About Soybean Rust

What is Asian soybean rust?

Two fungal species, Phakopsora pachyrhizi, also known commonly as Asian soybean rust, and Phakopsora meibomiae, the New World species, produce windborne spores that can travel long distances in a short period of time.

The disease attacks the lower canopy initially, causing premature defoliation and reduction in photosynthetic area. The reduction in energy production results in reduced yields from unfilled pods, seeds per pods and fewer pods in general. Soybean rust may also develop on petioles, pods and stems.

As a biotrophic organism – it must be associated with a living host to grow and reproduce – there are more than 31 species in 17 genera of legumes in nature other than soybeans that can serve as a host for soybean rust. Some common hosts include kudzu, yellow sweet clover, medic, vetch, lupine, green and kidney bean, lima or butter bean and cowpea or black-eyed pea. For example, the weed kudzu in the southern United States is likely to serve as a host for soybean rust during the winter season, which will allow it to spread to soybeans and alternate host plants throughout the country during the following growing season.

The life cycle of soybean rust starts with direct penetration onto a plant’s lower leaves while they are wet. A spore requires a minimum of six hours of wetness, either dew or rain, and will infect within a maximum of 10 to 12 hours. Once a spore infects a leaf, it incubates for five to 10 days before pustules are visible.

Within a pustule, spores reproduce within 10 to 21 days and then are released. One pustule can produce a vast number of spores over weeks. Spores not attached to a susceptible host are believed to be viable for up to 60 days, but cease to be viable once the host plants have died from a hard freeze.

Visual signs start as small lesions that increase in size and change from gray to tan or reddish brown. The disease needs favorable conditions to replicate, with temperatures between 60 F and 80 F and 75 percent to 80 percent relative humidity.

Soybean rust is thought to have the greatest impact on yields between the R1 (flowering) to R6 (full seed) stages of soybean plants.

The first soybean rust case in the United States was confirmed in Louisiana in November 2004. Cases were also confirmed in eight other states – Mississippi, Alabama, Georgia, Florida, Arkansas, Tennessee, Missouri and South Carolina – through November and December.

Click here to learn how Laredo^® fungicide helps growers battle soybean rust. Laredo is available through 2006 for control of soybean rust in soybeans through Section 18 exemptions.¹