In cucumber, nearly all crosses of powdery mildew resistant (PMR) x susceptible parents yield fully susceptible F1s. In a backcross program to transfer PMR from Spartan Salad to Marketmore 70, Munger observed one F1 progeny in which some plants showed partial resistance (1). Omara, by selfing nearly 100 plants of Spartan Salad and testcrossing them to Marketmore 70, found that one plant, 77-717 gave an F1 with uniformly low-intermediate PMR about equivalent to Tablegreen 65, while several others gave segregating F1 progenies. He also found that PI 197088 produced a similar F1 but it had more tendency to lose resistance as it matured. When Tablegreen 65 was crossed with either the PI 197088 or 77-717, PMR in the F1 is increased to a high-intermediate level similar to Poinsett.
In the present study, PMR from 77-717 has been carried through three consecutive backcrosses to Marketmore 70 without selfing until the BC3. In the F2 of the BC3 several plants with high-intermediate and high resistance were tested by crossing with Marketmore 70 and Tablegreen 65.
Only the F2 parents with high resistance produced F1 plants with more resistance than the other parent. The inheritance of this partially dominant resistance is not clear but is probably relatively simple since populations of 25 to 30 plants per generation have been adequate to permit its selection through 3 backcrosses. Some F3 progenies from the third backcross appear to have PMR equivalent to Marketmore 76 or Poinsett. This suggests that the incorporation of PMR by repeated backcrossing can be speeded considerably by using 77-717 as the resistant parent, thereby eliminating the need to self-pollinate the progeny after each cross. Furthermore, PMR lines bred in this way should produce more resistant F1s than previous PMR parents.
- Munger, H. M., Sadig Omara, and Abad Morales. 1979. Partially dominant genes for resistance to powdery mildew in cucumber. (Abstract). HortScience 14:447.