By Shannon Friesen, PAg, Crops Extension Specialist, Moose Jaw
Noticeable yellow plants and patches in the field typically warrant a closer look. Yellowing plants can be caused by a number of factors, including disease, environmental stress or herbicide misapplication. In the case of pulse crops such as peas and lentils, a combination of disease, field history and environmental factors can be the cause of the problem.
Root rots are a common issue in pulse fields and often cause damage both above and below ground. There are several different root rots, such as Pythium, Rhizoctonia, Fusarium and Aphanomyces. They can appear in the field relatively early in the growing season and often get worse as the crops get established. It is common for later infections to appear, even as the crop is maturing.
Depending on the severity of the infection and type of pathogen responsible, yield loss can be quite significant. For example, research has shown that in wet years, up to 70 per cent yield loss can be expected if infection by Aphanomyces is high. Further yield loss may be caused by thin plant stands that are unable to compete with weeds; harvest challenges are common as late-season infections can delay maturity and cause lodging issues.
When scouting for root rots in the field, always practice good biosecurity measures to prevent further spread of the disease. This includes wearing disposable booties or shoes that can easily be sanitized with a bleach solution. Look for areas in the field, especially low spots, that crops appear stunted and thin or have a yellow or lime-green appearance. You may notice that individual plants remain a healthy green colour while its neighbour is yellowing, or that there is premature ripening. Pull some plants and examine the stems and roots. For plants with root rots, nodulation ability is often compromised so the nodules may be very small, rotting or even non-existent. Different root rots have different symptoms and you may see roots that are honey-brown or caramel coloured, or even dark brown or black.
Unfortunately, by the time symptoms are seen in the field, it is too late to do anything about it. However, scouting root rots, can help disease reduction for the future. There are several easy ways to reduce future infection from root rots, including practicing a minimum six-year crop rotation if Aphanomyces is present, introducing alternative pulse crops that are less susceptible, selecting lighter-textured fields with good drainage, ensuring that a good fertility package is applied, applying appropriate seed treatments and proper pesticides as needed and minimizing compaction in the field as much as possible.
If the infection is severe enough in a field, you may want to consider taking a soil sample and sending it to a lab for confirmation of the pathogen. The Crop Protection Lab is also available to help diagnose live plant samples.