Severidade de ocorrência das manchas de alternária e septoriose em girassol semeado em diferentes épocas no Rio Grande do Sul

the sunflower cultivation has received great attention because of their agronomic characteristics and the possibility of inclusion in crop rotation systems. however, the different climatic conditions prevailing in the growing season determine the occurrence or not of the disease, so that this knowledge is critical for choosing the best growing season. this study aimed to determine the severity of alternaria and septoria leaf spots on sunflower (helianthus annuus) at different sowing dates. experiments were carried out during four seasons, from 2007 to 2011, in santa maria, state of rio grande do sul, brazil. the experiments had a completely randomized design with two-factor (sowing dates vs. sunflower hybrids). the measurements of plant growth and severity of leaf spots caused by alternaria and septoria were taken once and twice a week, respectively. the final severity (svfo) and the variables area under the disease progress curve (aacpd) and days of healthy leaf area (dafs) were determined and used for comparing hybrids and sowing dates. in the crop seasons of 2007/2008, 2008/2009 and 2010/2011, the severity of disease was lower for early sowing dates from august to september, while the later ones tended to increase the disease severities. in 2009/2010 growing season, period under el ni？o influence and with rainfall above normal, all sowing dates resulted in high disease severity, especially negative for the first sowing dates because of the highest rainfall in years under the phenomenon influence. therefore, the late sowing or sowing during el ni？o years are favorable to high alternaria and septoria leaf spots severity in sunflower crop and then the sowing should be avoided under those conditions.