Downy mildew in sunflower: New virulent races Downy mildew caused by the fungus Plasmopara halstedii is a
common disease affecting sunflower in Manitoba. This pathogen is widely
spread in Manitoba, and favoured by high soil moisture conditions and
temperatures of 15-18 °C prevailing at the seedling stage of the crop.
The fungus invades the plants through the roots and systemic infection
progresses through the plants resulting in stunting, chlorosis of the
leaves from the top to bottom, and complete death of the infected plants.
Under high relative humidity, gray-colored myclia grow on the lower side
of the leaves producing large mass of conidia. Field survey of sunflower
diseases in Manitoba in 2011 revealed the presence of downy mildew in
9 out of the 11 fields (82%) with disease severity ranging from trace
to 10% infected plants in some fields. A total of 36 isolates of P.
halstedii have been collected from infected plants, pathogen isolated
and increased on susceptible plants in growth chambers, and assessed for
race identification using a universal set of 9 sunflower differential
genotypes. Preliminary results showed that the most prevalent races (75%)
are the 700 group including the races 730, 720, and 700 followed by the
race 330. Most commercial hybrids are susceptible to these virulent races;
however, the most recently released hybrids have better resistance to
downy mildew than the old hybrids. Seed treatment with fungicides to protect
sunflower seedlings from downy mildew infections is a common practice
for all sunflower hybrid seed available in Canada.
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