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Downy Mildew of Sunflower: Systemic and Secondary Infections (07/09/15)

We have been hearing a lot of reports about downy mildew of sunflower appearing in many fields in the area.

Downy Mildew of Sunflower: Systemic and Secondary Infections

We have been hearing a lot of reports about downy mildew of sunflower appearing in many fields in the area.  Unusually, much of this is secondary infection – also called local lesions.  While the systemically infected plants are stunted and will not likely yield, plants with secondary infections are likely just fine.

Systemic Infection

Systemic infection occurs in roots and very early in the season. If the roots of the germinating sunflower come into contact with pathogen spores in the soil they will become infected. This infection will progress through the whole plant, and after a couple weeks, chlorosis and sporulation on new leaves appears to progress through the leaf (Figures 1). Plants are typically heavily stunted and will not yield (Figure 2). Systemic infection usually occurs when there are saturated soils right after planting.

ppth.markell gilley humann.1.downy mildew

ppth.markell gilley humann.2.downy mildew

Secondary Infection

Secondary infection occurs only on the leaves, and is observed shortly after systemic infections begin to appear. Secondary infection occurs when windblown zoospores from one leaf (usually from a systemically infected plant) land on the leaves of a nearby plant. These spores cause small angular lesions on the leaf (Figure 3). These lesions will become chlorotic and sporulate but will not spread through the plant. We have never seen secondary infections cause yield loss.

ppth.markell gilley humann.3.downy mildew

Managing Downy Mildew

Systemic infections are the only economically important part of the downy mildew disease cycle. However, management has to occur before planting; resistant hybrids, seed treatments, for example.

Secondary infections are not considered economically important and no action is needed. The infection will not spread and new leaves should not be infected. Consequently, the sunflowers will appear to ‘grow out’ of the disease.

For more information please see the Downy Mildew of Sunflower NDSU publication PP-1402.

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

 

Ryan Humann

Ph.D. Candidate

Department of Plant Pathology

 

Michelle Gilley

M.S. graduate student

Department of Plant Pathology

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