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February 25, 2008
Source:
Kentucky Pest News #1153
by Paul Vincelli
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[i] 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 and
seedling 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.
[1] Characterization of Pythium spp. associated with corn and
soybean seed and seedling disease in Ohio.
Broders, K. D., Lipps, P. E., Paul, P. A., and Dorrance, A. E.
2007.
Plant Dis. 91:727-735. |
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