|
Downy Mildew (Plasmopara halstedii)
|
Downy mildew is common and economically important in environments with cool and wet conditions at planting. The disease is particularly common in the northern Great Plains of North America, including North Dakota, northern Minnesota and Manitoba. The disease does occur in the US High Plains and Southern Great Plains, but is less frequent. Root infection of seedlings results in stunted, systemically infected plants, many of which die. Plants that survive will have minimal seed set. However, total yield loss in a field depends on the number of plants infected and their distribution.
The pathogen is host specific and soil-borne, and can survive many years in the soil. The disease is favored by cool and wet/waterlogged soil conditions when the seeds are germinating and seedlings are emerging. Infection begins when motile zoospores, which swim in water, infect the roots of germinating seeds. Infected seedlings may die pre- or postemergence.
Surviving plants display systemic chlorosis on the upper side of emerging leaves, which radiates outward from the petioles (Figures 1 and 2). During periods of dew or rain, fluffy white growth will appear on the underside of the leaves, opposite the chlorosis (Figure 3). This is diagnostic and distinguishes the disease from powdery mildew (with white sporulation on upper leaf surface), herbicide damage and other abiotic or biotic ailments causing chlorotic leaves.
|