Sinclair, Lyons' book "Diseases of Trees and Shrubs" from Cornell Press. Also one on "Insect pests of Trees and Shrubs". Then go through and highlight those diseases and insects that you will find in your area. We do not need to look for bagworms or any of the diseases that require high humidity in North Dakota. I have seen apple scab twice in 14 years, it was an annual event in central Illinois. After this look at the last 30 pages or so on abiotic disorders and tell them to start there since 90% of what they see starts there and can be ended there. It has great images, the index is categorized by tree and by disease. The pictures tell where they were taken and what time of year. Each disease is going to be present or show up within a very narrow time frame, so this is a great diagnostic tool.
Remember 40 below and 20% humidity keeps some of the riff-raff out!!!!
Bob