Identifying Asian Soybean Rust

Disease Symptoms

Asian soybean rust symptoms are most commonly found on the underside of the lower leaves, starting around the flowering stage. But under severe disease pressure, infection of leaves and even the cotyledons of early vegetative-stage soybeans has been reported. Initially, symptoms of ASR infection are difficult to detect, and can be mistaken for bacterial pustule, bacterial blight and Septoria brown spot (see section on scouting for details). The first symptoms of ASR infection are small, poorly defined discolorations on leaves in the middle to lower canopy (Figure 1).

Later, small brown-gray lesions appear predominately on the underside of the leaves. As the lesions mature, they increase in size, and their color changes to tan or reddish brown. Mature lesions are 2 to 5 millimeters2 in size (for reference, 2 mm2 = , 5 mm2 = ), tan to dark brown in color, and polygonal in shape (Figure 2).

At the center of the mature lesions, uredinia are formed. Uredinia have the appearance of pustules (Figure 3).

Inside these pustules, fungal spores are produced. Initially, the spores are released through a circular opening in the center of the pustule (Figure 4). On more mature lesions, the grayish-tan spores are released through irregular ruptures of the epidermis (Figures 5 and 6).

There are two lesion types. Tan lesions, when mature, consist of small pustules (uredinia) surrounded by a slightly discolored necrotic area with masses of tan-grayish spores on the lower leaf surface (Figure 7).

Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules and few visible spores on the lower leaf surface (Figure 8).

As ASR severity increases, premature defoliation and early maturation are common. Lesions can also be found on petioles, pods and stems, but are most common on leaves.

Asian soybean rust can only be managed by scouting and timely fungicide applications. No soybean varieties with resistance or tolerance to ASR are available.

Scouting

Early disease detection is the basis of a successful control program. Scouting should be initiated when:

• ASR has been reported in your area
• Anytime after flowering
• When large weather systems from southerly directions have moved through the area

After initial discovery of ASR in your area, scouting should be done on a weekly basis. If ASR is suspected, laboratory confirmation may be needed for positive ID.

Field Scouting Guidelines

• Examine areas conducive to ASR development that have a high likelihood of showing the symptoms first. These are areas that promote leaf wetness and high relative humidity such as:|
  - Low lying areas
  - Wet spots
  - Areas along tree lines
  
• In general, evaluate 20 locations in a field examining five plants per location (scouting patterns may vary depending on the purpose)
• Look in the lower soybean canopy
• Know your other diseases (bacterial blight, bacterial pustule, brown spot)
• Use a 20X handheld lens to greatly enhance your ability to correctly identify ASR lesions.

Sampling Guidelines

• Collect 20 leaflets, flatten and place between paper towels
• Place in sample bags and clearly mark pertinent information: date, location, name, phone number, etc.

It is important to remember that initial lesions of ASR can be misdiagnosed as bacterial pustule, bacterial blight, and Septoria brown spot. The best way to differentiate these diseases is the characteristic spore production of ASR. Spores are round to oblong, hyaline to grayish color, and protrude through the blister profusely, first through the circular opening in the blister, and later through the irregular ruptured blister cover. The best opportunity to observe sporulation on the leaves is during the early morning hours, or after prolonged leaf wetness periods.

More tips for differentiating Asian soybean rust from other common soybean leaf diseases are shown in Table 1 below. In general, Septoria brown spot and ASR start in the lower canopy, but Septoria brown spot doesn’t have pustules. Both bacterial diseases start in the upper canopy. Bacterial pustule produces pustules, but does not produce spores.

Table 1. Characteristics differentiating Asian soybean rust from other soybean leaf diseases.