There is not a one-size-fits-all answer, and that’s good! North Dakota has a wealth of diversity in crop production, soil types, cultural practices, crop inputs and many other factors. One way to help simplify the thought process and understand risk is to think about the corners of the disease triangle (host, pathogen and environment); we will use leaf rust of rye as our example.
Host: What is the host range for leaf rust of rye? Rust pathogens tend to be very host specific, meaning that the rust we find on rye will not cause disease on other crops (i.e., sunflower, dry beans, wheat and barley). Just knowing that leaf rust of rye will be confined to rye drastically reduces potential disease risk. Other diseases that might be associated with cover crops can have a much larger host range (i.e., wheat streak mosaic, ergot, soybean cyst nematode). I encourage everyone to do homework on the pathogens (diseases) affecting a cover crop plant species and determine whether a relationship exists with a field crop.
Pathogen: How and where does the pathogen survive? The leaf rust pathogen of rye is not able to survive in North Dakota. This pathogen has a complex life cycle (two hosts and multiple spore stages) and overwinters in the southern United States. Southerly winds blow spores northward during the growing season. Therefore, we often do not observe leaf rust of rye until later stages of grain development or, in some cases, in the rye crop that was seeded in the fall. Knowing the leaf rust pathogen does not overwinter in North Dakota tells us the spores observed in the fall will not be the cause of infections observed in the following spring. For other pathogens found in cover crops that can overwinter in North Dakota (such as soybean cyst nematode), determine the management tools that are already in place for the field crop of concern and whether if you need to employ more.
Environment: Arguably, the biggest driver of disease epidemics in North Dakota is the environment. Development of leaf rust of rye is favored by warm temperatures and prolonged periods of leaf wetness (dew or rain). These conditions are more common starting in June and helps explain why we see leaf rust near grain fill and/or during the fall. In North Dakota, diseases can be found in hot, cool, dry and wet environments, so understanding the environmental profile of a pathogen helps determine the associated disease risk.
Final thought: When determining disease risk associated with cover crops, consider the most problematic diseases in your field crops and start asking questions if a disease risk relationship exists. Then, use the disease triangle to help guide conversations related to the topic.