Usefulness of local varieties for developing resistant varieties to Philippine downy mildew disease

Evaluation of local varieties as genetic resources having resistant gene(s) to Philippine downy mildew disease, Sclerospora philippinensis Weston, has been carried out. The materials used in the present investigation were as follows: (NE#1 x Ph 9 DMR)F1, Aroman, MIT VAR 2, Mimies(MIT), Tiniguib(CMU), Cebu, Marianas, Maguindanao Red, Cotabato, White Glutinous DMR, Isabela(CVIT), La Granja Popcorn, and UPCA VAR 3. MIT VAR 2, White Glutinous DMR, and UPCA VAR 3 are improved varieties, (NE#1 x Ph 9 DMR)F1 is a single cross, and the other nine are local varieties with provincial names of the Philippines.

Experimental design was randomized complete blocks with three replications in which each plot had 50 plants. Inoculation was conducted by the method of Yamada et al. (JARQ 10:168, 1976) with 30 x 103 conidia/ml at 1.5-2.0 leaf stage of seedlings. This test was carried out at an experimental field of University of Philippines at Los Banos, College of Agriculture, where no natural infection could take place since there were no infected plants around. Diallel analysis on the inheritance of susceptibility of all entries was done by the program DIALLA and DIALLB coded by Kumagai (Bul. Comput. Cen. Res. Agr. For. Fish. A 1:273, 1968).

Infection percentages, based upon number of seedlings with systemic symptoms obtained on the 26th day after the inoculation, were compiled and their statistical parameters were calculated after arc-sine transformation. In Table 1, a diallel table of means in every array and other parameters is laid out. Infection percentages of UPCA VAR 3 and La Granja Popcorn used here as the susceptibles were not so high as expected, 60.34(75.5%) and 72.75 (91.2%), respectively. Correlation coefficients among genetic parameters are tabulated in Table 2. These data indicated that there were extremely high correlations between Mid-parent and F1 means or G.C.A., between F1 means and G.C.A., and between Heterosis and S.C.A.

The Wr/Vr graph in Fig. 1 indicates that the degree of dominance was partial with average dominance 0.594 and with regression coefficient of almost unity (b = 0.945 ± 0.136). Moreover, there was significant correlation (r = 0.697) between parental value and (Vr+Wr). The Vr-!Wr relationship suggests that the two susceptible materials, UPCA VAR 3 and La Granja Popcorn, were the recessive parents and the other nine varieties, except Marianas and Cebu, had dominant gene(s).

Table 1.

Mochizuki (Trop. Agr. Res. Ser. 8:179, 1975) pointed out that resistance at the open-pollinated variety level was controlled by polygenic systems accompanied by no dominance, based on results reported in several papers. The test reported here was followed by the complete diallel table on recommended varieties and local open-pollinated ones, particularly collected at heavily infected areas, and was conducted under artificial inoculation. Results obtained seem to be different from those already published data. Since degree of dominance for resistance to Philippine downy mildew disease is regarded as partial dominance, it is concluded that the susceptibility was governed by recessive gene(s). Owing to high correlation between MP or F1 and G.C.A., it seems possible to introduce resistant gene(s) from local varieties for breeding. In this test, a limited number of local Varieties was used as materials for diallel analysis, regardless of whether they were typical local varieties or not. So, based on the results here it is expected to collect further the local varieties of maize as genetic resources having resistant gene(s) to Philippine downy mildew.

Fig. 1.

Minoru Yamada and Bliss A. Aday


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