Plant Diseases
Downy Mildew
Click her for more information about Downy Mildew
→ Although it is about grapes, it will give you the basic understanding of the disease.
Phytophthora
These are all photos of Phytophthora disease. The photos are of watermelons and eggplants.
Septoria Brown Spot on Soybean Leaves
Downy Mildew on Soybeans
Back side of leaf. 
Bacterial Soft Rot in Potatoes and Peppers
Black Rot on Grapes
Powdery Mildew
Bacterial Spot
Here you see bacterial spot on a pepper leaf.
In this photo you see bacterial spot on a tomato.
Click here for more information on bacterial spot and a few other diseases.
