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Downy Mildew Appearing In Sunflower Fields

By Febina Mathew
 
Downy mildew is one of the most common diseases that can affect sunflowers early in the season. It is present in areas with poor drainage, where water can collect and spores of the causal pathogen (Plasmopara halstedii) can swim and infect the plants. Yield losses can be substantial depending on the percentage of infected plants and their distribution within the sunflower field. 
 
Disease Profile
 
 
 
Figure 1. Chlorosis (yellowing) on the upper side of the leaves from downy mildew infection. 
 
In South Dakota, given the cool, wet weather in June, we are seeing downy mildew develop in a few sunflower fields. However, the infected plants are scattered randomly in these affected fields and yield losses will be minimal, if they occur at all. Typically, downy mildew develops as a systemic infection caused by the pathogen early in the season. The spores enter the sunflower’s roots and invade the rest of plant, causing chlorosis (yellowing) on the upper side of the leaves (Figure 1). White cottony masses (mycelium) appear on the underside of the infected leaves (Figure 2). Sunflower plants may appear stunted and will produce little or no seed (Figure 3).
 
 
 
Fig. 2. White cottony masses (mycelium) on the underside of infected leaves. 
 
 
 
Fig. 3. Plants may appear stunted (notice differences in height between healthy plant and systemically infected plant) from downy mildew infection.
 
Delayed or secondary infections from the downy mildew pathogen can also occur on sunflower plants when zoospores are windblown from an infected plant onto the leaves of a nearby plant. However, secondary infections are not economically important and if the infection occurs, the sunflowers will “grow out” of the disease.
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