Possible Limits To The Value Of Seed-Treatment Fungicides In Corn
LEXINGTON, KY.
Pythium species are fungal
like organisms
commonly found in
agricultural soils. These are
the primary cause of seed
rot and seedling damping off
in corn in Kentucky, and
they are typically most active
in cool, wet soils.
The trend towards earlier planting dates increases
disease pressure on seedlings, because
earlier planted corn is more likely to sit in cool,
wet soil before successfully establishing itself.
The increased use of conservation tillage also
increases pressure from Pythium seed and
seedling diseases, since residue protected soil
does not dry out as quickly as plowed soil. The
importance of effective fungicidal treatment of
corn seed has increased because of these two
trends.
A recent study 1 by plant pathologists at The
Ohio State University closely examined the
Pythium organisms associated with seed and
seedling problems in corn and soybean in Ohio.
This article focuses on their findings for corn,
which are summarized as follows:
1. The most common species isolated from
diseased corn seeds and seedlings were
Pythium sylvaticum and Pythium dissotocum.
Less common were Pythium torulosum, Pythium
irregulare and Pythium inflatum. One interesting
find was that Pythium ultimum, the organism
that typically has been regarded far and
away as the Number 1 Pythium in corn, was infrequent
in their surveys. Perhaps changes in
cultural practices account for this shift; perhaps
something else is at work; but either way,
it is interesting how P. ultimum was so far down
the list now.
2. Of the five most common Pythiums found
in diseased corn seeds and seedlings, none
were highly aggressive on corn. Two were moderately
aggressive: P. sylvaticum and P. irregulare.
These two species were relatively sensitive
to the seed treatment fungicides mefenoxam
and captan but insensitive to the QoI fungicides
azoxystrobin and trifloxystrobin.
3. P. dissotocum, P. torulosum, and P. inflatum
were slightly aggressive on corn seeds and
seedlings. Based on their results, less than
complete control of P. dissotocum and P. inflatum
would be provided by mefenoxam, trifloxystrobin,
or captan. P. torulosum would be
difficult to control completely with mefenoxam
or captan.
4. Pythium graminicola was isolated less commonly
than the five listed above, but it was aggressive
on corn and insensitive to both
mefenoxam and trifloxystrobin.
Significance
These results suggest that a diversity of
Pythium organisms is responsible for seed and
seedling disease in corn under current production
practices. Significantly, these Pythiums
are not all controlled by a single fungicide used
for seed treatment. Because of this diversity,
improving drainage and planting when soil
temperatures exceed 50˚F remain important
cultural practices for minimizing seed andseedling
diseases in corn. For fields and farms
with a history of seed establishment problems
in cool, wet soils, consider using a mixture of
seed treatment fungicides to assure the best
chance of success in stand establishment. Δ
1 Broders, K. D., Lipps, P. E., Paul, P. A., and
Dorrance, A. E. 2007. Characterization of
Pythium spp. associated with corn and soybean
seed and seedling disease in Ohio. Plant Dis.
91:727 735.
Dr. Paul Vincelli is Extension Plant Pathologist
with the University of Kentucky at Lexington.