|
| Figure 14. Barley yellow dwarf is caused by an aphid-transmitted virus. Yield losses occur when seedlings become infected soon after planting. Early-planted wheat (right) is more likely to be affected than wheat planted after the Hessian fly-safe date (left). |
|
| Figure 15. Plants affected by barley yellow dwarf are usually stunted, produce few heads, and have characteristic reddish to yellowish leaf tips. |
Planting date can have a profound effect on the incidence and severity of many diseases. Wheat producers are advised to plant wheat after the Hessian fly-safe date for their area. The optimum planting date for wheat planted no-till is from the fly-safe date to 10 days later. Early fall planting increases the risk of yield losses from powdery mildew, leaf rust, the Stag-ono-spora diseases, take-all, Pythium seedling blight, Fusarium seedling blight from seed-borne scab, barley yellow dwarf, and wheat spindle streak mosaic. The warm soil temperatures favor the development of seed rots and seedling blights caused by soilborne pathogens, Pythium and Fusarium.
Planting in early to mid-September allows sufficient time for plants to become diseased during the warm periods of late summer. The fungi causing powdery mildew, leaf rust, Stagonospora, and Septoria diseases may infect the leaves at this time. The fungi survive over winter in diseased plants where there is protection and food. Then, in the spring, when weather conditions become favorable for their growth and spread, an epidemic may be readily initiated.
Devastating virus diseases, like barley yellow dwarf (Figures 14 and 15), can be effectively controlled by planting after the Hessian fly-safe date. The cooler temperatures of late September and early October reduce the activity of the aphids that transmit the virus. In research trials examining the effect of planting date on yield, wheat seeded one week before the fly-safe date yielded 16% less than that planted one week after the fly-safe date (Figure 16). The primary cause of this yield loss was barley yellow dwarf. Yield losses from virus diseases are always greatest when plants become infected in the fall.
 |
| Figure 16. The effect of planting date on barley yellow dwarf incidence and yield of six wheat varieties is shown here. |