Keys to Successful Leaf Disease Management

The NDSU forecast system for leaf disease management extends from jointing to the end of flowering or the watery milk growth stages. For information on control of early-season leaf diseases, see the NDSU Extension Bulletin on tan spot or consult with your crop advisor. Foliar infections that occur in the middle milk stage and afterward happen too late to affect crop yield or quality significantly.

The fungi that cause tan spot and Stagonospora (Septoria) blotch are capable of attacking the head and cause diseases known as pink smudge and glume blotch, which reduce seed size and grain quality. So in addition to Fusarium head blight, a fungicide applied after heading can control both leaf and head infections. The fungus that causes leaf rust is capable of both long-distance movement and rapid development. Thus, if your cultivar is susceptible to leaf rust, you should be aware that crop rotation/black dirt tillage is largely ineffective and you should inspect the crop for rust frequently.

The leaf disease forecaster has four elements or steps, which are listed below, that work together to maximize your potential economic return by protecting the health of the top two leaves. In order for the forecasting model to work as intended, the disease-causing agent must be present. (Disease = pathogen + crop + environment). Thus, each field first must be scouted for the presence of leaf diseases. Disease incidence on lower leaves engages the computerized infection period models, which predict infection on upper leaves. As a check, fields are scouted once again to establish disease progress. Lastly, the weather forecast should be checked to see if conditions might be favorable for further disease development. The information below is also presented as a brief flow diagram.

Step 1: At late jointing, look for disease lesions on the Flag-2 leaf (leaf 5) on the main tiller. At this early growth stage, lesions probably will be small brown spots, perhaps surrounded by a faint yellow halo. Tan spot and Stagonospora (Septoria) blotch should be fairly uniform throughout the field but you'll want to avoid atypical areas, especially in regard to the presence or absence of grain stubble. The two diseases may be hard to distinguish but be sure the spots are not due to a bacterial disease or insect feeding, which are effects not controllable by a fungicide. Scout about 2-5 acres, stopping at random spots. Proceed to step #2 when 50% of 40 leaves have lesions, otherwise repeat this step every three days or as often as practical. In the case of leaf rust, some cultivars possess adult plant resistance and lesions on F-2 should not be of concern. Check with your extension agent or crop consultant if you are uncertain whether your cultivar has adult resistance.

Step 2: Mark the date when the disease incidence exceeded 50%. Use the information provided by the computer models to accumulate six to eight infection periods, depending on how much risk you are willing to accept. Count leaves infected by multiple pathogens only once.

Step 3: A fungicide application should suppress disease development but first consider today's weather conditions and check the weather forecast. Hot, dry weather can stop an epidemic and lessen the potential return from a fungicide application. Conversely, rainy, humid weather with long dew periods can accelerate disease progress.  See this link for recommended fungicides registered in South Dakota( .pdf | .doc).

Source: NDSU Small Grains Plant Pathology program