Resistance in Cucumis sativus L. to Tetranychus urticae Koch

Chapter 1The role of plant breeding and particularly of host plant resistance in integrated control is discussed. Host plant resistance to insects and mites, especially to Tetranychus urticae is reviewed. A standard terminology for disease and pest resistance is recommended.Chapter 2The relationship between the twospotted spider mite and cucumber has been studied on plants and on leaf disks of a number of varieties with different levels of resistance. Existing laboratory tests are critically discussed and it appears that they are only reliable if many factors are taken into account. A new, more efficient laboratory test for resistance, measuring acceptance and reproduction is described.Chapter 3After a first screening of 800 cucumber varieties, a laboratory test and a practical test, the following nine varieties have been selected for their distinguishable level of resistance to the twospotted spider mite: PI 220860, 'Hybrid Long Green Pickle', PI 178885, 'Ohio MR 200', 'Taipei no 1', 'Robin 50', 'Aodai', PI 163222 and PI 218036. Besides resistance tolerance for the twospotted spider mite also appears to occur. Related Cucumis species do not seem to possess higher levels of resistance than the most resistant C. sativus varieties. The resistance-parameters acceptance and reproduction appear to be positively correlated. On resistant varieties the metabolism of the spider mites is clearly disturbed, but this seems not to be caused mainly by bitter principles.Chapter 4For several generations subpopulations of the twospotted spider mite were reared on cucumber varieties previously selected as partially resistant. Subsequently resistance tests were carried out in the laboratory and the glasshouse with mites from these subpopulations and concurrently with mites from a basic population reared on a susceptible cucumber line. In these tests neither acceptance nor net reproduction or damage index on the partially resistant varieties depended on the kind of mite populations used as inoculum. Therefore it is concluded that the resistance is genuine. The stability of the resistance is discussed.Chapter 5After crossing partially resistant varieties some lines with a markedly higher resistance level were selected. This transgression for resistance indicates a polygenic inheritance of the resistance. On the most resistant F 5 lines selected, oviposition was reduced by 50 to 60 and the economic damage threshold was reached 5 to 8 weeks later than in the susceptible control. The level of acceptance was not influenced by the selection.The selection of individual F 2 plants was hampered by low heritabilities, whereas the heritabilities of F 3 - and subsequent line means were generally high enough. Selection for higher resistance levels was attended by an unintended increase in cucurbitacin content. This is explained by linkage of genes for resistance and bitterness rather than by identity of these genes.Chapter 6The bitter cucumber variety Improved Long Green did not differ in resistance to the twospotted spider mite from its near isogenic non-bitter mutant, the source of all non-bitter varieties. Four pairs of near isogenic bitter and non-bitter varieties gave similar results. Therefore any causal relation between bitterness and resistance is denied.Chapter 7The inheritance of resistance to the twospotted spider mite and of bitterness in cucumber has been studied in three sets of P 1 , P 2 , F 1 , F 2 , B 11 and B 12 of crosses between three bitter, resistant lines and one non-bitter, susceptible line. Resistance to the twospotted spider mite as measured by acceptance and oviposition appeared to be determined by several to many genes, which are inherited mainly in an additive fashion. Bitterness is basically governed by the gene Bi, which, contrary to earlier reports, is inherited in an intermediary way, while the expression of Bi is influenced by additively inherited intensifier genes. Whereas Bi and the bitterness intensifier genes are not related to the resistance factors acceptance and oviposition, they are related to resistance or tolerance as measured by the damage index. This relation is explained by linkage rather than by identity of the genes concerned. Changes in the test methods and breeding consequences are discussed.