Rapid identification of plant disease is essential for food security. Deep learning, the latest breakthrough in computer vision, is promising for plant disease severity classification, as the method avoids the labor-intensive feature engineering and threshold-based segmentation. Using a public dataset of 54,306 images of diseased and healthy plant leaves collected under controlled conditions, a deep convolutional neural network and unsupervised methods are used to identify 14 crop species and 26 diseases. The trained model achieves an accuracy of 89.83% on a held-out test set, demonstrating the feasibility of this approach.