agar), MA (malt and agar), OA (oat and agar) and CA (corn flour and agar). Disks (5 mm in diameter) were placed in the center of Petri dishes containing the respective culture media. The dishes were incubated at 24°C and a 12-hour photoperiod for 10 days. Colony traits and conidia measurements were evaluated in PSA, while radial growth was evaluated in the four media. There were differences between all isolates with regard to the culture media. Spore size varied from 3-4 × 27-33 μm in width and length, respectively, and 80% of the isolates had a light color (pink) and a coriaceous and wrinkled aspect; the highest mycelial growth was observed in the PSA medium. A diagrammatic scale was developed by randomly collecting 110 diseased leaves in the field, discarding 20 cm from the leaf tip, and using the next 20 cm. Measurements were made for leaf area and the area containing lesions in order to determine minimum and maximum severity limits of the disease; intermediate levels were calculated mathematically. The severity levels observed in the field followed logarithmic increases, representing: 0.9; 2.0; 4.0; 9.0; 18.0; 32.0; and 51.0% of leaf area with lesions. Scale validation consisted of two steps: in the first, 10 evaluators experienced in plant disease evaluations assessed 30 leaves with different severity levels, both using and not using the diagrammatic scale. In the second step, another 10 naive (without experience) evaluators performed the same evaluations, both using and not using the diagrammatic scale. Actual and estimated severity values were compared by linear regression, and evaluator accuracy and precision were analyzed. Better accuracy and precision were obtained for visual estimates made with the diagrammatic scale. The reaction of corn genotypes to eyespot was evaluated in the city of Pato Branco-PR during the 2006/2007 growing season using 33 hybrids, and during the 2007/2008 growing season using 10 hybrids at two distinct sowing seasons, and in the city of Palma Sola-SC during the 2007/2008 growing season with 8 hybrids. Hybrids AG 9020 and SPRINT were the most susceptible to the disease in relation to AS 1565 and DKB 234; this behavior was observed in both years and planting seasons. The peroxidase enzyme was observed at higher concentrations in leaves of corn hybrids with the lowest eyespot severity.