August 3, 2007

Figure 1. Southern corn rust (Puccinia polysora) was confirmed this week in three counties in southern Nebraska.

Figure 2. Gray leaf spot on lower leaf with heavy dew.

Southern rust (Figure 1) was confirmed Thursday (August 2) on corn leaf samples submitted from Kearney and Phelps counties and today on a sample submitted from Gage County to the UNL Plant and Pest Diagnostic Clinic. Warm temperatures, high humidity, and recent showers are expected to promote development and spread of both southern rust and gray leaf spot (Figure 2).

At this time, southern rust is not as severe as it was last year, but it could increase with rainfall and prolonged periods of high relative humidity. Last year, in August 2006, frequent rain in south central Nebraska exacerbated the disease.

It is not unusual to have southern rust in Nebraska at low severity. Last year's epidemic was more severe than in previous years and fields should be monitored regularly for its spread. The characteristics used for differentiating between common rust and southern rust are described in the recent NebGuide, Rust Diseases of Corn in Nebraska (G1680).

Gray leaf spot, which is also being reported, requires several days for lesion development after fungal infection, so the disease can advance without being immediately obvious to the observer.

Management

Fungicide applications made several weeks ago likely are no longer providing protection from fungal foliar diseases. The preventative activity for foliar fungicides usually lasts two to three weeks. The pre-harvest interval for some fungicides (in the triazole class) restricts their use after the development of brown silks. Consult a current label for pre-harvest intervals because changes have occurred for some products.

Tamra Jackson
Extension Plant Pathologist
Amy Ziems
UNL Plant and Pest Diagnostic Clinic
Jenny Rees
Extension Educator, Clay and Webster Counties