By Pierce Paul
It has been cool, wet, and very humid so far this spring - perfect conditions for early season diseases like Septoria tritici blotch and powdery mildew to develop. Both of these diseases usually become established in the fall, thus getting an early start in the spring, particularly if winter conditions are mild. Here are a few more facts about Septoria tritici blotch and powdery mildew.
- Symptoms first appear on lower leaves (those close to the soil) as irregularly-shaped lesions with a tan center and yellowish margin. Under favorable conditions, lesions spread up the plant, taking on more of a rectangular appearance on upper leaves, without the clear yellowish margin. Stems may also be affects.
- The presence of distinct black dots or specks in the center of the lesions is the easiest way to tell Septoria apart from other diseases like Stagonospora leaf blotch and tan spot.
- Septoria is favored by cool, humid weather, with frequent rainfall (every 2-3 days). Rain splashes spores of the fungus from lower to upper leaves or to nearby healthy plants, causing the disease to spread quickly (within 7-14 days).
- As the name suggests, powdery mildew develops as white, powdery lesions (called pustules), beginning on the surface of lower leaves and stems. As the plant and lesions mature, pustules take on a grayish appearance, with distinct brownish-black dots.
- Powdery mildew is favored by cool, humid weather, and high nitrogen.
- Wheat tends to be most susceptible during rapid growth following spring topdressing.
Yield loss and management:
The highest yield losses (due to small, light-weight grain or reduction in the number of heads in the case of powdery mildew) are seen in Ohio when susceptible varieties are planted and conditions remain cool and humid during grain development. Warm weather slows both powdery mildew and Septoria down.
Resistance is the best approach for managing Septoria and powdery mildew, but if your variety is susceptible, a fungicide application between flag leaf and head emergence will provide very good control. Most of the available fungicides are equally effective against both diseases. But scout fields before applying a fungicide to see if lesions are moving up the plant – in most years unfavorable hot, dry late-spring conditions provide the best control of Septoria and powdery mildew in Ohio, saving you the money.