Resistance of corn inbred lines to foliar diseases in two planting dates
Abstract
The aim of this study was to attempt to identify elite inbred lines resistant to tropical rust, southern rust, gray leaf spot, northern leaf blight, physoderma brown spot and phaeosphaeria leaf spot. Fifty inbred lines were evaluated, in two seasons, in randomized blocks with 3 replications for AUDPC (Area Under the Disease Progress Curve), obtained by evaluation of the disease at 45, 60, 75 and 90 days after planting. Rating of at 1, 2, 3, 4, 5, 6, 7, 8 and 9 corresponded t o 0, 1, 10, 20, 30, 40, 60, 80 and > 80% of leaf symptoms, respectively. The joint analysis of variance had significant differences between inbred lines for tropical rust, southern rust, gray leaf spot and phaeosphaeria leaf spot. There was a significant interaction between inbred lines and seasons for both tropical rust and southern rust. In season 1, tropical rust, southern rust and gray leaf spot differed significantly. In season 2, significant differences occurred for southern rust and phaeosphaeria leaf spot. We found 12 inbred lines with improved levels of resistance to tropical rust, southern rust and gray leaf spot. For phaeosphaeria leaf spot, 38 inbred lines had satisfactory resistance.
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Introduction
Big losses in grain yield in corn are associated with the incidence of diseases. Disease monitoring studies have demonstrated that rust, gray leaf spot and Phaeosphaeria leaf spot are among the major diseases that affect the corn crop in Brazil (Carson 2005, Casela et al. 2006).
Due to the characteristics of corn growing in Brazil, such as plant height, the length of the planting season and economic yield and, in some cases, continuous planting of corn years around the most viable measure to control the disease is the use of genetic resistance. For nearly two decades chemical control was practically viable only in seed production fields (Gianasi et al. 1996). Currently, crops grown with the highest level of technology, with higher income potential, can often economically use chemical control for these diseases, although genetic resistance is still preferred. For the farmer, the desired resistance is in the hybrid planted, but breeders have to also worry about resistance in the parental lines that give rise to these hybrids. In addition, resistant inbred lines can be used for adding resistance to other inbred lines and better performance in future hybrids. There is also the possibility of using synthetics from resistant inbreds as commercial varieties for corn producers with lower technological level.
Many reports in the literature indicate that there is genetic variability in cultivars in disease resistance (Nihei and Ferreira 2012, Vieira et al. 2012, Zambrano et al. 2014); however, few papers discuss genetic resistance to diseases in inbred lines. Colombo et al. (2014) reported that the Area Under the Disease Progress Curve (AUDPC) can quantify the progression of disease during a certain period, and it has been frequently used to evaluate the level of resistance in field conditions. The objective of this study was to identify inbred lines resistant to tropical rust (Physopella zeae (Mains) Cummins & Ramachar.), southern rust (Puccinia polysora Underw), gray leaf spot (Cercospora zeae-maydis Tehon & E.Y. Daniels), northern leaf blight (Exserohilum turcicum (Pass.) Leonard & Suggs), physoderma brown spot (Physoderma maydis) and phaeosphaeria leaf spot (Phaeosphaeria maydis in association with Pantoeae ananas).
Conclusion
The resistant inbred lines based on Area Under Disease Progress Curve (AUDPC) for southern rust, tropical rust, gray leaf spot, northern leaf blight, phaeosphaeria leaf spot and physoderma brown spot were IVF1 -3, IVF1 -9, IVF1 -10, IVF1 -11, IVF1 -25, IVF1 -230, IVD1 -2-1, IVD1 -12, 2F, 3F, 6F and 4C. The results of this study suggest the need for further assessment, in other months of planting, for the correct evaluation of symptoms of northern leaf blight and physoderma brown spot.