Common Rust
Beginner
Scientific Name
Puccinia sorghi
Identification
- Reddish-brown (rusty) pustules develop on the upper leaf surface
- The size of each lesion ranges from 1- 3 mm (1/25- 1/8 in.)
- Older pustules turn blackish-brown
Often Confused With
N/A
Period of Activity
Rust is not usually a problem until late summer/early fall. Infections may begin earlier during cool, wet summers. Corn at the whorl stage is particularly susceptible. Moisture often collects in the whorl, providing the ideal conditions for rust germination.
Scouting Notes
Inspect all varieties of sweet corn, including resistant ones, for signs of rust from the mid-whorl stage onwards. Examine 10 groups of 10 plants across the field. Look for symptoms on the lower leaves and in the whorl. Record the percentage of plants infected and the number of lesions per infected leaf. Continue scouting on a weekly basis.
Thresholds
Prior to silking, plants with more than 6 pustules per leaf will benefit from a fungicide application.
Advanced
Scientific Name
Puccinia sorghi
Identification
Reddish-brown (rusty) pustules develop on the upper leaf surface. The size of each lesion ranges from 1- 3 mm (1/25- 1/8 in.). Older pustules turn blackish-brown.
Often Confused With
N/A
Biology
Rust requires a living host to overwinter. Each year a new crop of spores is produced in the southwestern United States and Mexico. Urediniospores are blown north by summer storm fronts.
Corn infected prior to tassel is at highest risk of yield loss. Studies at the University of Illinois have found that sweet corn yield is reduced by 6% for each 10% of the leaf area infected at harvest. Corn infected after pollen shed will not usually suffer yield loss. Rust has a 7-day latency period. If the first sign of infection occurs after tasseling there is little chance that the fungus will develop to yield-reducing levels before harvest.
The rust fungus thrives in cooler weather conditions. A temperature range of 16 to 24°C (60 to 75°F) poses the highest risk for corn infection. Rust also needs high humidity. Infection will occur if the leaves are wet for more than 3 to 6 hours. Even a light shower or heavy dew can provide enough wetness for infection when air movement is restricted by a full crop canopy.
Many sweet corn varieties are resistant to common rust. These varieties provide good protection against rust infections; however even resistant varieties may become infected under heavy rust pressure, or shifting rust pathotypes.
Period of Activity
Rust is not usually a problem until late summer/early fall. Infections may begin earlier during cool, wet summers. Corn at the whorl stage is particularly susceptible. Moisture often collects in the whorl, providing the ideal conditions for rust germination.
Scouting Notes
Inspect all varieties of sweet corn, including resistant ones, for signs of rust from the mid-whorl stage onwards. Examine 10 groups of 10 plants across the field. Look for symptoms on the lower leaves and in the whorl. Record the percentage of plants infected and the number of lesions per infected leaf. Continue scouting on a weekly basis.
Thresholds
Prior to silking, plants with more than 6 pustules per leaf will benefit from a fungicide application.
Management Notes
- There is a wide range of rust resistance in sweet corn cultivars. Commercial corn hybrids have good tolerance, whereas many seed corn inbreds, sweet corn and speciality corn hybrids are very susceptible to the disease.
- Rust infections occurring after silk emergence are unlikely to result in yield losses.
