Characteristics of Soybean Foliar Diseases from Bacterial Blight to Rust

Dean Malvick

Dean Malvick

Assistant Professor of Plant Pathology and Extension Specialist

Phone: (217) 265-5166




Although root and stem diseases have generally been the most important diseases of soybean in Illinois (Table 1), much concern has been raised recently about foliar diseases, especially soybean rust. In this context, foliar diseases are considered those that primarily infect and damage soybean leaves. They do not include diseases that infect the roots and stems and have secondary effects on leaves, such as BSR, SDS, and white mold. Although viruses can also cause important foliar diseases, this presentation will focus on fungal and bacterial diseases.

Foliar diseases are getting increased attention for several reasons. Foremost, is the concern about soybean rust, which has the potential to cause considerable yield loss. Second, weather conditions were quite favorable in Illinois for many foliar diseases during the 2004 growing season. These included Septoria brown spot, downy mildew, bacterial blight, Cercospora leaf blight, and frogeye leaf spot. Many questions have arisen regarding the diagnosis, potential for yield losses, and management of these foliar diseases. A third reason is that fungicide spray programs are being increasingly used, sometimes with mixed results, and discussed for soybeans. However, the actual effects of the fungicides on foliar diseases are generally poorly understood.

With the increased awareness of soybean rust, many soybean fields have been getting (and will get) increased attention. Most foliar diseases are fairly easy to see compared to many other diseases and have been noted frequently. The exception to this is soybean rust, which is very difficult to see and detect during early stages of infection and disease development. Furthermore, as efforts are made to increase soybean yields, foliar diseases and other factors are being considered as possible yield reducing problems that can potentially be managed to increase yields.

These factors have combined to increase interest and awareness of soybean foliar diseases and the need for increased understanding of diagnosis, biology, damage potential, and disease management. This presentation will cover diagnosis and characteristics of soybean rust as well as common soybean foliar diseases, and will discuss potential relative yield losses due to these diseases. Yield losses due foliar diseases are highly variable from year to year and location to location depending in a large part on environment. This information will not only be beneficial in understanding and diagnosing common foliar diseases, but will help in recognizing and managing soybean rust if and when this disease arrives in Illinois.

Descriptions, Characteristics, and Distinguishing Features of Foliar Soybean Rust and Common Foliar Diseases of Soybean

The following are examples of leaf diseases that are common in Illinois and which may be confused with soybean rust. Other diseases of soybean leaves and stems that are important and could potentially be partially controlled by foliar fungicide applications include frogeye leaf spot, pod and stem blight, and stem canker. Yield loss potential is indicated, with the caveat that yield loss estimates are very difficult to achieve with accuracy and the high estimates are generally extreme situations that are uncommon. Different varieties and cropping systems clearly can influence yield losses.

Figure 1

Asian Soybean Rust

���������������������������������������������   Caused by the fungus Phakopsora pachyrhizi.
  • Occurrence: Has been a problem in Africa and South America in recent years, but was reported for the first time in the continental U.S.A. (Louisiana) in small plots in November 2004. Infection requires 6–7 hr of continual wetness with an optimal 12 hr dew period at temperatures of 66° to 80°F.
  • Symptoms: Common symptoms of soybean rust on soybean are 1/10 to 1/5 in2 tan to dark brown polygonal lesions. One to several raised, circular pustules form within each of the lesions. Tiny rust spores (urediniospores) are released through a pore in the pustule and can often be seen on the pustule with a hand lense at 10-20×. Lesions are most common on leaves, but also may be found on petioles, pods, and stems. Premature defoliation and early maturation of plants often occurs as rust severity increases.
  • Potential yield losses: Yield losses have been reported from 13% to 80% or more in severe cases.
Figure 2

Septoria brown spot

���������������������������������������������   Caused by the fungus Septoria glycines.
  • Occurrence: This disease may appear on the first pair of true leaves in the spring and progress throughout the season. It is often most common with warm and wet weather, continuous soybeans, and minimum tillage.
  • Symptoms: Infections usually begin on the lower leaves. Diseased areas are angular to irregular and dark brown and are often associated with chlorosis. They range in size from pinpoint to 1/5–inch diameter and may be more prominent on lower leaf surfaces. Infected leaves turn yellow and drop prematurely, progressing from the bottom upward.
  • Potential yield losses:This is a ubiquitous disease that normally causes minimal if any yield losses, however, yield loss estimates of 8–34% have been reported when much defoliation occurred.
Figure 3

Downy mildew

���������������������������������������������   Caused by the fungal–like pathogen Peronospora manshurica.
  • Occurrence: Plants of all ages are susceptible, and disease is favored by infested seeds, narrow rows, cool, and moist conditions.
  • Symptoms: Symptoms of downy mildew on soybeans begin as indefinitely–shaped small light green spots (not water–soaked) on the upper and lower surfaces of leaves. Small raised tan tufts of fungal growth often develop in the lesions, especially on the underside of leaves during wet or humid conditions. The lesions can grow together into large irregular brown areas that may ultimately rip away from the leaf. Seed coats and the inside of pods can also become covered with white to tan fungal growth, but infection of the outer part of the pod may not be obvious.
  • Potential yield losses: Yield loss is usually minimal, but in severe cases yield losses have been reported to be 9–18%.
Figure 4

Bacterial blight

���������������������������������������������   Caused by the bacterium Pseudomonas syringae pv. glycinea.
  • Occurrence: All season. Mainly a cool, wet weather disease; spread by wind and rain within a field as well as by cultivation when foliage is wet.
  • Symptoms: Small, angular, water-soaked spots turn yellow, then brown as the tissue dies. Several spots may merge and the dead tissue drops out giving the leaves a ragged appearance. The lesions usually do not cross leaf veins. Leaves usually remain on the plant.
  • Potential yield losses: Studies have reported yield loss potential for this disease to range from 4% to as high as 40% under extreme conditions. This disease often occurs at low levels that don't cause yield losses.
Figure 5

Bacterial Pustule

���������������������������������������������   Caused by the bacterium Xanthomonas campestris pv. glycines.
  • Time of Occurrence: All season, mainly during warm, wet weather conditions.
  • Symptoms: Small yellow–green spots with brown centers, are noticeable mainly on the upper leaf surface. A small, raised pustule develops in the center of the lesion, particularly on the underside of the leaf. Spores are not produced. Spots may merge and tissue may fall out, giving the leaves a ragged appearance. There is no water–soaked appearance to the spots.
  • Potential yield losses:Studies have reported yield loss potential for this disease to range from 4% to as high as 35% under extreme conditions, but this disease rarely reaches high levels of severity.
Figure 6

Cercospora leaf blight

���������������������������������������������   Caused by the fungus Cercospora kikuchii.
  • Occurrence: from flowering to maturity, and especially during times of high humidity and warm temperature.
  • Symptoms: Purple seed stain is recognized as purple to pink areas on seeds ranging in size from small spots to large patches. Infected seeds do not always express symptoms. Symptoms on leaves primarily begin after flowering. Infected leaves appear "sun–burned." Redbrown spots may develop on leaves, stems, and pods, and these spots may merge to form large lesions.
  • Potential yield losses: Yield losses are typically considered to be minimal in the U.S., but they are not well understood. The economic loss from the purple seed stain phase of this disease is generally considered to be more significant than the leaf blight phase.

Scouting and Diagnosis

It may be wise to increase scouting for foliar diseases to determine what is affecting different soybean fields. This is especially important after the report of soybean rust in the southern continental U.S.A. Look high on the plants and look low, and include the upper and lower sides of leaves in your observations. Start scouting early in the season because some foliar diseases, including Septoria leaf spot and soybean rust, can occur as early as the seedling stages of crop development.

Diagnosis of foliar diseases can be a challenge, but look for the symptoms that are described above and use a good guide with additional descriptive information and clear color photographs. For thorough diagnosis, send samples to the University of Illinois Plant Clinic in Urbana. With the exception of soybean rust, these diseases can be seen in many circumstances with the naked eye. A hand lens of 10–20× will be critical for detection of soybean rust, especially in the early stages of infection. In many cases the foliar diseases of soybean may not be significant, but they may be significant under some situations and we should monitor when and where severe situations occur.

Tables & Figures

Figure 7
Table 1. Six most yield–reducing diseases of soybean in the top five soybean–producing countries in 1998 (estimated). Even before the introduction of Asian soybean rust to South America, foliar diseases were considered major problems in some countries.
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