August 10, 2007

Scab, or Fusarium head blight, is a disease that presented a new challenge for many growers in 2007. This is because it has not occurred over many years in Nebraska due to drier than normal conditions. However, excessively wet conditions during the 2007 growing season caused outbreaks across much of the wheat growing area east of North Platte and in many irrigated fields in southwestern Nebraska.

There are several approaches to managing scab. The first is to plant seed that is free of the scab fungus. This will prevent seedling blights caused by the fungus. It also will reduce inoculum or the amount of fungus that survives in soil and on plant debris until flowering when it infects wheat heads.

The second approach is to apply a fungicide at the beginning of flowering to prevent the fungus from infecting heads. The challenge here is that most growers will already have applied fungicide earlier in the growing season to control powdery mildew, leaf spots, and the rusts. Spraying to control scab will mean an additional fungicide application and, hence, additional costs. Based on the experience we had this year, this additional spray will be worth the investment, especially in high yielding varieties and in irrigated fields. Monitoring the weather at least one week to 10 days before the beginning of flowering will be critical in making the decision to spray for scab.

A third management strategy for scab is to avoid planting wheat after corn. The scab fungus also causes stalk rots in corn and survives very well on corn stubble. Therefore, severity of scab usually is greater in wheat planted after corn.

In general, to prepare for wheat diseases:

1. Start with fungicide treated, pathogen free seed.

    

2. Control volunteer wheat and other volunteer cereals. These plants must be completely dead at least two weeks before planting in the fall. Controlling volunteer will reduce or eliminate the risk of wheat streak mosaic virus and barley yellow dwarf virus.

    

3. Avoid planting earlier than the recommended date for your area. This will reduce damage caused by disease infections that occur during the fall, especially wheat streak mosaic virus and barley yellow dwarf virus.

    

4. Monitor the weather and scout fields for early disease detection starting in early spring. Preventive fungicide sprays applied soon after disease is detected are most effective. For powdery mildew, leaf spots, leaf rust, and stripe rust, fungicide sprays should be timed to protect the flag leaf. For scab, apply fungicide at the beginning of flowering (approximately 10% to 20% flowering).

    

5. Consider applying these fungicides labeled for early season disease control at reduced rates for early disease control, especially in high yielding varieties, susceptible varieties, in irrigated fields, or if weather is favorable to disease development.

    

6. Consider planting several different varieties differing in genetic background. This will increase the chances that in the event of a severe disease epidemic, some varieties may still give profitable yields.

Stephen Wegulo
Extension Plant Pathologist