In general, root rots are much less problematic in dry conditions than in wet conditions.  Some root rot pathogens are not impacted much by dry conditions, while others all but disappear. 

Pathogens in the near-fungal group ‘oomycetes’ are inhibited by dry conditions.  They have swimming spores that need water to move, to cause infection and for disease to develop or for an epidemic to spread.  This group includes pathogens that cause downy mildew on sunflower, phytophthora on soybeans, pythium on many crops, late blight on potatoes, aphanomyces on sugar beets, and other less common diseases.  

Root rot pathogens that are true-fungi, such as Fusarium and Rhizoctonia, are less dependent on moisture to cause problems.  For the most part they need some moisture to get going, but soils do not need to be saturated for long periods of time to have problems.  Also, warm temperatures are not necessarily inhibitive for disease development.  For example, Rhizoctonia in sugar beet is minimally active until the soil temperatures reach the mid-60’s ^oF or greater.

When assessing root rots in dry years another thing to consider is the damage potential.  If roots rot pathogens cause infections early in the plants growth stage and the weather becomes very dry, the compromised roots are going to have a much harder time collecting moisture than if the soil was wet.  As a result, yield loss can occur from root rots in dry years; but this all depends on the development of the infections earlier in the season.

                 Sam Markell

Extension Plant Pathologist

samuel.markell@ndsu.edu