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Soybean Rust Origin
Description
Detection
Life Cycle
Look-a-like Diseases
Soybean
Rust Origin
Asian soybean rust,
caused by the Phakopsora pachyrhizi fungus, is a major soybean disease that was discovered for the first time in the continental United States in nine southeastern/Midwestern states in November and December, 2004. It has for some years been active in Asia and Australia, causing periodic epidemic outbreaks in some regions, and had in recent years spread to Hawaii and Puerto Rico.
First detected in Japan in 1902, Asian soybean rust can cause premature
defoliation, with yield losses of up to 70% reported in farms in Asia.
Phakopsora meibomiae is the less-virulent fungal species found in South America that also causes soybean rust.
Description
Asian soybean
rust causes lesions on cotyledons, stems, petioles, leaves and pods of
soybean and other host plants. The main effects on the soybean plant are
destruction of photosynthetic tissue, which in turn causes premature defoliation,
early maturation, and severe yield reductions through reduction in the
number of pods and seeds, and decreased seed weight.
Detection
Scouting for soybean
rust may begin at anytime during the growing season, but detection is
typically easiest just prior to or during the flowering stages. Infection
usually begins on leaves low in the canopy, and chlorotic regions begin
to appear along with lesions found mainly on the lower leaf surface, although
they can appear anywhere on the plant. Symptoms and signs will move to
the middle and upper plant canopy as the severity of the infection increases.
Signs of infection
include tan or reddish lesions containing raised uredinia postules, mainly
on the underside of leaves. Areas surrounding reddish lesions may be necrotic.
Uredinia may also be found on upper leaf surfaces, though they are typically
smaller and less abundant. Lesions are restricted by leaf veins, and may
be as large as 2-3 square millimeters.
Life
Cycle
The soybean rust life
cycle begins with inoculum in the form of urediospores being blown onto
healthy soybean plants. Under prime conditions, uredinia mature 6-7 days
after infection of healthy soybean plants, and urediospores are produced
for the next 10-11 days. These urediospores may germinate and develop,
increasing the amount of infection on the same plant, or be carried to
and infect another soybean plant (nearby or far away) or an alternate
host plant.
Telium and teliospores
may also be produced during the infection period, but this is a rare occurrence
in nature. The role of teliospores in the soybean rust life cycle is currently
being investigated.
The short life cycle
of the soybean rust fungus increases the likelihood of many generations
being produced within a short period of time, as well as creating a short
window of opportunity to put control measures in place before the infection
increases dramatically.
Look-a-likes
Several other diseases of soybean exhibit similar symptomology including
bacterial pustule (Xanthomonas axonopodis pv. glycines),
bacterial blight (Pseudomonas savastanoi pv. glycinea) and
the fungal disease septoria brown spot (Septoria glycines).
The bacterial look-a-likes may be distinguished from soybean rust by examining
postules under magnification. Soybean rust pustules will have a pore at
the top when mature; bacterial pustules will have a fissure on top instead.
Bacterial pustules may also be surrounded by a yellow halo that is absent
in soybean rust pustules.
Brown spot doesn't have pustules as the bacterial diseases and soybean
rust do. However, it does begin its infection in the lower canopy like
soybean rust does, so this may increase the chance of a misdiagnosis.
The table below compares traits of soybean rust and common look-a-likes.
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