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How to know when to spray for rust
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By Cheryl Rainford, News Editor
Agriculture Online
Optimal fungicide application timing depends on how well you assess the risk of the disease during the growing season
11/30/2004 -- Because fungicides are the only available means to control soybean rust (SBR), correct assessment of the risk of the disease during the growing season is key to applying fungicides effectively and economically, says X.B. Yang, plant pathologist at Iowa State University.
"Spraying too early may increase the number of applications needed. Later application does not do the job. Deciding when to apply chemical is critical to effective and economic control of the disease," he says.
The most reliable way to determine when to apply fungicides is to detect the disease at its early arrival, Yang says. Keep in mind spore movement is not the same as disease movement. Airborne spores spread faster than the disease moves, he notes. The disease may not be detected until many days after spores arrive. An experienced detector can find SBR at a low level, and therefore find the disease earlier than others do.
Predicting the spread of rust
Like corn rusts, SBR is expected to spread from south to north during the growing season, Yang says. The risk of the disease can be assessed progressively over the course of a growing season. "Climate predictions for July to August and occurrences of rust in the Gulf Coast regions have to be considered in risk assessment," he says, adding models projecting spore movement are under development.
If history can shed light on the potential spread of soybean rust, data from the southern corn leaf blight epidemic in 1970, recorded by Dr. William Moore that same year, are the best available, Yang says. Because corn and soybeans grow in the same geographic areas and have a very similar growing season, there are similarities between the two diseases. Further, in 1970, corn showed no resistance to southern leaf blight, and there is no resistance to rust in soybeans now. "This is a key similarity," Yang says.
In the 1970 epidemic, the movement of disease was recorded over time during the season as shown in the following five maps, Yang says. The red areas are areas infested by southern corn leaf blight. The disease outbreak was first noticed in March in Florida. By May 20 outbreaks were reported in Mississippi and Alabama coastal areas (see the top left map below). By July 15 (see top right map), the disease was found in Iowa, Illinois and Minnesota.
In the coming season, there are three key factors that will determine the risk of soybean rust in the northern states, Yang says:
1. The occurrence of soybean rust during the spring and early summer in the Gulf Coast areas. This determines the amount of spores available to blow northward.
2. July-August climate conditions in the north, and
3. Northward movement of soybean rust spores.
Yang offers a set of tips for northern producers to assess the risk of seasonal outbreaks. "Assessments made before March have little meaning for seasonal production," he says.
1. In March, monitor the information on the occurrence of SBR in Florida and southern Texas. This would be an early indication of the likelihood of rust spore movement to the Gulf Coast states. Also monitor rust outbreaks in the Caribbean Islands and Mexico.
2. April, May and June are critical periods. Watch for soybean rust occurrence in Louisiana, Mississippi and Alabama, which are regions that potentially would act as a rust pathway to the north. Texas and Georgia are also regions to watch, but could have less of an impact compared with the other three states. If outbreaks occur on soybean plants or kudzu there during that period, the spores are likely to reach northern soybean regions as early as July.
3. April and May are also important time periods to check the Climate Prediction Center at NOAA.
July and August are critical months for rust outbreak in the northcentral region. The lead time for climate prediction varies from one-half month to one year. Yang says he likes the 3-month lead time. A prediction that includes normal to below normal temperatures and normal to higher than normal moisture levels would be suitable for rust occurrence in Iowa.
[Dr. Yang thanks Drs. Emerson Del Ponte, Palle Peterson, Shimon Pivonia and Claudia Goody for reviewing his comments, and Barb McBreen and Jean McGuire for editing them.]
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