Yang: China Rust Similar to U.S. 01/03 14:47
By X.B. Yang
Department of Plant Pathology
Iowa State University
The risk of soybean rust in Brazil is far greater than in the U.S. because of its unique tropic and sub-tropic climates.
But since China and the U.S. have similar geographical size and latitude range for soybean production, a comparison between those two countries is more meaningful to what might happen in America.
Soybean rust overwinters in southern China, and the southern U.S. is a potential overwintering area. After the pathogen establishes in North America, soybean rust epidemics are expected to depend on the long-distance dispersal of spores northward from the southern areas, a similar way they spread in China.
The long-term risks of soybean rust to the United States can be determined by comparing the main risk factors affecting soybean rust epidemics.
Overwintering
Soybean rust is assumed to overwinter in southern China and neighboring countries south of China. In the United States, overwintering might be possible in parts of Florida and southern Texas and other coastal areas near the Gulf of Mexico during mild winters. In northern and central Mexico, potential overwintering areas are mainly in the eastern costal regions. The fungus can overwinter on Caribbean Islands, but the risk of it doing so is smaller than the risk of it overwintering in China. Growing season.
Along the southern coast of China, soybeans are grown year-round. Soybean rust appears in the winter and spring on Hainan Island and in southern parts of the Gaungdong Province. In coastal areas, soybean rust occurs in spring and fall and the disease usually is observed in summer and fall in the Yangtze River area of central China. Summer is too hot in southern China for soybean rust infection. Continuous soybean production helps maintain high levels of inoculum. In the United States soybeans are grown almost exclusively as a summer crop.
In China, excessive rainfalls have been associated with soybean rust epidemics, similar to what's reported in Brazil. To have a moderate epidemic - losses up to 30 percent – a minimum of 22 rain days in two months are needed.
In the U.S., the average rain days in the Midwest are less than 20 days in July and August. But the U.S. has sufficient dew periods and cool temperatures during summer, which are favorable to rust infection. Two schools of opinions are popular if epidemics can occur in a season with plenty of dew but fewer rain days. This risk factor is yet to be understood.
Wind pattern for spore dispersal.
Compared with China, winds in the United States during a growing season are stronger and more southerly due to the north–south orientation of the Rocky Mountains.
The nocturnal low-level jets with high nighttime humidity can facilitate the northward dispersal of soybean rust. This nocturnal jet during summer growing season is weaker and less frequent in China. Wind in the U.S. is more favorable to rust spore movement. Alternative hosts.
The kudzu situation in the US is unique and remains a great uncertainty in the risk equation. The plant is a very good host in Asia and South America.
Tests for soybean rust spores on U.S. kudzu have been mostly inconsistent, but spores were found on kudzu in Georgia in November.
If significant portions of kudzu are susceptible in the U.S., its importance to potential soybean rust epidemics cannot be underestimated.
Kudzu covers large areas of in the southern to central U.S. and might serve as an overwintering host because its leaves remain green during winters.
In regions where soybean rust cannot overwinter, kudzu leaves that grow a month before soybean emerges in the spring may serve as a bridging host for spore increase and northward spread.
(TD)
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