Introduction
The Asian soybean rust, Phakopsora pachyrhizi, has
been known to drastically reduce yields. In areas where the
pathogen occurs commonly, yield losses up to 80% have been
reported. The pathogen has been limited to the Eastern hemisphere
until it was found in Hawaii in 1994 and the United States
in 2004. The distribution of P. pachyrhizi includes
Africa, Asia, Australia, South America, and North American.
The rapid spread of P. pachyrhizi and potential for
severe yield losses makes this the most destructive foliar
disease of soybean. As soybean rust continues to infect soybean
in the U.S., it could have a major impact on both total soybean
production and production costs.
In general, conditions that promote good growth and full canopy
development of the soybean crop are also the conditions most
suitable for development of soybean rust. Temperatures in
the range of 15 to 28°C are optimal for infection and
spread of the pathogen as long as there is at least 6 to 12
hours of moisture (11). Moisture, as rain or dew, promotes
the spread of the pathogen once infection has taken place.
The normal temperatures and moistures found in the North Central
region of the U.S. are well within the ranges that would result
in severe yield losses. In 1984, an economic risk analysis
projected that the potential losses in the U.S. would be $7.1
billion per year, once soybean rust becomes established in
the main soybean growing area of the U.S. (9). A conservative
prediction indicated yield losses greater than 10% in nearly
all the U.S. soybean growing areas with losses up to 50% in
the Mississippi delta and southeastern costal states (19).
Geographic
distribution
The first report of the disease was from Japan in 1902. By
1934 the pathogen had been found in several other Asian countries
and as far south as Australia (4). In India, soybean rust
was first reported on soybean in 1951 (16). There have been
several early reports of soybean rust in equatorial Africa
(2, 7), but the first confirmed report of P. pachyrhizi
on the African continent was in 1996 from Kenya, Rwanda, and
Uganda. Since then, the pathogen has spread south with reports
from Zambia and Zimbabwe in 1998, Mozambique in 2000 and South
Africa in 2001 (10). The westward movement of the pathogen
on the African continent was reported from Nigeria in 1999
(1). The first detection of P. pachyrhizi in the
new world was in Paraguay in February of 2001. The disease
was found on soybeans in a limited number of fields in the
Parana River basin bordering Brazil. By 2002, soybean rust
was widespread throughout Paraguay and in limited areas of
Brazil bordering Paraguay, with reports of severe disease
in some fields in both countries (12). The pathogen also was
found in a limited area in northern Argentina (14). During
the 2003 growing season, the pathogen was detected in most
of the soybean growing regions of Brazil with a conservative
yield loss estimate of 2.2 MMT, or approximately 5% of the
annual production. In Paraguay, yield loses from rust were
limited due to dry conditions, while in Argentina the disease
did not spread to the major production areas. The disease
was found in Hawaii in 1994 on cultivated soybeans on the
islands of Oahu, Kakaha, Kauai, and Hilo (8). With this earlier
introduction into the U.S., a workshop was organized to discuss
issues related to the possible introduction of soybean rust
into the continental U.S. (17). In 2004, with the introduction
of soybean rust in the U.S., the USDA constructed the Coordinated
Framework for Soybean Rust Surveillance, Reporting,
Prediction, Management and Outreach. In 2005, a national
soybean rust sentinel plot system was implemented.
Host Range
Phakopsora pachyrhizi infects over 95 species of
plants from more than 42 genera (3), including soybean and
related Glycine species (13, 15, 18). Included in
the list are many of the wild and edible legumes. The full
host range of both rust species has not been clearly identified
and may be complicated by pathotypes and differential reactions
within host species (5, 6). Included in the list is Kudzu
(Pueraria lobata), which is widespread in the U.S.
and could serve as an inoculum reservoir or bridge host for
P. pachyrhizi in the southern U.S. Kudzu, growing
along roadsides and in ditch banks in Brazil and Paraguay,
was severely infected with rust and showed no apparent loss
of plant vigor (M. Miles, pers. obs.). The broad host range
of the two species of Phakopsora that infect soybean
is unusual among rust pathogens, as most rust species have
a narrow host range that is limited to a few plant species.
The large number of host species increases the likelihood
that this pathogen will survive and over winter in the southern
U.S., as well as in Central America.
Acknowledgement
and Disclaimer
Photo credits: USDA-Agricultural Research Service except where
noted from collaborators in the figure legend.
Names are necessary to report factually on available data;
however, the USDA neither guarantees nor warrants the standard
of the product, and the use of the name by the USDA implies
no approval of the product to the exclusion of others that
may also be suitable
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