ALBERT

Notes: Summary In three trials of cucumber varieties differences were observed in the degree of infection by the red spider mite Tetranychus urticae Koch. The possibilities of utilising a certain level of resistance in breeding are discussed.

Notes: Summary A number of investigations have been made to ascertain whether it is possible to select for taste in tomato. The taste of different cultivars and selections was assessed organoleptically, while comparative chemical analyses were carried out, both on whole fruits and on fruit parts. From these investigations the conclusion was drawn that under certain conditions selection for a better taste may be possible, but that for this purpose an organoleptic taste test is required, which cannot, for the time being, be replaced by indirect methods.

Notes: Summary In a test on bean seeds for their germinating capacity at low temperatures good germination was found in Phaseolus trilobus Ait. and P. coccineus L. and also in a variety of P. vulgaris, viz Comtesse de Chambord. Particularly at soil temperatures below 15°C this latter variety germinated better than the other vulgaris varieties. The ability of Comtesse de Chambord to germinate at low temperatures could for a large part be retained in an attempt to combine this feature with good cultural properties of snap beans.

Notes: Summary In crosses between highly resistant and susceptible parents, the resulting segregation ratios indicated that three genes govern a high level of resistance to cucumber mosaic, caused by Cucumis virus 1. The F1 between highly resistant and susceptible is intermediate. The present results were compared with the contradictory results in the literature.

Notes: Summary When searching for resistance to Cucumis virus 2 in cucumber a variety was encountered which, after infection, did not show disease symptoms, but it did show a decrease in yield. This reduction in yield did not differ significantly from that of a susceptible variety showing symptoms. Attention is drawn to the risk of using such a variety, which acts as a carrier, as basic material for breeding.

Notes: Abstract After the introduction, in 1961, of breeding lines of tomato with complete resistance to all races of Cladosporium fulvum known until then, it was found in 1963 that this resistance could be broken by the occurrence of new races. Resistance to the new races was shown to be also available and to be dependent on one dominant factor. Assumedly a new relation between host and parasite had occurred. Consequences for breeding are discussed.

Notes: Summary The possibility of sucessfully crossing snapbean (Phaseolus vulgaris) and Ph. ritensis was examined because it was expected that in this manner a transfer of tolerance to low temperatures from the latter species to the cultivated snap bean varieties might be effected. Hybrid plants were only obtained when Ph. vulgaris was used as the female parent and artificial culture of the embryos was applied. Total sterility in the interspecific hybrid could be overcome by chromosome doubling. Embryo culture was also necessary to raise the triploid plants produced in the first backcross to the snap bean. From the second backcross generation 13- and 14-lines were obtained by self-pollination. This plant meterial is being tested for tolerance to low temperatures.

Notes: Summary Powdery mildew in cucumbers (Cucumis sativus L.) can be caused by two fungi, Erysiphe cichoracearum and Sphaerotheca fuliginea. In The Netherlands we have been mainly concerned with the latter, at least during the last few years. A high degree of resistance to S. fuliginea was obtained by combinations of progenitors that are in themselves insufficiently resistant. It was made plausible that the resistance obtained is governed by three recessive genes. The relation to the resistance against E. cichoracearum, mentioned elsewhere, is discussed.

Notes: Abstract In growing parthenocarpic cucumber varieties the occurrence of bullnecks due to pollination by bees is a problem. To prevent the formation of these bullnecked cucumbers the possibilities of breeding female-flowered varieties were explored. Use was made of material derived from the gynoecious line MSU 713-5. Femaleness is caused by the gene Acr, which accelerates the production of female flowers. Within the type which, genetically, must be regarded as female, a variation occurs from total absence of male flowers to rather numerous axils with male flowers, mainly on the lower part of the stem. On the one hand this variation is also genetically controlled, and we may speak of “weakly” female and “strongly” female. On the other, environmental factors play an important part. The formation of male flowers in preponderantly female-flowered material is promoted by a long day, high night temperature and particularly, at least with our material, low light intensity; furthermore by a low pH of the soil and rich dressings of nitrogen. In order to be able to distinguish during selection between the weakly and strongly female plants, the environmental conditions should be favourable for the formation of male flowers. The maintenance of female plants is possible, because the formation of male flowers can be induced by spraying with gibberellin, after which selfing can be practised. Old plants respond less readily to gibberellin than young plants. Also, the response from strongly female plants is less pronounced than that from weakly female ones, which offers another possibility of selection. The extent to which male flowers may occur when growing female varieties depends on the cultivation method used. With the conventional method fruiting is forced to begin mainly on the laterals and any male flowers, if confined to the main stem, could be removed early. However, in view of fluctuations caused by the environment a rather pronounced female character will be desirable. Other cultivation methods are conceivable, e.g. short-term cultivation of larger numbers of plants per area unit, allowing the plants to come into bearing rapidly. The total absence of male flowers is then indispensable. With the aid of the femaleness it is also possible to simplify the production of hybrid seed, as hand pollination can be replaced by bee pollination. The combination gynoecious × monoecious, which in itself is the most favourable for this purpose, has the drawback that the resulting hybrid is usually insufficiently female. Therefore, use will generally have to be made of the combination gynoecious × gynoecious, spraying the latter parent with gibberellin.

Notes: Summary The flesh of ripe cucumber fruits can show different colours, varying from intense or dingy white to yellow and orange. These colours appear to be governed by two genes, for which the symbols V and W are proposed. If both genes are recessive, an orange colour is produced; if both are dominant they give a dirty white colour. No linkage with the genes for mildew resistance could be demonstrated. A very strong linkage was observed between mildew resistance and a dull green exterior fruit colour governed by one gene for which the symbol D is proposed.