By Dr. Charla Hollingsworth, Plant Pathologist, U of Minnesota Extension
and Dr. Sam Markell, Plant Pathologist, NDSU Extension Service
This past week, the fungus that causes rust on sunflower, Puccinia helianthi, was identified on wild and volunteer sunflowers in Minnesota and North Dakota. The rust fungus is known as a "macrocyclic" pathogen because it produces five successive types of spores during its lifecycle. While all five types of spores are produced on sunflower, only one type is responsible for causing rust epidemics.
The first spores produced of the growing season (basidiospores), have already infected sunflower, resulting in "pycnia". Tiny and bright orange, these structures have been found in volunteer sunflowers north of Fisher, MN by Craig Hanson, BASF. Later scouting turned up more pycnia structures east of Crookston and again on sunflower volunteers (Fig 1A). The same week, Kyle Schepp and Kent McKay (Vision Research), Mike Hutter (Northern Ag Management), and Jack Rasmussen (NDSU Plant Pathology) also noticed pycnia on cultivated and wild sunflower in North Dakota. Pycnia produce the third type of structure called "aecia" (Fig 1B), which will eventually lead to uredial pustules. These cinnamon-brown uredial pustules produce masses of spores and are responsible for rust epidemics and crop loss (Fig 1C). Finally, at the end of the growing season the fifth type of spore is produced. Darkly-colored teliospores are resilient enough to survive some Minnesota and North Dakota winters. The evidence is in -- they survived the 2008-09 winter.
Now that we know that the pathogen is established and cycling through its spore stages, we must stay aware of the health of our sunflower crop. After sunflowers emerge and establish, routine scouting will go a long way in preventing nasty disease-related surprises later. During 2008, sunflower rust was widespread across North Dakota while Minnesota had isolated fields that were severely diseased.
It is very important to control volunteer sunflowers to reduce the number of pre-uredial spores being produced. This is particularly true if 2009 sunflowers are planted near a 2008 sunflower production field. Wild sunflowers are another consideration. Since they are also a susceptible host, wild populations should be destroyed, or be kept mowed. Airborne rust spores travel from field to field easily.
A period of approximately two weeks generally occurs after plant infection and before rust pustules develop. Rust is most likely to cause economic damage when it establishes on the crop early and environmental conditions promote disease development. There is some good news. The disease can be managed with fungicide. Currently, three fungicides, Quadris (FRAC 11), Folicur (FRAC 3) and Headline (FRAC 11), are labeled on sunflowers for rust control in Minnesota and North Dakota. If the disease establishes relatively late in the growing season (after flower), yield loss may not occur.
The Minnesota Dept. of Agriculture (MDA) Plant Pest Survey Program is mindful of the potential for trouble by this disease. This year MDA surveyors will target sunflower fields so disease development is monitored in the State. Survey information will be posted at: