Registration of 27 Maize Parental Inbred Lines Resistant to Maize Mosaic Virus

Twenty-seven inbred lines of maize (Zea mays L.) (Reg. no. PL-327 to PL-353, PI 641224 to 641250) have been released by Hawaii Foundation Seeds (HFS) of the College of Tropical Agriculture andHuman Resources of the University of Hawaii. They were bred specifically to incorporate, by backcrossing, resistance to maize mosaic virus (MMV), a major disease in tropical ecosystems (Brewbaker, 1981). From 8 to 22 generations of breeding were involved (average 5 13.0). This completes our series of 70 such conversions, including 13 inbreds released in 1997 (Brewbaker, 1997) and many that were not formally released (Brewbaker et al., 1989). Registration numbers, PI numbers, parentages and latitudes of origin are summarized in Table 1. The inbreds were chosen on the basis of their outstanding performance in international hybrids and in trials coordinated by HFS (Brewbaker et al., 1989). Inbreds Hi42 to Hi45 are solely from breeding in Hawaii, while the others originated from public corn breeders in ten countries. The converted inbreds represent five or more backcrosses for conversion to the incompletely dominantMv allele (Chrom. 3:801) that governsMMV resistance (Ming et al., 1997). Backcrossing was followed by several generations of sib and self pollinations to achieve homozygosity. Most cycles of selection were performed under controlled field epiphytotics of this virus in Hawaii. MMV is transmitted by the leafhopper Peregrinus maidis, and both virus and leafhopper are essentially limited to the maize crop (Brewbaker, 1981). We discovered that the Mv allele occurred only among six Caribbean races that were often grown year-round by Arawak and Carib (Brewbaker, 1979). The disease is particularly severe for sweet corn growers in Hawaii, who plant weekly year-round, and is becoming more serious throughout the tropics as year-round production of maize increases. Descriptions of the inbreds (Table 2) are based primarily on performance data from trials in Hawaii and Nigeria (Brewbaker et al., 1991). The days to silking ranged from 55 to 65, under daylengths of 12 to 13 h and temperatures of 24 to 308C typical for the tropics. Familiar temperate inbreds Hi47 (B73) and Hi60 (Mo17) averaged 58 d to silk. Most inbreds were yellow flints (f) or semi-flints (d/f). Two African lines, Hi56 and Hi66, were white flinty-dents (f/d). White Zimbabwe inbred Hi61 was retained after conversion both as white and yellow sublines. Cob color was white for all inbreds except those based on B37, Mo17 and SETG117. Data for plant and ear heights, filled kernels per row (kpr), 100-kernel weight (100 kw), kernel depth (kd), ear lengths and diameters, and grain yield varied greatly due to seasonal differences in the tropics (Brewbaker, 2003) and represent averages for summer conditions in Hawaii (Table 2). Numbers of husks and of kernel rows per ear were not affected by daylength and temperature variations encountered. Parental lines for inbreds Hi54 and Hi55 carried a dwarf gene, brachytic-2, that was eliminated in backcrossing. Modifications during backcrossing and inbreeding affected several quantitative traits of these parents, including improved ear height, lodging tolerance, and tolerance to fusarium rots, rusts and blights common in Hawaii. Inbreds Hi42 and Hi43 were developed from composite Suwan1-C7 in Hawaii, a composite that segregates for MMV resistance. Hi44 and Hi45 were outstanding inbreds among our SET G RILs (Ming et al., 1997), based on parents Hi31 (a B68 conversion, resistant to MMV) and Hi58. All other inbreds represent conversions of publicly available lines that were included in the MIR (Maize Inbred Resistance) studies since 1975 (Kim et al., 1987; Brewbaker et al., 1989). A MIR database summarizing agronomic traits and resistance to diseases, pests, and stresses of over 150 inbreds included in these studies is available on compact disk from Hawaii Foundation Seeds. All inbreds were chosen for their recognized high general combining ability and many of them originated before 1970 (e.g., Hi46 to Hi56, Hi60 and Hi61, Hi65 to Hi68). They represent many different heterotic groups and have appeared in marketed public hybrids worldwide. The inbreds were characterized for general resistance to diseases and pests that are common in the tropics (Table 3). These resistance scores are averages frommany trials in Hawaii and several other countries before the Mv conversion. Converted inbreds have been retested, insofar as possible, to assure no loss of resistance. Rating for resistance is presented on an empirical scale of 15 highly resistant to 95 highly susceptible. These numbers correspond approximately to damage in percent of the means of pertinent experimental trials, with 1 (, 60%), 2 (60–70%), 3 (70–80%), 4 (80–90%), 5 (90–100%), 6 (100–110%), 7 (110–120%), 8 (120–130%) and 9 (. 130%). In most cases scores of 7 to 9 represented major loss of grain yield of the inbred. High genetic tolerance to common rust (caused by Puccinia sorghi Schw.) and southern rust (caused by Puccinia polysora Underw.) characterized most inbreds of tropical origin (Table 3). These diseases are severe annually in Hawaii, providing many data to supplement those from five other countries in our earlier studies. Northern blight [caused by Exserohilum turcicum (Pass.) L&S] is severe in Hawaii’s highlands, where many inbreds could be classified as intermediate or low in tolerance. Tropical inbreds normally showed resistance to southern blight [caused by Bipolaris maydis (Nisik.) Shoem.] and to kernel rots (caused by Fusarium verticilliodes Table 1. Hawaii inbreds with PI number, Registration number, parentage, and latitude of origin.