Revista Brasileira de Milho e Sorgo (Dec 2008)
EFFECT OF GRAY LEAF SPOT ON SECOND-SEASON MAIZE GRAIN YIELD IN SÃO PAULO STATE
Abstract
Gray leaf spot is an important maize disease in São Paulo State. Due to its relevance, the effect of this disease on maize grain yield was studied during the 2004 to 2008 autumn-winter seasons. Experiments were carried out to evaluate corn cultivars, comprising 44 to 60 simple and triple hybrids, being 19 trials in 2004, 15 in 2005, 16 in 2006, 16 in 2007 and 15 in 2008. The evaluations of disease severity were done at the kernel dough stage, ascribing notes 1 to 9 (for 0%; 1%; 2.5%; 5%; 10%; 25%; 50%; 75% and more than 75% affected leaf area, respectively). Significant negative correlation between gray leaf spot severity and grain yield was detected in 16 of the 35 trials where the disease occurred. In order to quantify the effect of gray leaf spot on yield of these 16 trials, attempting to minimize the dispersion caused by differences of yield potential, cultivars tolerance to the disease and other factors, cultivars were grouped into categories of disease intensity based on the Scott-Knott test at 5%, and means for severity and yield were calculated for each category. The adjustment of linear equations to these data allowed to estimate the yield reduction as a function of disease severity, for the intervals studied in each trial, that was on average 238 kg ha-1 (4%) with note 2, 634 kg ha-1 (11%) with note 3, 1006 kg ha-1 (17%) with note 4 and 1574 kg ha-1 with note 5, in relation to the groups with greatest gray leaf spot resistance which showed mean note 1.4 and mean grain yield 5655 kg ha-1. These results indicate that even with low disease severity a significant maize yield loss can be observed, demonstrating the importance of the use of genetic resistance in the control of gray leaf spot and to reduce losses in maize grain yield.
