An experiment to locate QTL conferring resistance against Peronosclerospora philippinensis causing Philippine downy mildew was observed in a BC1F2 population developed from a cross between the susceptible inbred line Pi 23 and the resistant inbred line P 345. Phenotypic data were collected in Los Baños, UPLB for disease incidence. A total of four regions were associated with disease incidence. Some of QTL associated with disease incidence have been reported previously, and the remaining QTL are described here for the first time. The results on QTL analysis using molecular markers also agree with the previous study using classical generation mean analysis of the cross combination Pi 23 x P 345.

No abstract

Resistance to Philippine downy mildew (Peronosclerospora philippinensis) was observed in a BC1F2 population developed from a cross between the susceptible inbred line Pi 23 and the resistant inbred line P 345. Five components of resistance namely, disease incidence, disease severity, onset of systemic symptoms, the-area- underdisease-progress- curve (AUDPC), and rate of downy mildew development affected host resistance reactions. All components were significantly correlated. The adult plant resistant reaction was evident in resistant progenies showing prolonged onset of systemic symptoms. Transgressive segregation that exceeded the high and low parent values was also observed for all components of quantitative resistance to Philippine downy mildew.

The genetics of resistance to Philippine downy mildew in maize (Zea mays L.) caused by Peronosclerospora philippinensis was studied in progeny derived from crosses between the resistant inbred P 345 and susceptible inbred Pi 17 and Pi 23. Plant generations used in this study were the PS (susceptible lines Pi 17 and Pi 23); PR (P 345); extensively F1, F2; F3; BS and BR. Plants at three leaf stages were artificially inoculated and evaluated for components of resistance, namely: disease incidence, disease severity, onset of systemic symptom, area under disease progress curve, and rate of downy mildew development. Analysis of generation means indicate that additive- dominance with epistasis gene effect play important role in all components of resistance to Philippines downy mildew.

One of the major disease problems affecting maize farming in the Philippines is bacterial stalk rot (BSR) caused by Pectobacterium chrysanthemi pv. zeae Burk., McFad. and Dim., which is formerly known as Erwinia chrysanthemi. The annual loss due to BSR is estimated at PhP. 20 million equivalent to Rp3.5 billion. At present, there is no effective control method against BSR and, therefore, varietal improvement through breeding resistant germplasms is needed. The present study aimed to determine the combining ability and the extent of additive and non-additive genetic effects in corn inbred lines with a range of reaction to BSR. Four resistant lines (S3YB 137-1-1-B, TUPI (S3) 5-1-B, TUPI (S3) 15-2-B, and 97-835) and two susceptible lines (CML 295 and 97-733) were used as genetic materials. Generation of test entries and evaluation of disease resistance were conducted at the experimental farm station of University of the Philippines Los Banos and Institute of Plant Breeding Los Banos, respectively, during 2002 to 2003 wet seasons. Griffing’s diallel mating system Model 1, Method 1 was followed in generating the test entries to make a total of 36 entries (six selfed parental lines and 15 each of F1 crosses and their reciprocal<br />crosses). The entries were then evaluated for disease resistance in a yield trial following a randomized complete block design (RCBD) with two replications. Results of diallel analysis<br />showed two lines, S3YB 137-1-1-B and TUPI (S3) 5-1-B, exhibited the best general combining ability (GCA) for resistance to BSR, while the crosses S3YB 137-1-1-B x TUPI (S3) 5-1-B and TUPI (S3) 5-1-B x 97-835 performed the best specific combining ability (SCA) for the resistance. GCA effect was greater than that of SCA. This indicated that additive gene effects were found to be more important than non-additive gene effects in the expression of resistance to BSR in the six corn lines used. Therefore, breeding programs towards recurrent selection that emphasize GCA would be more appropriate for<br />BSR resistance improvement involving those six lines.