Virginia Asian Soybean Rust

In addition to Phakopsora pachyrhizi, the causal agent of Asian soybean rust, another rust fungus, Phakopsora meibomiae, also causes rust on soybean and other legumes. P. meibomiae is found throughout South and Central America and the Caribbean. Fortunately, P. meibomiae is not as virulent as P. pachyrhizi.

Symptoms typically develop on lower leaves first. Chlorotic flecking of leaves is an early symptom of soybean rust. Spots are typically small (2 to 5 mm in diameter). Positioning a leaf so that it is backlit (hold the leaf up to the sky) will help to visualize these tiny spots when scouting the field. Later leaf spots may appear tan, dark red-brown or, less frequently, gray-green.
When sporulation begins pustules form in these spots, mostly on the lower leaf surface. Sporulation typically occurs in about 9 days after infection when favorable environmental conditions are present

Magnified pustules resemble microscopic volcanos.
Microscopic examination reveals urediniospores erupting through a circular opening in the pustule. These spores can be produced on a plant in repeating cycles of about every 9 days when environmental conditions are favorable for spore production and infection (i.e. temperatures from 59° to 82° F and high humidity or rainy weather). These spores initiate new infections and are readily carried by wind to new plants, fields or long distance to new geographic areas. Near the end of the growing season telia and teliospores may be produced. The waxy and darkly pigmented telia burst through the leaf epidermis .
Ureidiniospores erupting from leaf surface on soybean.
Asian soybean rust urediniospores viewed at high magnification under a compound microscope. These spores were found on soybean in Virginia during the 2006 growing season. (Image courtesy of Patrick Phipps, Tidewater AREC, Virginia Tech)
As Asian soybean rust progresses, leaves yellow and premature defoliation occurs. This results in decreased pod production and fill. Disease can progress very rapidly with defoliation occurring within a month or month and a half after the initial infection.
Reddish-brown lesions have been associated with decreased spore production (i.e. a resistant reaction).
Mottling and chlorosis may also be associated with lesions.

Asian soybean rust spores collected in Seminole County, Georgia from volunteer soybean, viewed at 400 times magnification.
The USDA Agricultural Research Service's Foreign Disease-Weed Science Research Unit, in cooperation with the Pictsweet Company, Mississippi State University and Iowa State University, has released images of Asian soybean rust on soybean and other hosts (see below) as an identification aid for Asian soybean rust. Please note that these images were taken from plants grown in greenhouse conditions and symptoms in the field may differ. For example, spores on plants outdoors would most likely be disseminated through wind and rain and it would be unlikely for spores to accumulate on leaves, as is observed in greenhouse conditions. Read more about the work of the research team.
Return to top
Look-a-Like Diseases on Soybean
Brown spot (Septoria glycines) is a common fungal disease on soybean in Virginia. Symptoms are brown, pinpoint to 4 mm size spots that occur on upper and lower leaf surfaces.
Downy mildew (Peronospora manshurica) symptoms may begin as pale green to irregular yellow spots on the upper leaf surface. As the disease progresses, grayish sporangiophores form on the lower leaf surface.
The following two images of downy mildew on soybean were from a sample submitted to the Plant Disease Clinic at Virginia Tech by the Clarke county extension office in 2005. Symptoms on this Pioneer variety 93M90 are small angular brown spots on both the upper and lower leaf surfaces.

Frogeye leaf spot (Cercospora sojina) initially appears as angular to circular leaf spots. Thecenters of these spots change to silver to tan with age and are encircled with reddish borders. This fungal disease was prevalent in soybean fields in Virginia in 2004.
Bacterial blight (Pseudomonas savastanoi pv. glycinea) produces small, angular, water-soaked, yellow to light brown spots on leaves. As the disease progresses, lesions turn reddish brown to black and are bordered with yellowish green halos. Centers of older lesions frequently drop out, resulting in a tattered appearance.
Early Bacterial pustule (Xanthomonas axonopodis pv. glycines) symptoms are pale green spots with raised centers that may occur on both leaf surfaces. Bacterial pustule is easily confused with Asian soybean rust due to small raised, light colored pustules that form in mature lesions, usually on undersides of leaves.
These look-a-like diseases may occur simultaneously with Asian soybean rust in a field and may cause Asian soybean rust to be overlooked in its early stages.
Return to top
Other Virginia Soybean Pests
The insect, thrips, was a common problem in dry soybean fields in Virginia during 2005. Although thrips injury looks quite different from symptoms of Asian soybean rust, the untrained observer may be uncertain. The soybean samples pictured below were submitted to the Plant Disease Clinic at Virginia Tech.
Silvery, white thrips damage on soybean leaves.
Brown and yellow flecking from thrips injury on upper soybean leaf surface.
Browning on lower leaf surface and along veins associated with thrips injury on soybean.
These leaves are showing browning from thrips injury.
The larval stage of thrips.
Return to top
Other Soybean Rust Hosts
A list of reported hosts of Phakopsora pachyrhizi has been compiled. See the pdf list. The USDA Agricultural Research Service's Foreign Disease-Weed Science Research Unit, in cooperation with the Pictsweet Company, Mississippi State University and Iowa State University, has released images of Asian soybean rust on cowpea, lima, jicama, lupine kidney, kudzu (low and high sporulation), green bean (low and high sporulation), to help scouts and others identify Asian soybean rust. Please note that these images were taken from plants grown in greenhouse conditions and symptoms in the field may differ. For example, spores on plants outdoors would most likely be disseminated through wind and rain and it would be unlikely for spores to accumulate on leaves, as is observed in greenhouse conditions. Read more about the work of the research team. Snap beans (Phaseolus vulgaris) are susceptible to Asian soybean rust. Information on symptoms, disease cycle, epidemiology, and management of rust on snap beans can be found in: 2005 Alert: Asian Soybean Rust Threat to Dry and Snap Beans (pdf). Asian soybean rust has a broad range of hosts in the legume family, including weeds, such as kudzu, and forage crops, such as winter vetch. Disease symptoms and severity vary among hosts.
Asian soybean rust symptoms on upper surface of kudzu leaf.
Asian soybean rust symptoms on lower surface of kudzu leaf.
Pustules on lower surface of kidney bean leaf.
Return to top
Look-a-Like Diseases on Other Hosts
Bacterial leaf spot on the upper and lower leaf surface of kudzu. During the 2005 growing season researchers in Arkansas also found bacterial leaf spot on young soybean and kudzu leaves. The unidentified bacterial leaf spot disease was commonly found while scouting. See their cautionary note on scouting.

An unidentified bacterial leaf spot on kuduz, Montgomery county, Virginia , July 2006.