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Soybean Rust At-A-Glance
Soybean rust: It would be hard to find two words that cause more concern for
U.S. soybean growers.
But what exactly is soybean rust? As much as growers may have heard about this
disease, few have seen it in their own fields. The first thing to
understand is that soybean rust is caused by not one, but two fungal pathogens:
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Phakospora pachyrhizi, or the Asian type of rust, is the most
destructive pathogen and the one that poses the biggest threat to the U.S.
crop. The name can be somewhat misleading, because it has been found in
Australia, Africa, South America and Hawaii as well as Asia. This is the
pathogen that is most commonly associated with soybean rust.
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Phakopsora meibomiae, by contrast, is less aggressive and is not known
to cause severe yield losses in soybeans.
Scientists continue to learn more about how soybean rust behaves and how it
damages crops. Several facts are known:
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Three factors are necessary. The presence of the soybean rust pathogen
alone is not sufficient to cause infection. Remember the disease triangle:
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Soybean rust is highly mobile. Wind currents carried spores from Uganda
to Zimbabwe - 1,200 miles - in 2001. From 2001 to 2003, it traveled more than
1,500 miles, from Paraguay to near the equator. Its presence in Brazil places
the United States within striking distance, either by wind or a land bridge
route.
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Soybean rust has several hosts. In addition to soybeans, the host range
for soybean rust is more than 90 species of legumes - including wild soybean,
yam bean, snap bean, dry bean, yellow lupine, cowpea, and kudzu in the southern
United States.
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It spreads quickly on plants. Soybean rust can defoliate infected plants
in as few as seven days. The rate of infection can
balloon from 5 percent to 90 percent within 21 days.
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It is not winter-hardy. The soybean rust pathogen may be able to
overwinter on alternate hosts in South Texas, southern Florida and other
coastal states. However, the majority of the U.S. crop likely would be
reinfected each spring and summer by spores blown in by southerly winds.
There still is much to learn about the characteristics of soybean rust. But the
more growers understand about this threat to their bottom line, the better
prepared they will be to protect their crops.
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