Source: Soybean Asian Rust Research Update - NCSRP, United Soybean Board and Plant Health Initiative.

Once a spore lands on a soybean leaf, and if the environmental conditions are right - warm temperatures and at least six hours of moisture - the spore begins to germinate.

It sprouts a germ tube, which penetrates the leaf. Once inside, the fungus, an obligate parasite, draws its nutrients from living plant cells. Initially, infection appears as tan spots, usually on the underside of the leaf.

Lesions (dead spots) appear five to eight days later as leaf tissue is destroyed. Within mature lesions, pustules form, which look like tiny volcanoes. Inside the pustules, new spores can develop in as little as nine days after the initial infection.

Spore productio can continue for up to three weeks, and they can remain viable for almost two months if humidity is high and ultraviolet radiation exposure is low.

KILLING SPORES

Scientists aren't completely sure what it takes to kill a soybean rust spore, but there is what they think so far:

- Spores die in 6-7 days when temperatures are under 32 degrees F
- Spores can survive off a host for about 50 days in ideal lab conditions
- Spores don't thrive at temperatures above 100 degrees F
- A combination of hot, dry and sunny weather is deadly

Other studies suggest that UV radiation may be more deadly to spore survival than extended exposure to very cold temperatures or very dry weather.