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Downy
Mildew of Sunflower
Compliments of
Agronomy Services Department
Downy Mildew is a disease farmers have had
no concern over since the advent of a fungicide seed treatment called
Apron. This fungicide has been so effective at eliminating Downy Mildew,
that many farmers do not even recognize the disease that is now invading
some sunflower fields this spring. Ironically, Apron's excellent control
of Downy Mildew is the reason we are now facing the disease once again.
When a species is threatened with extinction, it often finds a way to
survive. Downy Mildew now has developed new strains that Apron seed
treatment is not effective against.
Downy Mildew is a fungus that is soil born.
Spores can be moved by wind, blowing dirt or moving water. Volunteer
sunflowers are often a perfect medium for the disease to increase and be
distributed to new production fields. Cool, water-saturated fields are
prime targets for Downy Mildew infection. A splashing rain on a new field
of sunflower at or shortly after emergence can cause heavy infection.
Typical symptoms in seedlings include
yellowing or chlorosis of the leaves (See Figure 1) accompanied by a white
cottony mass on the underside of these leaves. (See Figure 2) Infected
plants will be dwarfed. Many seedlings are killed by the fungus, but those
that survive the disease will be stunted and produce very small heads that
face upright.
Sunflowers are susceptible to the disease
from the seedling stage and during the first several weeks of growth. The
fungus may survive in the soil for five to ten years. Fields that have
never had sunflower on them have been known to become diseased because of
the ease that the fungus is spread by wind.
Yield loss will generally be light if
infection is 15% or less. The disease is usually spread evenly across the
field and surrounding plants will compensate for seedlings that die early.
If infections of 25% or more are detected, then economic yield losses
occur.
Current management strategies include
planting on well drained, warm soil. Good crop rotation practices will
help and early control of volunteer sunflowers can help reduce the spread
of fungal spores.
New fungicides are being tested, with at
least one that appears promising in controlling the disease. Fungicide
options for Downy Mildew look to be at least two years away. Conventional
breeding techniques have resulted in some hybrids that may have total
resistance to all current known strains of Downy Mildew. Mycogen Seeds has
two such hybrids in strip trials this summer. One or the other, if not
both, should be available within the next two years. In the meantime, good
management will be required along with some help from Mother Nature to
help control Downy Mildew.
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