The Leaf Doctor app

Leaf Doctor is a free iPhone and Adroid app for pathometry, the quantitative assessment of plant disease intensity.

Download Leaf Doctor at

This interactive app allows users to collect or submit photographs of symptomatic plant organs (e.g., leaves) and measure the percentage of tissue area diseased. The assessment data and photographs may be sent by e-mail to any recipient. This site describes the app and provides instructions for using it.

The Leaf Doctor app Home and About screens

1. PHOTOGRAPHS (Analyze):

The assessment algorithm is based on the user-guided recognition of pixel colors for healthy plant tissues. You may use your iPhone to take a photograph or analyze photographs already saved to your gallery. 

Leaves must be photographed individually against a solid black background, such as a black piece of paper or cloth. 

Remember to develop and use an appropriate sampling plan to gather plant organs for analysis by The Leaf Doctor. Comparative statistics and mathematical models for disease severity are only as reliable or as the data from which they derive.

Alternately, a photograph may be modified using Adobe Photoshop or other software to create a black background to surround an individual leaf or leaves. Such photographs can be e-mailed to the app user, who saves it to his gallery for disease assessment via The Leaf Doctor. 

Take photographs under shade, such as that provided by a dark umbrella or a dense plant canopy. This will help to minimize reflected light on leaf surfaces that can complicate the app’s disease measurement algorithm. Do not use a camera flash.

Select an image from the gallery or use the camera
Touch healthy areas to specify up to eight colors for healthy tissues


Touch a healthy area within the photograph to begin a disease assessment. This specifies the color of healthy tissue. Use the circular magnifying lens that appears when you touch the screen to select and specify colors precisely. 

You may select several different colors to represent the healthy areas by touching consecutive areas of the photograph that correspond to the healthy tissue. For example, healthy veins may be a different color than a leaf blade, and should be touched so they are no grouped with diseased tissues in the analysis.

After removing the background (i.e., the black color), the algorithm analyzes the entire image, pixel by pixel, to determine the proximity of a pixel’s color to the healthy color(s). 

The maximum distance from a healthy color can be adjusted by using the threshold slider. At any time, additional healthy tissues may be touched if they are revealed incorrectly as diseased when you slide the threshold bar.

After you are satisfied with the result by visual inspection, the app computes the percentages of healthy and diseased pixels and displays the result.


Slide the threshold bar to shade the diseased areas for calculation of % disease


You may send assessment data and photographs by e-mail singly or as a s

et to a recipient. You must specify the recipient's address by entering the text in the e-mail header.

Within the text of the e-mail, you may add more information to embellish the data, such as the names of host and disease, the location, or the experimental treatment. Data may also be deleted from the history.

Data may also be deleted singly or entirely.  


E-mail or delete the results singly or as a group

Slide a cell to the left to reveal the red delete box ('X') for individual data

Applications for data derived from The Leaf Doctor:

  • Quantify the progress of plant disease epidemics in space and/or time;
  • Compare treatments used for plant disease management using analysis of variance and means separation;
  • Evaluate host resistance or compare host varieties and environments for disease development;
  • Develop standard area diagrams for plant disease assessment.

Limitations of Version 1:

Although the current algorithm is robust, the Version 1 of the Leaf Doctor app was developed on a limited budget and there are limitations.

  • Lesions with a dark black color may not be recognized as part of the diseased tissue.
  • The background color used for the plant organ (e.g., leaf) must be pure black, not gray or another color.
  • There is an element of subjectivity in recognition of diseased tissue. However, this can be a positive if an evaluator wishes to recognize yellow as non-healthy. The algorithm performs better for some diseases than for others.

We intend to improve the algorithm after we receive additional funding for development of the Version 2.

Advantages of Version 1:

  • Low cost (free; compare cost with PC software, Assess 2.0)
  • Fast and easy to use
  • User interactivity
  • Simple and elegant algorithm
  • Portability (iPhone)
  • Can be installed and used on several mobile iOS devices
  • High accuracy is possible, compare with the use of standard area diagrams


Support for development was provided by USDA National Institute of Food and Agriculture Hatch funds managed by the College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. 

The Leaf Doctor was created by Dr. Scot Nelson of the University of Hawaii at Manoa, College of Tropical Agriculture and Human Resources (UH-CTAHR), and Dr. Sarah Pethybridge (Cornell University).

Contact or questions about The Leaf Doctor app: 

Scot C. Nelson

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