| Wheat breeding for scab resistance Hideo GOCHO Kyushu National Agricultural Experiment Station, Chikugo, Fukuoka 833, Japan More than three thousands wheat varieties and strains collected from home and abroad so far were screened for scab resistance. However, no vareity was immune or highly resistant under optimum conditions for the pathogen. Wheat group from Asia and Japan were recognized to contain many varieties resistant to the disease, comparing with those from Oceania, South America, Africa, Mediterenean countries and Europe. In home grown wheat varieties, Nobeokabozu-komugi showed highest resistance to scab. This variety is a local variety in Miyazaki Prefecture in Kyushu region where the disease causes most severe damage. Some of relatives of Einkornreihe and Emmerreihe and some of strains of Aegilopus and Agropyron, which were tested, were not contained immune or highly resistant strains. Judging from the results of genetical analysis on F1, F2, B1, B2 and F3 hybrids, wheat scab resistance seemes to be conditioned by polygene. Favorable genetic correlation between scab resistance and other agronomic characters was not found. The selection for scab resistance by generation of hybrid population showed that line selection in later generation is more effective than individual selection in early generation. For the purpose of increasing genetic variation of scab resistance in hybrid population, seeds of three varieties, Norin 61, Sumai 3 and Nobeokabozu-komugi, were irradiated with 20k rad from the 60Co source, and M4 and M5 generation were screened to select more resistant line than unirradiated parent. Only one resistant line was selected from the population Nobeokabozu-komugi irrdiated. However, the degree of the resistance of the selected line was not so high compared with that of original parent, whereas it was statistically significant. Up to now, little success have been obtained in breeding of comercial vareity resistant to scab at regional agricultural experiment station under juridiction of the Agriculture, Forestry and Fitheries Research Council, MAFF. The scab resistance selection experiment was carried out to get more resistant line from the crosses among relatively high resistant varieties. In the results, no more resistant line than parent varieties was selected but some selected lines were recognized to have the same resistance as that of higher resistant parent. Major causes not selected scab resistance line with desirable agronomic characters, seemed to be that most of the resistant varieties have some inferior agronomic characters such as long stem and late maturity, and that scab resistance is governed by polygenes which are separated in succeeding generation after cross. Therefore, most effective method for scab resistance breeding would be ones containing cycles of selection intercrossing, which woule allow greater chance for recombination between favorable genes. In a case where two varieties, deriving from different genealogy, have medium scab resistance but many superior agronomic characters, the crossing between these varieties might be expected to induce transgression that accumulated genes for scab resistance of the parent. |