Last week Dr. Sam Markell, Extension plant pathologist at NDSU released a cautionary press release to North Dakota sunflower producers concerning the increasing risk for sunflower rust to cause significant damage to that state’s crop.  Locations of most concern were western North Dakota extending as far east as the Mohall area.  Additionally, Dr. Markell stated that sunflower rust was present near the North Dakota-Minnesota border at much lower levels. 

At about the same time, Vince Ulstad, BASF Technical representative, reported finding rust pustules on sunflower plants (growth stages R1-R2) located within fields near main roadways extending from Grand Forks east to Thief River Falls - St. Hilaire.  He reported that pustules were widespread across fields (high incidence), while few were found on individual plants (low severity). 

I did a quick field survey on Saturday (July 19) northeast of Warren and came up rust-free.  Bob Koch, Head of the Minnesota Dept. of Ag (MDA) Plant Pest Survey Program has alerted state surveyors to the potential risk, as well.  Sunflower fields will be targeted by MDA surveyors during the next two weeks to establish disease levels in our state and determine risks for crop injury.  Producers and consultants should also scout for rust and be prepared to treat, if needed. 

Disease Development Stats

Much like rusts in other crops, a pustule on sunflower can emerge from tissues in about one week if weather conditions favor infection and disease development (60-80˚F, 6 hr. leaf wetness).  Pustules release huge numbers of microscopic spores that are capable of spreading disease to other leaves, plants, and fields (Figure 1).  They are readily transported via air currents.  The disease often establishes in the lower canopy and works its way up as new leaves emerge.  Sunflower rust is not confined to leaf tissues.  It is also commonly found on heads, as well. 

Disease Management

Few fungicides are labeled for use on sunflower.  However, Headline (pyraclostrobin) and Quadris (azoxystrobin) can be used to manage rust before severity has reached the damage threshold.  Previous research indicates that a fungicide treatment should be scheduled before the upper four leaves have pustules covering 3% of surface area.   

Strobilurin fungicides, such as Headline and Quadris, do not arrest fungal development from infection established prior to application.  These active ingredients largely protect plants for a time from being infected by spores arriving after the application is made.  Consequently, leaf injury caused by pustules already present, as well as pustules developing within leaf tissues, will not be reversed.  Therefore, it is important to treat in a timely manner.  With this said, if disease development is slow enough, try to avoid treating plants before the crop has reached the flowering growth stage.  An application at this stage will best protect upper leaves during a time when seed development is critical. 

Note:  Headline has a 21 day, and Quadris a 30 day, pre-harvest index.  Always read and follow fungicide label directions. 

For additional information on soybean rust, try these references: