PRELIMINARY REPORT ON BREEDING EXPERIMENTS WITH THE CUCUMBER(Cucumis sativus L.)

PEILAN KAO

Cucumber(Cucumis sativus L.), one of the important vegetable crops of China, is affected seriously by downy mildew (Peronoplasmopara cubensis) in every part of the, country where this crop is grown. Although different commercial varieties differ in their susceptibility to this disease, yet none has been proved to be satisfactory to the growers. Repeated sprays of Bordeaux mixture or copper-soap solution may give partial control but they frequently cause spray injury. The experiments herein reported were carried out with the purpose of obtaining, by hy- bridization, new varieties of cucumber which would be more resistant to downy mildew as well as high in yield and quality. Work was started in summer, 1950, when pollen grains from Luffa cylindrica Roem, were applied to the female flowers of Peking Da-tz-gua, a variety of cucumber widely grown in Peking with fruits of very high quality but particularly susceptible to downy mildew. Luffa cylindrica was chosen as one of the parents because it is highly resistant to downy mildew, powdery mildew and aphids. The plants grow vigorously; they are high in yield and have a long harvesing season. As these two parents belong to different genus, the work met with complete failure, no fruit being obtained. In the summer of 1951,following Michurin's method of preliminary vegetative approxima- tion, young shoots from the seedlings of Peking Da-tz-gua and Bien-gua (another variety of cucumber grown in Peking of inferior quality but with higher resistance to downy mildew)were grafted respectively onto plants of Luf fa cylindrica. The female flowers of the scions were pollinated with mixed pollen grains of Luffa cytindrica and those of the scion itself. Fruits were obtained from the scions of both Da-tz-gua and Bien-gua, but those from the former were abnormal in shape and their seeds were either unable to germinate or gave very weak seedlings which died at the stage having 6-7 leaves. On the contrary, fruits set on the scions of Bien-gua developed nor- mally. Their seeds gave healthy plants. These seeds were sown in the greenhouse in winter, 1951. In order to obtain hybrids of the Da-tz-gua type, their flowers were pollinated with pollen grains of Da-tz-gua mixed with their own. Fruits set normally and good seeds were obtained. From 1952 to 1955, progenies of this cross were grown and studied in the field. With the purpose of testing their adaptabi- lity to forcing culture, supplementary observations were also carried out in the greenhouse. The results of 4-year's observations indicate that with regard to resistance to downy mildew, the hybrids, both of the Tz-gua type and the Bien-gua type, are superior to their parents. However, the hybrids are also affected by the disease, but not as serious as the parents and after spraying, they recover rather quickly. Thus, for instance, in 1955, the variety used as check (Peking Da-tz-gua) was seriously affected and in spite of spraying, little improvement was observed. As a result, beginning from the early part of July, yield from the check plots gradually dropped and all the plants had to be pulled out by the middle and later part of July. The hybrid cucumber was also affected by downy mildew, but after similar sprays as applied to the check, they recovered and continued to give yield up to the later part of August. With regard to yield, results obtained during the 3-year period from 1953-1955 show that both types of hybrid cucumber are higher than their respective check, expressed either in terms of the highest or average yield per plant. With the Tz-gua type, the highest yield per plant of the hybrid is 43.4-45.2% and the average yield per plant 47.9-77.1% higher than the check. With the Bien-gua type, the figures are 39.9-62.4% and 21.4-47.5% respectively. The hybrid cucumbers also have a longer period of harvest than the check, being 33-42 days longer with the Tz-gua type and 3-16 days for the Bien-gua type. The hybrid cucumbers show in some instances a fruiting habit quite different from the commercial varieties. On some plants, starting from the 6th or 8th node upward from the basal part of the stem, each node bears female flowers continuously for 5 or 6 or even 10 nodes. On some other plants, 2-3 fruits are borne in the same leaf axil and these fruits mature simultane- ously. The seeds of the hybrid cucumbers are larger and heavier than their parents. With the Tz-gua type. the average weight of 1,000 seeds is 31.44 g. for the hybrid and 25.92 g. for the check. With the Bien-gua type, it is 33.10g. for the hybrid and 24.27g. for the check. In order to ascertain the value of the hybrid cucumbers concerning their disease resistance, yield, quality and other characters, it is planned that variety test will be undertaken during the following years.