Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Why does resistance to some pathogens not result in disease and for other pathogens disease still occurs? A complete discussion of the complex physiological, genetic, and phytochemical responses involved in resistance is not something that can be covered in the WCU. I will instead provide some very general information that may help clarify what you may see in field crops.
First, remember that we have two basic groups of pathogens — some need a host to survive and reproduce (obligate pathogens) and some can continue to develop in the absence of a plant host (non-obligate). Examples of obligate pathogens in agronomic crops include rusts, powdery mildew, downy mildew, viruses, and smuts. Examples of non-obligate pathogens include Fusarium diseases, Rhizoctonia, and leaf spots (Grey, Northern, Southern, Septoria, Stagonospora, Frogeye).
In many pathosystems involving the obligate pathogens, resistance is what we call vertical or race specific. This means that the host plant carries one or several resistance (R) genes that enable detection of the pathogen and defense against specific races (genetic groups) of a pathogen. Think of these R genes as triggers — they are tripped and result in a rapid response by the plant that can quickly stop the pathogen before it can set up shop. Sometimes this interaction is observed as tiny flecks on foliage. Each fleck might be a spot where a spore from a rust or powdery mildew landed on the leaf and was corralled before it could establish a connection with the host. This rapid response is known as a hypersensitive reaction. If a pathogen race moves in and the host lacks an appropriate R gene, then disease will progress normally. Examples of such occurrences include powdery mildew in wheat and barley, stripe rust, and southern rust in corn.
In other pathosystems involving nonobligate pathogens plants depend on the action of many genes. The sum activity of these genes results in the overall resistance to a pathogen. This resistance is often called horizontal resistance. Horizontal resistance does not use R genes. Instead, the genes involved tend to produce defensive structures or chemicals. Because horizontal resistance does not involve a hypersensitive reaction, the response is slower. Consequently, disease can still develop, although it is often greatly reduced. Unlike vertical resistance, races don’t tend to overcome this type of resistance and the environment can influence the degree of defense.
Thus, not all resistance is created equal. This is why resistance to powdery mildew and rusts seems so absolute, but for leaf spots and head blight it is more variable. If you see some leaf spots on a variety that was rated as resistant, this does not indicate that the resistance is failing.