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Downy mildew on cucumber. Note the yellow angular spots. These will eventually turn necrotic as seen near the center of the photo.
Downy mildew on cantaloupe. Note chlorotic (yellow) halos around the lesions. This is diagnostic of the disease on cantaloupe. However, positive identification of downy mildew requires visualization of the causal fungus which can be found on the leaf underside.
The underside of a cantaloupe leaf showing downy mildew lesions. Often, spore masses are not visible to the naked eye, but are nonetheless present. They can be seen with a 20X hand lens.
Downy Mildew on acorn squash.
Downy mildew on the underside of a butternut squash leaf. Note the typical angular lesions with heavy sporuluation.
Downy mildew on the underside of a cucumber leaf as seen through a low-power microscope (40x magnification). The dark pepper-like flecks are spores of the causal agent Pseudoperonospora cubensis.
The downy mildew fungus (Pseudoperonospora cubensis) magnified 100 times. The spores (sporangia) are lemon-shaped and the tree-like structure (sporangiophore) is where the sporangia are produced. Sporangia measure 20-40 ´ 14-25 µm. Sporangia detach from their sporangiophores very easily.
Downy mildew can cause a variety of symptoms. On this pumpkin leaf, massive infection occurred after heavy rains. Each chlorotic (yellow) spot on the leaf upperside, corresponds to a visible fungus on the leaf underside.
Downy Mildew on watermelon. Anthracnose symptoms (see below) are very similar and can be easily misdiagnosed. Diagnosis must be done using a microscope.
Anthracnose symptoms on watermelon. Note how closely they resemble downy mildew symptoms. This can be easily misdiagnosed and proper diagnosis must be done utilizing a microscope.
Gummy stem blight lesion (caused by Didymella brioniae) on cucumber. Note the concentric rings caused by the variable growth rate of the fungus through host tissue.