Mechanism of resistance and presence of different resistance genes to Ramularia areola in two cotton genotypes

Abstract

Earlier studies showed that the resistance of cotton genotype FMT 02102996 to Ramularia areola is governed by one dominant gene. More recently, the resistance of another genotype CNPA BA 2003-2059 to R. areola was detected under field and glasshouse conditions. Present investigation was conducted to verify the mechanism of resistance of the genotype CNPA BA 2003-2059 and to find out if the resistance of these two genotypes is governed by the same or by different genes. Segregating plant populations derived from the cross between the resistant genotype CNPA BA 2003-2059 and the susceptible genotype FMT 701, the back cross populations, as well as those derived from the cross between the two resistant genotypes were evaluated for disease severity by artificial inoculations under glasshouse conditions. The ratio of plants segregating for resistance and susceptibility was studied by χ2 test. The results indicated that the resistance to R. areola in genotype CNPA BA 2003-2059 is governed by one dominant gene and that the resistance in each one of the resistant genotypes is governed by a different dominant gene. These results may assist the local breeding programs aimed at pyramiding resistance genes to this pathogen and may form the basis for genetic mapping of resistance genes. Key words: Gossypium hirsutum, genetics, inheritance of resistance. RESUMO Mecanismo de resistência e presença de genes diferentes de resistência a Ramularia areola em dois genótipos de algodoeiroA mancha-de-ramulária, causada por Ramularia areola, é uma das doenças de importância econômica para o Brasil. Estudos anteriores demonstraram que a herança da resistência do genótipo FMT 02102996 a R. areola é governada por um gene dominante. Recentemente, a resistência do genótipo CNPA BA 2003-2059 a este patógeno foi verificada em casa de vegetação e no campo. O presente estudo foi realizado para verificar o mecanismo de resistência deste genótipo e verificar se os genes de resistência dos dois genótipos resistentes são os mesmos. Foram avaliadas populações segregantes derivadas do cruzamento entre o genótipo resistente CNPA BA 2003-2059 e o genótipo suscetível FMT 701, dos retrocruzamentos, e também do cruzamento entre os dois genótipos resistentes. As plantas foram classificadas como resistentes ou suscetíveis por meio de inoculação artificial. Com base nos resultados, constatou-se que a resistência do genótipo CNPA BA 2003-2059 é condicionada por um gene dominante e que os dois genótipos possuem um gene diferente de resistência. Estes resultados podem auxiliar o planejamento dos programas de melhoramento do algodoeiro visando piramidar genes de resistência a R. areola em novas cultivares, ao mesmo tempo constituindo informação básica para o início de trabalhos de mapeamento genético deste gene de resistência. Palavras-chave: Gossypium hirsutum, genética, herança de resistência.

Identification of Phenotypic and Genotypic Variability among the Isolates of <i>Ramularia areola</i> of Brazilian Cotton

“…Although the resistance of genotypes CNPA BA-2003-2059 and FMT 02102996 as identified in earlier studies is governed by two different genes [2,4], these genotypes showed a susceptible reaction to three isolates 13.2, 17.5 and 58.4 originated from three different Brazilian States ( Table 1). With the exception of isolates 22.3 and 42.7, genotype FMT 701 was susceptible to all the 16 isolates of R. areola.…”

“…Leaf blight of cotton caused by Ramularia areola is economically important especially for Brazil, causing heavy yield losses [1][2][3][4]. In Brazil, some years ago it was considered as a secondary and late season disease, however, in recent years the disease is of principal concern especially because now it attacks cotton during the entire crop cycle.…”

“…In Brazil, some years ago it was considered as a secondary and late season disease, however, in recent years the disease is of principal concern especially because now it attacks cotton during the entire crop cycle. In the Cerrado region of Brazil the yield losses caused by this disease are estimated to be around 30% but in severe cases they can be up to 75% in the State of Mato Grosso [1,2,4]. Yield losses of over 60% are also recorded in Madagascar [5].…”

“…At present the disease is partially controlled in Brazil by 5 -8 applications of fungicides thereby adding to the cost of cultivation and at the same time making the intensive use of fungicides almost unsustainable in the course of time. Resistance of some genotypes was detected under field conditions but their resistance was not found to be consistent presumably because of the genetic variability among the pathogen population [1][2][3][4].…”

Efeito de fungicidas para o controle da Ramularia areola na cultura do algodoeiro

“…O custo com defensivos químicos é muito alto na cultura do algodoeiro, ao considerar que de modo geral são necessárias pelo menos quatro a cinco aplicações durante o ciclo (23). No entanto, o controle da mancha de ramulária tem exigido número crescente de aplicações, não raro em torno de dez, o que pode ser um indício de resistência do patógeno ao uso repetido de um fungicida ou de fungicidas quimicamente relacionados que expressam um mecanismo comum de ação antifúngica (6,8).…”