Non-Sikkim Cucumber Accessions Resistant to Downy Mildew (Pseudoperonospora cubensis)

Downy mildew caused by the oomycete Pseudoperonospora cubensis is a devastating disease of cucurbits. Cucumis species are attacked by pathotype 3 (clade 2) of the pathogen, while Cucurbita species are attacked by pathotype 6 (clade 1). The Sikkim-type cucumbers PI 197088 and PI 330628 express high levels of resistance against both pathotypes (clades) of the pathogen but no green-fruit cucumber cultivars resistant to the disease are available on the market. Here we report on several non-Sikkim accessions of cucumber that show resistance against downy mildew in four consecutive seasons. Mean % foliage attacked with downy mildew in the susceptible controls Ilan and SMR-18 was 93% and 71%, respectively, as against 0.2% and 1.8% in the Sikkim-type resistant controls PI 197088 and PI 330628, respectively. Twenty-four green fruit accessions were significantly more resistant than the susceptible cucumber controls. Five accessions showed less than 10% infected leaf area with downy mildew as follows: PI 432870—5%, PI 390266—7.5%, PI 418964—8.5%, PI 390258—8.8%, and G12—10%. PI 390258 and PI 390266 were susceptible to race 1 of powdery mildew but resistant to race 2, whereas PI 418964 was resistant to both races. These accessions may be used in breeding programs to accelerate the production of green-fruit, disease-resistant cucumbers.