SA Graan April 2015

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Downy mildew / Sunflowers
Downy mildew of sunflower:
Uncommon, but can cause localised damage
Downy mildew is caused by the soil-borne fungal pathogen
Plasmopara halstedii, which occurs in the major sunflower
production areas of South Africa.
Previously, minor downy mildew epidemics have been recorded in
the Rustenburg, Brits and Potchefstroom areas of the North West
Province of South Africa in commercial fields and research plots.
Epidemics are reliant on infection by primary inoculum from in-
fected seed or soil. Yield losses can be low to moderate, depending
on the percentage of infected plants and their distribution within
the field.
Epidemiology of the disease
The causal pathogen, P. halstedii, can survive for up to ten years
in contaminated soil as oospores. These oospores germinate when
soils are cool, accompanied by the presence of saturated water.
These oospores form zoosporangia, which will give rise to mobile
zoospores, which move through the soil.
Initial infections occur when these zoospores infect sunflower
seedlings. Infected plants usually die during the early growth stages,
but if plants do survive, it still grows and generally produces white
zoosporangia on the underside of the leaves prompting secondary
infection.
This is not always the case as dwarfed plants showing normal symp-
toms of downy mildew have been observed to not have the typical
leaf lesions and spore production on the underside of the leaf.
Symptoms of the disease
Early infected plants usually die, causing reduction in plant stand
and thus resulting in bare patches in the field. This has often been
the case where epidemics in South Africa have been recorded.
Systemically infected sunflower plants are usually dwarfed or
stunted (Photo 1) with shortened internodes and the sunflower
head pointing straight upward.
Surviving stunted, infected plants show a thickening and yellowing
of the leaves, which usually borders the veins of the leaves, but
can also be present on the whole leaf. White fungal mycelium and
spores appear on the underside of these leaves (Photo 2).
Disease control
Planting of resistant hybrids is recommended in areas where downy
mildew is a problem. Resistance is often race-related, and to deter-
mine adequate resistance, a survey of sunflower downy mildew in-
fected plants is necessary.
This will enable us to determine which P. halstedii races occur in
South Africa and which resistance genes need to be included into
local cultivars.
Volunteer sunflowers serve as alternate hosts of the downy mildew
causal pathogen; therefore, proper weed control in crop rotation
sequences can help reduce the disease. Crop rotation is important
because it prevents disease build-up in the field by interfering
with the life cycle of the disease. However, crop rotation will have
a minimal impact on downy mildew as the disease can survive in
the soil for more than ten years. Crop rotation programmes to re-
duce downy mildew need to be carefully planned.
The pathogen is seed- and soil-borne, therefore, fungicide seed
treatments can help minimise the disease. In the USA, metalaxyl re-
sistance is common and known to reduce seed treatment efficacy.
Metalaxyl resistant races of the pathogen have yet to be recorded in
South Africa so we assume at this point that metalaxyl is effective
for controlling infections. Alternate seed treatment fungicides are
used in the USA to counter this resistance. Foliar fungicide appli-
cations are neither effective nor economical.
The authors request that should any producer observe the above-
mentioned symptoms, to please contact them. They require isolates
for race determination studies to fully understand which resistan-
ce genes are effective to control this disease in local breeding
programmes.
Contact them at or FlettB@
arc.agric.za.
67
April 2015
ON FARM LEVEL Integrated pest control
1: Stunted downy mildew infected sunflower.
2: White fungal mycelium appearing on the underside of downy mildew
infected leaves.
MOSES RAMUSI and BRADLEY FLETT, ARC-Grain Crops Institute
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