Experts seem resigned to the fact that Asian soybean rust will make it to the United States. It is a disease that could have major implications for growers in the Midwest. Here's what we know so far.

For those who make their living by sticking seeds in the ground, there will always be a specter on the horizon. The only question is which in the panoply of weather, disease, insect or foul market will assert itself during a given year. And now that Asian soybean rust is well established in South America, growers have begun to wonder when that particular foe of soybean production will join in for a nick at their production.

At winter meetings and seminars farmers asked, "Is this the year we'll see Asian soybean rust make it from the south to our fields?" The answer from the experts (and it is an honest one) is, "We don't know." But there is a certain resignation in the experts' follow up. They say it is a more of a question of when than if.

There are actually a couple types of soybean rust. The one feared for its ability to travel and severely damage yield is Phakopsora pachyrhizi-commonly referred to as Asian soybean rust. Pathologists offer a range of yield loss for infected soybeans. The low-end estimate is 10 percent yield loss, with some reports suggesting upper limits of 80 percent. Anywhere on that spectrum of damage makes Asian soybean rust a serious economic threat.

University of Missouri Commercial Ag plant pathologist Laura Sweets, who traveled to Brazil and Paraguay in March, said that while Asian rust now covers large swaths of the South American soybean growing region, it's not entirely clear how it got there.

"There are theories," said Sweets, "but nothing that's really proven. We do know that it is disseminated in air currents and might survive for short periods of time on plant debris." A look at world prevailing wind patterns lends credence to the theory that rust moved via air from its sources in China and Japan to Australia, Africa and now South America.

Sweets pointed out that in areas like Brazil, where the climate is conducive to fungal growth, Asian rust grows year round, surviving on other host plants when soybeans are not present. Regardless of how it arrived, that means air currents readily spread it in the growing season.

That's one reason that growers, processors and folks from the Animal and Plant Health Inspection Service (APHIS) are keen to keep Asian rust out of the country. Once it is here, Asian rust is likely to survive on host plants in our warmer climates and hitchhike on strong air currents to move into soybean regions for the growing season. Unfortunately, Asian rust isn't finicky for host plants. It survives on some 30 types of legumes (sweet clover, medic, cowpea, etc.) and an unknown range of weeds. Abundant in South America, kudzu is a typical host, and could be here, too.

Research shows that Asian rust is not seed borne; it isn't carried inside the seed. But there is some chance that spores could travel on foreign matter in seed lots. In February, APHIS released a report that said seed is a theoretical but highly unlikely source of travel for Asian rust. The reasoning is that soybeans are harvested after the plant defoliates, and it is leaves that hold the largest concentration of rust. Also, commercial seed practices require seed cleaning, which typically reduces any foreign matter to 2 percent or less. Since Asian rust has been on its radar screen, APHIS has worked with seed producers to set standards that limit the risk of importation of the fungus.

Damage inflicted
According to Sweets, Asian rust will typically begin to show damage to soybean plants around flower stage. But seasonal circumstances can affect fungus growth. In Brazil, where Sweets saw the rust in fields, the soybean season is wide enough that seedling fields exist in the same area as fields with much more mature soybeans, allowing for earlier infection and damage.

She said an eye-catching feature of most fields she saw was the season-long spray rig tracks in the field.

"Of course, much of Brazil and the soybean growing parts of South America has an environment that is friendly to fungal diseases," said Sweets. They're accustomed to the fact each crop will require multiple application passes.

A challenge in controlling rust on soybean plants is that the fungus typically starts low on the plant. By the time afflicted plants are diagnosed, there is likely to be a canopy, making it a challenge to get fungicides to the fungus. Moreover, in most U.S. soybean growing practices, there's no need to make such a late season application pass. So spraying will require the cost of the fungicide and the cost of at least one extra application.

Speculating about how the fungus might be treated in the Midwest, Sweets said there might be a chance for some late-planted soybeans to receive a fungicide at the same time as herbicide application.

That's assuming there is enough fungicide to spray. The Minnesota Department of Agriculture and South Dakota extension plant pathologists have filled out the paperwork to obtain an EPA section 18 exemption for a group of fungicides to control Asian soybean rust. While the exemption is written for those states in particular, it could serve as a boilerplate application for other soybean-producing states should an outbreak of Asian rust occur.

Right now there are only two fungicides labeled for nation-wide use on soybean crops-azoxystrobin (Amistar, Quadris and other trade names) and chlorothalonil (Echo, Bravo, etc.). The section 18 exemption request suggests an additional group of fungicides be permitted for use on soybeans. In the event of a widespread rust infection, the additional chemistries would be needed for the obvious reasons-availability and price.

While a few strokes of the pen can get new fungicides on the market and in the field, finding soybean lines with genetic tolerance or resistance to Asian rust is more difficult. There is plenty of work going on to find such varieties, but as of now, they don't exist. That's one thing that has growers worried. Plenty of them are old enough to remember 1970 and Southern corn leaf blight.

Scouting and identification
For now, growers ought to keep vigilant about what's happening in their fields. In a year that is conducive to other types of fungal growth, there might be some challenge for the non-familiar to identify rust.

As you can see in the photos and chart that accompany this story, the disease causes small brown spots on both the top and underside of the leaf. These lesions are most often seen on leaves but can be found on stems and pods as well. As the rust progresses, damage becomes more obvious with a brown "dust" appearing on leaves. Under severe rust conditions, the soybean plant defoliates and dies.

As with any pest, regular scouting is important in that timely and proper identification of the problem relates to more effective and economic control.

If (or when) soybean rust arrives, understanding the disease will help mitigate its damage.