The invention is directed to methods of diagnosing a Pseudoperonospora cubensis infection in a cucurbit plant, comprising detecting at least one nucleotide sequence selected from the group of nucleotide sequences of SEQ ID NOs: 1-52, or any combination thereof, in a sample from a cucurbit plant or in an environmental sample, thereby diagnosing a P. cubensis infection. Also provided is a method of selecting a cucurbit plant comprising at least one resistance gene, the method comprising introducing into the cucurbit plant a nucleotide sequence of any one of SEQ ID NOs: 1-52, wherein the expression of the nucleotide sequence produces a hypersensitive response in a cucurbit plant comprising a resistance gene, thereby identifying the cucurbit plant as comprising a resistance gene; and selecting the plant having the hypersensitive response and identified as comprising said resistance gene.