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| Author(s): |
Josué Maldonado Ferreira
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Piracicaba. |
| Institution: | Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC) |
| Defense date: | 1999-04-29 |
| Advisor: | Jose Branco de Miranda Filho |
| Abstract | |
The importance of leaf diseases in maize increased in the 90 th decade in Brazil, as a consequence of economical losses. The southern corn rust is considered the most important leaf disease in Brazil, in comparison with others rusts, concerning its high potential of attack and damage. The objectives of this work were: a) to screen resistant populations from 140 improved varieties formerly evaluated in the NAP Project and supported by NAP-MILHO (Núcleo de Apoio à Pesquisa do Milho) of ESALQ/USP; b) to evaluate selected populations in crosses and to synthesize composites of varieties with high standard for disease resistance and acceptable agronomic values; c) to study the patterns of inheritance and genetic values related to resistance to the southern corn rust in maize populations. From NAP-MILHO results, thirty six populations were selected and divided in two groups: Group I ten populations selected for P.polysora resistance and yield; and Group II twenty six populations selected for P. polysora resistance and other leaf diseases with lower emphasis to grain yield. Fifty full-sib families from each of the ten populations in Group I were evaluated in two locations ( Ribeirão Preto, SP: Rio Verde, GO). The same populations were crossed in a diallel scheme. Populations from Group II plus nine populations of Group I were crossed in the topcross scheme, using the whole set of the parent populations as tester. In both experiments representing diallel and topcrosses the parent varieties were included and the experiments were evaluated in the mentioned locations in 1996/97. Two commercial hybrids (T1 and T2) with divergent patterns of resistance to P. polysora (POLY) were used as checks. The evaluated traits were: resistance to P. polysora (POLY), Physopella zea (PHIS), Phaeospheria maydis (PHAE), Exserohilum turcicum (ET), corn stunt complex (CSTUN) and percentage of damage ears (%ED), stand (ST), number of ears per plant (NE/P), stalk and root lodging (%QUE and %AC), corrected ear weight (PEC), corrected grain weight (PGC), plant height (AP), ear height (AE), ear placement in the stalk (PRE) and flowering dates (FLOR). The patterns of resistance for POLY, PHYS and PHAE were based on a rating scale for damages varying from 1 to 9, where the lower rate represented the highest resistance. Populations represented in the full-sib progeny experiments showed an excellent pattern of resistance to P. polysora (2.1 to 2.9), in comparison with T1 (2.4 to 3.1) and T2 (4.4 and 6.7). For PHYS and PHAE, the average rates observed in the additive genetic variance ( σ2A), heritability (h2), genetic coefficient of variation (CVg%) and index of variation (b) were obtained and indicated sufficient genetic variability and a good potential to be used in breeding programs in general and particularly for resistance to PHYS and PHAE. Progeny means indicated good yielding potential for most pf the populations in the study, as compared to T1 and T2. In the topcrosses and trials treatments were resistant to POLY and in the range of resistant to median resistant for PHYS and PHAE. The analysis of variance for PHYS and PHAE indicated the additive effects as the most important source of variation in the control of genetic resistance. The dominance effects were important to %ED and, in lower proportion, to PHAE. Significant heterosis effects were observed for yield traits in the diallel and topcross analysis, indicating the existence of genetic divergence among populations. Among the populations evaluated in diallel cross, five were selected for synthesis of a new composite: NAP21 (BR105), NAP48 (CMS 58 ND), NAP 49 (CMS 59 Sintético Elite), NAP 97 (IAPAR 51) e NAP 114 (PMI 9401). In the same way, five populations were selected among those evaluated in topcrosses: NAP07 (CMS 14C), NAP22 (BR 106), NAP44 (CMS 54), NAP97 (IAPAR 51) e NAP114 (PMI 9401). After recombination of the newly formed composites, they will be used in recurrent selection programs for yield and disease resistance. (AU) | |