Gregory L. Tylka

Robert N. Wisner

Asian soybean rust is a disease that causes leaf spots (lesions) and eventual defoliation of soybean plants, sometimes in a matter of a few weeks if untreated. Soybean yields are reduced due to poor pod set and seed filling because the infection reduces the plant’s ability to perform photosynthesis. The fungus that causes the disease is Phakopsora pachyrhizi. It reproduces prolifically by producing extremely small spores that are disseminated by the wind.

Asian soybean rust was first discovered in Hawaii in 1994, but the disease was not found in the continental United States until November 10, 2004, when the U.S. Dept. of Agriculture announced discovery of Asian soybean rust in Louisiana. In the succeeding weeks, it was found in eight additional southern U.S. states. Through late April, it had not been found on 2005-crop U.S. soybeans.

The severity of Asian soybean rust in the U.S. will be affected by several factors. There likely will be year-to-year and geographical variation in disease development, severity, and yield loss. The fungus must maintain itself on living green leaf tissue, so survival over winter will occur on weed hosts in the extreme southern U.S. or on host crops and weeds in Mexico and parts of Central America. The extent and specific location of over-winter survival will greatly affect disease development. Once spring arrives, spore production will increase and likely begin moving north into the U.S. How early in the season spore movement begins and exactly where spores are transported by the winds will affect the extent of the disease. Once spores are deposited onto a host soybean crop, moderate temperatures (60–80°F) and 6–8 hours of continuous moisture are needed for infection, so local environmental condition also will be a factor in disease occurrence and progression.

Numerous fungicides are available to U.S. soybean growers for management of this disease. Each fungicide application will add 5–10% to the cost of production, and multiple applications may be needed if successful disease management is not achieved. Successful control will depend on proper timing of fungicide application with respect to movement of spores into or development of disease in a field. Also, the fungicides are not very mobile within the plant, so proper application, deep into the crop canopy and covering the leaves, is necessary for successful control of the disease.

There are several implications for the food processing industry.

With Asian rust as a threat to the U.S. soybean crop, soybean and soy product prices will remain quite volatile until at least mid-August. Potential volatility is heightened by a sharp reduction from earlier potential in Brazil’s spring 2005 harvest. Drought reduced its production by 300–370 million bushels, equivalent to 10–12% of U.S. production. With last fall’s huge U.S. crop and excellent yields in Argentina and other countries, Brazil’s smaller crop is not creating a shortage of soybeans. But to prevent a tightening of world supplies next fall and winter, U.S. yields will need to be near normal.

Commodity-fund buying of soybean futures also has increased market volatility and likely will continue to do so. Some investment funds focus on short-term trading profits. Others include soybeans in a portfolio of numerous commodities designed to hedge against inflation. Any indication of Asian rust in southern U.S. soybeans would bring fund buyers aggressively into the soybean market.

USDA’s planting intentions survey indicated that farmers in some southern states have cut soybean plantings sharply, while most Midwest growers are keeping acreage near year-ago levels. A few Great Plains states, where risk of rust is low, are increasing soybean acreage. U.S. plantings are down about 1% from 2004. Widespread rust or lack of it this year will affect acreage trends over the next few years. In the South, early infestations could add $25–40 per acre to production costs and would encourage farmers to shift acreage to other crops in the next few years.

Serious 2005 Asian rust problems would have the potential to push soybean and soybean product prices up to levels of the first half of 2004. During that period, western Corn Belt cash soybean prices were $9–10 per bushel, about 50–65% above late April 2005 levels. Central Illinois crude soybean oil prices were in the upper 20 to mid-30¢ per bushel range (25–50% above late April 2005), with soybean meal prices at $295–315 per ton. These prices would boost costs for many processed food items and at fast-food establishments, but would encourage farmers to improve their disease-management skills. Expensive soybean meal would reduce profits for milk, pork, and poultry farmers, slowing the increased production in those commodities.

Gregory L. Tylka ([email protected]) is Professor of Plant Pathology and Robert N. Wisner ([email protected]) is University Professor of Agricultural Economics, Iowa State University, Ames, IA 50010.