Leaf Doctor performs quantitative assessments for plant diseases on plant organs such as leaves. Users collect or submit photographs of diseased plant organs and calculate the percentage of diseased tissue. Through a user’s touching of the device screen, the algorithm employs user-specified values for up to eight colors of healthy tissues in the photograph. The color of each pixel is then evaluated for its distance from the healthy colors and assigned a status of either healthy or diseased. Users may slide a threshold bar until satisfied that diseased tissues are represented accurately before the percentage calculation. The assessment data and photographs may be sent by e-mail to any recipients. Instructions for using the app are provided on the About screen. The Leaf Doctor was produced by Cornell University and the University of Hawaii at Manoa, College of Tropical Agriculture and Human Resources.