Tree Disease Illustrations - An On-line Slide Show.

Robert W. Stack

North Dakota State University, Fargo, ND.

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For more than 20 years I taught an upper division undergraduate course on "Tree Diseases" mainly for majors in horticulture, forestry, and landscape design. It was designed as a second course in Plant Pathology, students having first taken the general plant pathology course in a previous term. In common with many instructors of similar "applied" courses, over the years I had accumulated numerous original slides taken specifically for instructional use. I used these in both lecture and laboratory settings. In the mid-1990's North Dakota State University began a program in Instructional Technology and provided support - expertise and equipment - for instructors who wished use network or web methods in their teaching ( At that time few quality illustrations of tree diseases were available on line and I started a project to place my slides on the web. Since then much illustrative material useful for instruction has become available, especially by public agencies, such as the US Forest Service, which have placed their publications on-line. ( Most instructors, myself included, make abundant use of such excellent publications. Sites presenting tree disease pictures organized in an instructional manner, as would be for a class lecture using slides, are much less common, however, and that is the purpose of my site. This format also makes these images available to other instructors and to users outside the classroom setting.


I chose only the slides which clearly show the symptom, sign, or situation of interest, and cropped the image as much as possible when I made the scan. I scanned the slides at 1200 x 1800 pixels resolution directly into Adobe Photoshop running on a PowerMac9600. The raw scans were saved as Photoshop files on cd. All image editing was also done in Photoshop.

One of the my considerations is to be backwards compatible. So I have kept with straight html and avoided the proprietary glitz and latest gimmicks. Anyone with even an older browser should be able to access my site. For the same reason I have limited the size of the images. Few students trying to study at home connected by a phone modem will wait the time it takes to download a 100kb jpeg file from a busy server. I have tried to follow some general rules: keep images to less than 40 kb - smaller if possible; use universal formats (GIF or JPEG); don't put more than a dozen pictures on any html page. I have found I need to do the editing (cropping, contrast and color adjustments, etc.) myself, since I know the effects I'm after and what I'm trying to show.

Each image has a brief accompanying explanation; image and text occupy one row of a table. In my original effort, not using tables, having them side by side was an insurmountable problem and resulted in a less-than-aesthetic format (see conifer diseases 1). More than 90% of all the slides I eventually intend to use are my own originals. For others, I obtained permission to use some borrowed ones in the first version of the site. There is so much public domain pictorial material available in on-line publications today, in contrast to just 4 years ago, that in future revisions, I will just provide a link to those sites rather than using any additional borrowed slides.


Students entering my site by the "front door" first encounter a welcome page which explains the organization of the site ( I have organized my site by type of disease and symptom, a structure for which I make no claim; it has been used many times before in forest pathology textbooks. The first image set illustrates examples of diseases of broadleaf trees. There are 31 slides in this set - too many images for a single html page. I divided the set into three pages, one featuring symptoms (, one of signs, and one of the effects of defoliation by disease. The 3 html pages are linked in a circular manner so that a viewer can enter anywhere and progress through the entire set. This is not intended to be a textbook but a slide show, so I have kept the text to a minimum, just sufficient to describe what's being shown. First completed in 1997, this set has been revised over the past year with an improved format.

I have several additional sets in various stages of development or revision. I have enough slides for eventually about eight sets of 30 to 50 pictures, each covering a different type of host or disease. The two other sets presently up are Conifer diseases (3 pages - 34 slides) and Abiotic disorders (4 pages - 42 slides). Both of these are still in the original format I started with in 1996-97.


The site has been well used by students in my class and in other classes on our campus. In the student evaluation at the end of the term, the web site ranked about equal with the slide sets viewed on a slide viewer in our library. Those students with their own newer computers liked the convenience of the web images better but those with older machines or no home computer or modem complained of crowding at campus computer clusters. There were some comments on the limited image quality compared to actual slides, but that concerned some students and not others. In addition to students on this campus, the site has been assigned by instructors at several other colleges as well as being picked by search engines. For 1999, the foliage diseases pages were requested about 100 times per month, based on statistics compiled by the NDSU computer center. I hope to continue development of this site and welcome any suggestions to improve it.


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