Scouting for Asian Soybean Rust

Scouting to detect fungal lesions as early as possible is key to an effective control program. Scouting should begin around the R1 growth stage. The first disease symptoms generally appear in the lower canopy.

Quick Tips – Scouting

Conditions conducive to ASR disease development is favored at temperatures between 68 to 75° F and leaf wetness periods greater than 6 hours. Rainfall plays a critical role in disease development and spread. ASR is highly dependent on leaf wetness and high humidity.

Keys to Effective Scouting

Timely observations – Start to scout from first flowering stage to R-6 stage of soybean plant.
Scout high-risk areas first. A high-risk area is an area where rust spores will germinate and develop first. They include:
- Areas where leaf wetness is present for a minimum of six hours
- Early planted fields (early canopy closure)
- Narrow-row fields (early canopy closure)
- Low-lying areas and protected areas/fields with prolonged dew periods.
Close-up examination – It is necessary to walk soybean fields and examine individual leaves. Remove leaves and look for symptoms under a magnified lens. (60 to 100 power field microscope is preferred)
Concentrate initial scouting in fields and/or areas with high potential for ASR infections. Early detection in high-risk areas will allow for more time to schedule a fungicide application before infections occur over a wide area.

Soybean leaves in a sealed plastic bag with a moist paper towel. This procedure can hasten the diagnosis of ASR if plants are infected but lesions are immature.

Suspect ASR? Immature, non-sporulating lesions of ASR are hard to identify. An easy method has been developed to help ensure correct diagnosis without the need for repeated trips to the field. Simply harvest leaves with suspected ASR lesions and put them in a sealed plastic bag containing moist paper. The plastic bag should then be stored at room temperature and shielded from direct sunlight. Observe leaves daily for spore production. As soon as spores are visible and correctly identified, appropriate control measures should be initiated. Sample 20 leaves (from the lower canopy) each in five areas of the field. Do not store the leaves in direct sunlight. Place sample bags in a cooler with an ice pack. Examine the leaves the next day using the lens

Tips for Scouting Asian Soybean Rust

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.

	Environmental Conditions	Early Infections	Typical Symptoms
Asian Soybean Rust	Cool and rainy	Lower leaves	Pustules
Bacterial pustule	Dry and hot	Upper leaves	Pustules
Bacterial blight	Cool and rainy	Upper leaves	No pustules
Brown spot	Rainy	Lower leaves	No pustules