Cucurbit Genetics Cooperative Report 10:1 (article 1) 1987
Henry M. Munger and David P. Lane
Departments of Plant Breeding and Vegetable Crops, Cornell University, Ithaca, NY 14853
Lane and Munger reported (1) an association between resistance to powdery mildew (PMR) and susceptibility to target leafspot (TLS) caused by Corynespora cassiicola (Berk. and Curt.) Wei. This unfortunate situation has been found in most U.S. cucumber varieties. Wisconsin 2757 from C.E. Peterson was the first U.S. source in which we found resistance to both diseases. Lane crossed it with Poinsett 83, our backcross derived cucumber mosaic resistant version of Poinsett 76. Having been told that in the South Poinsett is extremely susceptible to target leafspot and finding the same true in greenhouse tests of Poinsett 83, we released the latter as germplasm only, pending incorporation of TLS resistance. This has now been accomplished through 5 backcrosses. Four sublines of Poinsett 83 with TLS are available as Poinsett 87, a germplasm rather than a variety release.
In another approach to resistance, linkage was broken in crosses between 2 near-isogenic pairs of lines with and without PMR, Cornell PMR551 x SR551 and Cornell PSMR18 x Wis.SMR18. Later, the same was done in the F2 of Marketmore 80 (PMS) x Marketmore 76 (PMR). We now have lines breeding true for combined resistance to mildew and Corynespora from all three crosses. In addition we backcrossed TLS resistance from W2757 into PMR551 and Marketmore 76 because of the possibility that a source of TLS resistance with PMR might give higher PMR than one originally linked with mildew susceptibility. This seems not to be the situation; the lines with TLS resistance from mildew susceptible Marketmore 70 and SR551 have as high PMR as those with resistance from W2757 which has a slightly higher level of PMR than either Marketmore 76 or PMR551. Likewise, Poinsett 87 has no greater PMR than Poinsett 83 even though the donor of TLS resistance, W2757, had much higher PMR.
It seems clear that the single dominant gene for Corynespora resistance is the same in all the sources we studied, and that it is near the locus that determines whether a cucumber has a minimal level of PMR. It is not associated with the modifier genes required for higher levels of PMR. Nevertheless, it may be advantageous to use the coupled resistances by selecting for the dominant TLS resistance gene and thereby carrying the basic recessive gene for PMR in most of the progeny.
Literature Cited
- Lane, David P. and Henry M. Munger. 1985. Linkage between Corynespora leafspot resistance and powdery mildew susceptibility in cucumber (Cucumis sativus). HortScience 20(3):593. (Abstr.).