Most problems of plant disease cannot be eradicated once they appear, at least not during the current growing season. For example, once you see scab on your crabapples, you cannot do much to help the tree that season. Likewise, diseases such as rusts, anthrac-nose, and needle casts must be managed before they appear.
Practically speaking, you can't take action against every disease that might occur. Instead, determine which diseases or what plants you wish to target. Start now, before the diseases occur, or it will once again be "too late for this year." List all the disease problems you remember from last year. Recall the amount of damage that occurred and whether you think control options are warranted: This may involve assessing the value of the plant, cost of management options, labor costs or availability, physical or aesthetic damage by the disease, etc. The goal is not 100%-disease-free plant material. A low level of disease is perfectly acceptable (at least to me). Tolerance for disease is something that varies with the grower, homeowner, landscaper, and nurseryman. Even I have my limits. I won't put up with a tree that is perennially defoliated by June.
As an example, maybe you or a client had problems with scab on a crabapple for the past 10 years. You are ready to take action for the upcoming season. Chemical sprays control the disease if initiated at the pink-bud stage of growth and continued according to label recommendations, but such control is effective for only one season. Consider the more lasting option of tree replacement with a scab-resistant species. You may need to do a little reading to determine disease severity and disease management options. Besides gardening and landscape texts, some very good disease fact sheets, called Reports on Plant Disease, available on the University of Illinois Extension Web site at www.ag.uiuc.edu/%7Evista/horticul.htm--free of charge. Other sources of information include past issues of this newsletter, references available in Extension offices, and the pest management guides for home growers and commercial growers.
Tired of powdery mildew on your zinnias? Ready to get rid of that rust-infested apple tree? For long-lasting disease management, use plant material with resistance to the disease pathogen you have identified. Many of the fact sheets from the university list resistant varieties or cultivars. This is a good starting point, but ultimately we have to work with the seed companies or nurseries to find out what is available for sale. Because it may take some time to track down the variety you want, get started now.