Downy and powdery mildew on the underside of a butternut squash leaf. The dark areas are masses of downy mildew spores produced by the fungus, Pseudoperonospora cubensis. The white powdery spots are masses of powdery mildew spores produced by the fungus Sphaerotheca fuliginea. Note how downy mildew lesions are more angular in shape (bound by leaf veins) whereas powdery mildew tends to spread out radially without regard for leaf veins.

Downy mildew on cantaloupe. Note chlorotic (yellow) halos around the lesions. This is diagnostic of the disease. 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 cucumber. Note the yellow angular spots. These will eventually turn necrotic as seen near the center of the photo.

Downy mildew on the underside of a cucumber leaf. Here sporulation is visible to the naked eye in the lesion near the center of the photo.

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.

Anthracnose on cucumber caused by the fungus Colletotrichum orbiculare. Note the tan spots characteristic of early infection.

Mature infection producing "classic" anthracnose symptoms. Note how the center of the lesions is cracking and dropping out.

Advanced anthracnose. It is not uncommon to see this disease virtually wipe out a field.

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.

Target spot on cucumber caused by Corynespora cassiicola. The symptoms of this disease resemble those of downy mildew. Note the angular shape of the leaf spots. The key to diagnosis lies in seeing the fungus. C. cassiicola is very different from P. cubensis.