As this season progresses, we will be confronted with many plant disease problems. Some will be harmless, some a nuisance, and some quite devastating. If we could choose a preferred method of disease control, it would be resistance. Of course, that is not always possible.
Disease resistance is the capacity of a plant to lessen the harmful effects of a pathogen. We see fewer disease symptoms on plants with some level of resistance. Resistance is an inherited trait. It seems most useful in preventing diseases such as rust, powdery mildew, vascular wilts, and scab; but there are available varieties resistant against many other diseases.
If a pathogen is able to cause only a small amount of disease on a plant, that plant is resistant. If the pathogen causes a large amount of disease, the host is susceptible. Resistance is a continuum from mild disease to severe disease, with all levels in between.
In terms of disease-control options, resistance is preferred over other methods because it
- Reduces expense (no labor or chemical costs)
- Eliminates inconvenience of other disease-control activities, such as pruning
- Eliminates side effects, such as impact of chemicals on the environment
- May be the only disease-control option, as may be the case with Verticillium wilt or crown gall diseases
Disease resistance may be rated in many ways. There is no uniform rating scale. The usual scales are numerical or ordinal. Many companies that rate their plants for resistance level use a 1-to-9 scale, with1 indicating most susceptible and 9 most resistant. Beware, however, that other companies use a 9-to-1 scale or other variation. Ordinal scales include ratings using words such as high, medium, or low disease resistance. Word scales are easy to understand but are not always as precise as numerical ratings. HR for highly resistant, R for resistant, MR for moderately resistant, MS for moderately susceptible, and S for susceptible are commonly used. When using resistant plant material, look for the rating provided by the seller, but also look for an explanation of the scale.
Where do we find disease-resistance information? There is no one central location. It can be found in a variety of places, some highly visible and some difficult to find. Web searches have made this information more available. Some sources to try include university publications; botanic garden trials; breeding and selection programs, such as the U.S. National Arboretum, land-grant universities, and the private sector; plant societies, such as the hosta society.
There are also textbooks and other publications that list resistance information. Some of this information will last for many years; and some will be short-lived, as new pathogen races develop. An excellent book that discusses available disease-resistant cultivars is Diseases of Woody Ornamentals & Trees in Nurseries, by APS Press, St. Paul, MN. Some journals and newsletters that discuss current resistance information include California Agriculture, Greenhouse Grower, Mycologist, Plant Disease, and the Ohio Florists Association Bulletin. Always be open to these and other sources of information. The U of I Report on Plant Disease series provides a starting point.
Who sells resistant plant material? Identifying the resistant plants is useless if the plants are not available. Often it is best to start with operators of local garden centers that know the disease problems in an area and try to find sources of resistance in locally adapted plants. A program called Chicagoland Grows, Inc., evaluates plants that do well in northern Illinois and provides a list of retail businesses that sell them. The Center for Development of Hardy Landscape Plants provides similar information in Minnesota. Undoubtedly, there are other such sources.
Try to find disease-resistant varieties in the plants you purchase this spring, especially for areas where you had problems in the past. As an example, there are many varieties of phlox with powdery mildew resistance. Some tomatoes have resistance to Verticillium and Fusarium infection. Most new crabapples are resistant to scab. There are hundreds of other examples, so look into disease resistance now, before you purchase or plant.