A LEAF-DISC BIOASSAY FOR SCREENING CUCUMBERS FOR RESISTANCE TO DOWNY MILDEW

A leaf-disc bioassay was used to screen cucumber breeding lines, F₁ hybrids, and advanced families for resistance to downy mildew caused by Pseudoperonospora cubensis. Leaf discs (2.2 cm diam.) were taken from the third leaf from the apex of young field-grown plants, placed on 0.5% water agar and inoculated with sporangia of P. cubensis. The disease index data obtained from leaf discs were positively correlated with the disease index data collected later in the field at an advanced stage of a natural downy mildew epidemic.