Leaf rust, leaf spot or brown spot and powdery mildew, caused by Puccinia recondita tritici, Bipolaris sorokiniana and Blumeria graminis tritici, respectively, are wheat diseases that may severely decrease crop yield. Several practices are recommended for the control of these diseases such as cultivar resistance, planting at a suitable time, and chemical control, without which the crop frequently becomes uneconomical. The objective of the present work was to evaluate, under field conditions, the response of six wheat cultivars, IAC 24, IAC 289, IAC 350, IAC 362, IAC 364 and IAC 370, in four experiments, carried out from 2000 to 2003 at Capão Bonito, State of São Paulo, Brazil. The experiments were set out in a randomized block design with four replications, and analyzed as a 2 x 6 factorial, being fungicide treatment (with and without) the factor 1 and cultivar the factor 2. Severity of the diseases was graded in a scale that ranged from 0 to 9, (0 = without symptoms and 9 > 60% of the foliar area affected by disease). Grain yield and the thousand kernel weight (TKW) were also assessed in each experiment. Powdery mildew was only observed in 2000, but leaf rust was detected in all years in high severity indexes (30 to 60% of leaf area affected by the disease). Leaf spot, detected in the last three years, from 2001 to 2003, showed the highest severity indexes in the rainiest year, 2001 (up to 60 % of the leaf affected by the disease). In all years the fungicides were efficient to control the diseases in all experiments and resulted in higher TKW and in grain yield increases. The cultivars with the longest crop cycles, IAC 370, IAC 289 and IAC 350, had the highest responses in yield to the chemical control, despite the fact that the last two cultivars were less affected by the diseases in the control plots, showing that not only cultivar resistance, but the crop cycle and the yield potential, characteristic of each cultivar, were important in the response to the chemical control.
Bipolaris sorokiniana; Puccinia recondita tritici; Blumeria graminis tritici; a thousand seed weight; yield