Plant Disease: Soybean Rust

Taxonomy:

Distribution:

• In the Eastern Hemisphere it occurs from Japan to Australia and westward to India and in China, Hawaii, and central and southern Africa.
• The Western Hemisphere it is found generally in South America.

Disease Symptoms:

• Lesions on leaves (most common), petioles, pods, and stems.
• Lesions are 2 to 5 mm 2 in diameter, and tan or red-brown in color.

Soybean Rust Pustules

• Within each lesion is one to many erumpent, globose uredinia.
• Urediniospores are released through the circular ostiole.
• As rust severity increases, premature defoliation and early maturation of plants is common.

Small Tan Colored Pustule

Favorable Conditions for Development:

• Temperature: 59° - 84° F/15° - 28° C
• Relative Humidity: >12 hours @ > 90% RH
• Leaf Wetness: > 6 hours of leaf wetness (from dew, rain, or irrigation), 10-12 hours ideal

Dispersal:

• Wind borne urediniospores (natural)
• Movement of infected live plants, plant material, pods, and stems (artificial).

Host Range:

• Potential hosts; more than 95 species of plants from more than 42 genera
• Primary Host is Soybean, Glycine max
• Other Hosts known in the U.S. are: Royal Poinciana, Hyacinth bean, Narrow-leaved lupine, Yellow lupine, Black medic, Yellow sweet clover, Kudzu, Colorado River hemp, Narrow-leaf vetch, Cowpea, Black-eyed pea.

Identification:

• Early symptoms of soybean rust resemble bacterial pustule and brown spot.
• Soybean rust can be distinguished from bacterial pustule and brown spot by examining the lesions.
• Mature soybean rust lesion contains cone-shaped pustules with a pore on the top with spores inside or on top of the cone.
• Use a 20X hand lens to view pustules for evaluation of spore presence.
• Molecular analysis provides rapid and accurate identification.

Management:

• Immediate management will rely on fungicide application.
• ong-term management will rely on the development of resistant or tolerant varieties.