Cucurbit downy mildew caused by the oomycete pathogen Pseudoperonospora cubensis

has been a sporadic disease concern in Wisconsin for many years. Since 2004, the United States

has seen a novel introduction of a variant of the pathogen which overcomes the dm1 cucumber

host resistance to downy mildew. The work of Drs. Lina Quesada Ocampo of North Carolina

State University, and Mary Hausbeck of Michigan State University has greatly elevated our

knowledge of cucurbit downy mildew in the US since 2005. The information that I provide in

this piece comes from their work. Because Wisconsin sees sporadic and infrequent cases of

cucurbit downy mildew, my fungicide recommendations typically reference the trial outcomes of

Dr. Mary Hausbeck at Michigan State Univ. due to proximity to our production region.

Currently, we have two groups of cucurbit downy mildew pathogens. One group, termed Clade

1 infects acorn squash, pumpkin, butternut squash, watermelon, bittermelon, and balsam apple.

The second group, termed Clade 2 is the newly introduced strain and it infects cucumber,

cantaloupe and Buffalo gourd. An additional notable difference between the clades is that Clade

2 very quickly becomes resistant to single site mode of action fungicides, rendering them useless.

Monitoring of these clades has become critical in the southeastern US where both clades

routinely are present. Clade 1 tends to arrive in NC fields later than Clade 2. Most cucumber

varieties commercially available are resistant to Clade 1 (old strain), but there is no complete

resistance available for Clade 2. For pickling cucumber, ‘Citadel’ and ‘Peacemaker’ from

Seminis are tolerant to Clade 2. For slicing cucumber, SV3462CS and SV4142CL are tolerant to

Clade 2. Greenhouse growers should use tolerant varieties since chemical options in a

greenhouse are very limited and the risk of fungicide resistance is very high.