Disease Reaction Changes from Tandem Selection for Multiple Disease Resistance in Two Maize Synthetics

Future maize (Zea mays L.) productivity increases require breeding materials with high grain yield potential and multiple disease resistance. As part of an integrated program to develop breeding populations with high grain yield potential and multiple disease resistance, two maize synthetics were reciprocally recurrently selected for yield and mass selected for multiple disease resistance. The objective of this study was to determine selection response of two maize synthetics to six cycles of tandem selection for multiple leaf diseases (MLD) and multiple stalk rots (MSR). Plants were inoculated each cycle and evaluated for MLD including their causal agents; northern corn leaf blight, (NCLB) [Exserohilum turcicum (Pass.) Leonard and Suggs, Races 0 and 1], southern corn leaf blight (SCLB) [Bipolaris maydis (Nisik) Shoem.], northern corn leaf spot (NCLS) [Bipolaris zeicola (Stout) Shoem. Races 1, 2, and 3], anthracnose leaf blight [Colletrotrichum graminicola (CES) G. W. Wils.], and eyespot (Kabatiella zeae Narita and Hiratsuka). Following anthesis, plants were inoculated and evaluated for resistance to MSR including their causal agents; diplodia stalk rot (DSR) [Stenocarpella maydis (BERK) Sutton = syn. Diploidia maydis (BERK)], anthracnose stalk rot (ASR) (Colletotrichum graminicola), gibberella stalk rot (GSR) [Gibberella zeae (Shw.) Petch.], and fusarium stalk rot [Fusarium moniloforme, Shield]. In 1993 and 1994, selection cycles 0, 2, 4, and 6 of synthetics RSSSC, RBS10, and their cycle crosses were evaluated. Selection response to MLD, NCLB, SCLB, NCLS, gray leaf spot (GLS; Cercospora zeae-maydis Tehon and Daniels), MSR, DSR, GSR, and ASR were measured in separate experiments. Decreases in leaf blight severity from Co to C6 in RSSSC was 29% for MLD, 23% for NCLB, 33% for SCLB, 28% for NCLS, and 21% for GLS. Decreases for RBS10 were 34% for MLD, 33% for NCLB, 37% for SCLB, 49% for NCLS, and 16% for GLS. Cycle crosses were usually intermediate in values for leaf blight reductions. For stalk rots, the reduction in percentage internode area discolored from C₀ to C₆ for RSSSC was 44% for MSR, 42% for DSR, 39% for GSR, and 18% for ASR. Reductions for RBS10 were 63% for MSR, 67% for DSR, 64% for GSR, and 63% for ASR. Selection for multiple disease resistance along with a reciprocal recurrent program for yield resulted in significant improvement in resistance to multiple and individual diseases in RSSSC, RBS10, and their crosses.