DOWNY MILDEW RESISTANCE IN CUCUMBERS*

Abstract

Downy mildew is a common disease of cucumbers in many sections of the United States. It spreads by means of wind-borne spores and may be controlled by copper dusts or sprays. The development of downy mildew-resistant varieties would offer an easy and cheap method of control. Out of more than 80 varieties and strains tested in 1939 only China and Puerto Rico No. 37 were highly resistant to downy mildew. These were crossed with Colorado, a good commercial variety, and the F2 generations were grown in 1941. The F1 plants were intermediate for resistance. The F3 is being grown for a genetic study of downy mildew resistance since an accurate classification of resistant types is difficult in the F2. Resistant plants of fairly good commercial type were isolated in the F2.