Crops
●
March 2008
Yellow-bean culprit
Expert View
By NATALIA ROGOVSKA
A
FARMER had patches of yellow
soybean plants growing in his
otherwise green fields every
summer. He was convinced the problem
was attributable to infestations of soybean cyst nematode.
Sure enough, soil tests for SCN confirmed his suspicion. Densities ranged
from 300 to a whopping 37,000 SCN eggs
per 100 cubic centimeters of soil.
He complained that planting SCNresistant soybean varieties did not do
much to alleviate the problem and didn’t
increase yields. He switched to several
years of continuous corn trying to bring
the nematode levels down. Imagine his
disappointment when he saw the same
patches the first year he planted the
field back to soybeans.
What that grower, as many others,
did not realize is that the visual symptoms commonly associated with SCN
infestations are similar to those associated with other yield-limiting factors
such as nutrient deficiencies.
Parts of Iowa, especially the Des
Moines Lobe landform which covers a
large soil association area from central
Iowa extending north into Minnesota,
have high pH, calcareous soils. Some
of the micronutrients such as iron, zinc
and copper become deficient in such
soils. Soybean plants are especially sensitive to deficiencies of iron.
Symptoms of iron deficiency chlorosis are very similar to the symptoms
produced by SCN infestation: stunted
plants, yellowing of leaves and consequently reduced yields.
Symptoms are confusing
One very distinctive symptom of SCN
infestation, though, lies underground.
You need to look for adult SCN females,
which can be visible to the naked eye as
small cream-colored objects the size of
sand grains. However, even when SCN
infestations are confirmed, you should
not rush to conclude that SCN is solely
responsible for yellow, stunted beans.
A recent survey of more than 20 Iowa
fields planted to SCN-resistant and susceptible soybean varieties showed that
degree of SCN infestation depends on
soil pH levels. The study was conducted
during a period of five years on fields
within the Des Moines Lobe.
The study found that both pH and
nematodes were responsible for soybean yield reduction even though sometimes no clear visual symptoms were
present.
In SCN-susceptible varieties, about
40% of the yield reduction was attrib-
SCN thrives in high pH soil
10,000
SCN densities, eggs/100g soil
24 www.WallacesFarmer.com
8,000
SCN-resistant
SCN-susceptible
uted to pH and 10% to SCN. When
fields were planted with SCN-resis6,000
tant varieties, 13% of the yield was
lost because of high soil pH and
4,000
nematodes did not have a significant effect on yields.
2,000
The much greater magnitude of
yield reduction for SCN-susceptible
0
soybean varieties due to pH can
6.5-7.4
7.5-8.4
5.3-6.4
be explained by the dual negative
Soil pH range
effect of iron deficiency and nema- Yield robber: The ISU study showed that the number of
tode injury.
nematodes in the soil increases as soil pH increases.
Moreover, as the graph shows,
the SCN population present in the soil you have SCN, plant a soybean variety
increased with the increase in soil pH. that is resistant to SCN. If you have iron
Soybean plants grown in soil with a pH chlorosis, select a soybean variety that
greater than 7.5 had the highest rate of is also tolerant to iron chlorosis.
nematode infestation, but those plants
Are there other options? When it
also suffered from iron deficiency chlo- comes to options to reduce severity
rosis as iron availability is the lowest in of iron chlorosis or SCN, there are not
the range of 7.5 to 8.4 soil pH.
many economically feasible treatments.
Soil fumigation to control SCN or foliar
Variety selection crucial
applications of iron to correct iron defiIt is very easy to attribute the yellow, ciency chlorosis can be quite costly.
stunted plants to nematode infestaThe practical solution is to select
tion, especially if you see the soybean bean varieties that are resistant to SCN
roots covered with female cysts, when and, at the same time, tolerant to iron
in reality both factors — iron deficiency chlorosis. If you do this, don’t expect
chlorosis and SCN — contribute to the 100-bushel bean yields, but it is common
reduced yields.
to see a 10% to 20% yield increase in seWhat is more important: nematodes verely-affected areas of the field.
or iron deficiency? That’s hard to tell,
As for that soybean producer, he’s
but one thing is clear. It is essential to still planting continuous corn.
identify the problem correctly and use
Rogovska is a research agronomist in
the appropriate corrective measures. If soil science at ISU.
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