Desenvolvimento de Resistência Múltipla a Doenças em Linhagens Avançadas de Algodoeiro

After 15 years of studies concerning the development of multiple resistance to five major diseases of cotton (Gossypium hirsutum) - Fusarium and Verticillium wilts, bacterial blight, ramulosis, and nematodes - in advanced lineages arising from the normal breeding program conduced in the State of Sao Paulo, Brazil, by Instituto Agronomico (IAC), it was revealed an accumulative process whose results were agronomically relevant. Based on an interactive scheme involving the selection of lineages exhibiting resistance to one or more of the considered diseases and posterior reselection within them, the process was shown to effectively exploit natural genetic variability existing in cotton genotypes stabilized for other agronomic and industrial traits. Tendency to the establishment of the positive correlation was observed only in the case of resistance to bacterial blight and nematodes. Otherwise, persistence of negative correlations, mainly resistance to nematodes x ramulosis and ramulosis x bacterial blight, even in the more resistant materials, indicated the possibility of resistance losses for certain diseases if excessive selection pressures are exerted for others.