Corn resistance to Physopella zeae (Mains) Cummins & Ramachar, causal agent of tropical rust

Obteve-se, através de autofecundação, progênies de milho da população composto amplo, com diferentes níveis de infecção foliar a Physopella zeae, agente causal da ferrugem tropical do milho. Elaborou-se uma escala descritiva para o estudo do desenvolvimento de lesões de P. zeae em milho. Realizaram-se estudos de parâmetros monocíclicos e policíclicos em plantas jovens de milho em condições de casa-de-vegetação e em plantas adultas no campo. O desenvolvimento de lesões de P. zeae caracteriza-se pela formação de uma pústula primária no início, com posterior formação de pústula primária. Foram avaliados os parâmetros: desenvolvimento de lesões individuais previamente marcadas, fase predominante de desenvolvimento de lesões, número de lesões, número de pústulas secundárias, comprimento de lesões e severidade de infecção foliar, que possibilitaram a diferentes níveis de resistência a o patógeno. A maior resistência a P. zeae, está relacionada com reduções no desenvolvimento de lesões, número de pústulas secundárias, cumprimento de lesões, número de lesões e severidade de infecção foliar da doençaCorn progenies of the Composto Amplo population were obtained by self-pollination. These progenies showed different leveIs of foliar infection by Physopella zeae, causal agent of com Tropical Rust. A descriptive scale was built for studying P. zeae lesions development in corn. Studies of monocyclic and polycyclic parameters, for young plants of com in greenhouse conditions and for adult plants of com in the field, were conducted. Plants at four-to five-leaf stage were inoculated with a P. zeae spores suspension, poured in drops into the whorl. The development of P. zeae lesions is characterized by the initial formation of a primary pustule. The parameters evaluated were the development of previously marked individual lesions; development phase of predominant lesion; number of secondary pustules; length of lesions; number of lesions; and leaf infection severity. These evaluations enabled genotypes differentiation for different leveIs of resistance to the pathogen. Lesion development, according to size and shape of lesions, and to presence of primary and secondary pustules, was slower for more resistant genotypes when compared to more susceptible hosts. Greater resistance to P. zeae presented by different genotypes, when compared to more susceptible genotypes, is related to reductions in lesions development, number of secondary pustules, length of lesions, number of lesions and severity of leaf infection with the disease. Different reaction types of corn pIants to P. zeae were observed. These types were characterized as High Resistance - circular chlorotic-necrotic lesions and/or small circular chlorotic lesions with primary pustule at the center of the lesions, presenting little sporulation; Resistance - small circular to elliptical lesions, absence or presence of few secondary pustules, with sporulation; Moderate Resistance - elliptical lesions, presenting primary and secondary pustules, with sporulation; Susceptibility - large rectangular lesions with chlorotic edges, presenting many secondary pustules around primary pustules, with abundant sporulation