Looking onto the fields now as the soils are becoming warm and dry in many parts of Illinois, one may speculate which plant diseases will take their toll on field crop yields this year. We can only guess in many respects because disease development is so highly dependent on weather conditions, and we obviously don't know what the weather conditions will be next week and certainly not in July. Although we know some of the regular major disease "players" to watch for this year in soybean and corn crops (SDS, SCN, Phytophthora rot, gray leaf spot, stalk and ear rots, etc.), a disease foremost in the minds of many is soybean rust. It is new and unknown, and may at some point become another significant disease player. We don't know if, when, or where soybean rust may first be found and cause significant damage in the continental United States.
Which diseases of corn, soybean, wheat, and alfalfa will we do battle with in Illinois this year? The word "battle" can only be used in a loose sense in the context of plant disease management because many of the crop management decisions that influence disease have already been made. Cultivar and hybrid selections have been made that will influence disease incidence and severity, many fields have been sampled and tested for SCN, decisions have been made regarding whether and what types of seed treatments will be used, and other decisions have been made based on past experience and knowledge of disease in an area. But still management decisions can be made to reduce problems with some diseases. Plant pathologists and nematologists with the University of Illinois Extension will write articles for the Bulletin throughout this 2003 growing season that, we hope, will help you understand and manage many of the diseases that influence field crop yields and quality in Illinois.
We can make lists of diseases to watch for in each crop in Illinois, and we can have some idea of the realistic range of damage levels that could occur based on past experience and reports from others for a particular part of the state. But soybean rust is different--it's new and unknown, and if it does arrive in the continental United States and Illinois, we have no sure idea of the damage it will cause. It could cause minor or major yield losses, and plant pathologists are diligently working to improve our abilities to model, predict, and manage soybean rust. The remainder of this article will focus on a brief summary of key information relating to soybean rust and will provide sources of more detailed information that will answer other questions that you may have.
Based on current models, soybean rust is likely to be dispersed up from the southern United States; hence this disease is likely to first appear in Illinois in the southern part of the state. The soybean rust pathogen is not seedborne, and the microscopic rust spores will most likely be spread by wind and rain or contaminated objects. Scouting for the early stages of this disease is somewhat challenging because the soybean rust lesions often develop first on the underside of leaves. Soybean rust is favored by wet, cool (~55 to 80°F) weather and will most likely appear in the middle or late summer, although it can appear at any time in the season. The lesions are pinhead-sized (1/12 to 1/6 inch diameter), tan to gray to brown in color, and have sharp edges that are bordered by leaf veins. If lesions are not carefully scrutinized, soybean rust may be confused with the disease bacterial pustule, which does occur in Illinois on a regular basis without causing significant damage. The soybean rust pustules are filled with brown spores, whereas the bacterial pustules contain many bacterial cells that can only be seen with a compound microscope.
Soybean rust is caused by two different rust fungi: Phakopsora meibomiae and Phakopsora pachyrhizi. P. pachyrhizi is the more aggressive of the two species and has been reported in Hawaii and recently in South America. This aggressive species is the one we are more concerned about, but it cannot be distinguished from the mild species (P. meibomiae) without detailed laboratory tests. If a plant is found that is suspected to have soybean rust, send samples to the Plant Clinic at the University of Illinois in Urbana (217-333-0519), where diagnosis and identification of the rust species will be made.
If soybean rust does come to Illinois, how will we manage it? Fungicides will probably be the short-term management strategy, and resistant cultivars are the long-term strategy. Fungicide efficacy trials have been conducted this winter in South America and Africa to establish the information needed to use fungicides for control of soybean rust. As of March 2003, at least two fungicides were labeled for control of soybean rust in the United States: Quadris (active ingredient azoxystrobin) and Bravo (active ingredient chlorothalonil). Additional fungicides will likely be available if and when they are needed for soybean rust control in the United States. Resistant cultivars will not be available for a number of years. The commercial soybean cultivars grown in the Midwest all appear to be susceptible to rust. There is hope that resistant soybean germplasm will be developed, and significant work is in progress to identify resistant germplasm.
Photographs and more information on soybean rust are available at the following Web sites: http://www.planthealth.info/rust/rust.htm and http://www.unitedsoybean.org/ (go to the "soybean producer workshop" section). Throughout 2003, articles will appear in the Bulletin to bring you new and current information on soybean rust and other disease problems. As you read the Bulletin, we will appreciate any comments and questions you have about any plant disease topics.--Dean Malvick