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During normal Nebraska winters, many plant pathogens that attack wheat stop growth and reproduction or are killed due to cold temperatures. Some fungal pathogens survive the winter as mycelium in debris, weed hosts, volunteer wheat, or fall-sown wheat. Others form fruiting or survival structures which can withstand cold winter temperatures. Pathogens that normally do not overwinter in Nebraska include those that cause the rust diseases and powdery mildew. These pathogens overwinter in southern states where warm temperatures prevail through the winter. In spring their spores are blown into the northern Great Plains states where they cause disease on spring and winter wheat.
This winter has been unusually warm in Nebraska. Unseasonably warm winter temperatures can enable pathogens that normally do not overwinter in Nebraska to survive and cause disease early in the growing season. In addition, during warm winters, the southern overwintering zone for the rust and powdery mildew pathogens extends further north, which increases the potential for spores blowing into the northern wheat growing areas earlier in the spring than normal. When diseases develop early in the growing season, the potential for yield loss is increased because damage can occur over a longer period.
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Because of the warm winter we have had, powdery mildew has been active (as of March 6, 2006) in wheat research plots at the South Central Agricultural Laboratory (SCAL) near Clay Center since January when it was first observed (Figure 1). Powdery mildew also was observed in a grower’s field near Clay Center on March 6. Leaf rust was observed in research plots at SCAL in November 2005. However, subfreezing temperatures during a brief period in December killed most of the leaf rust. On March 6, there were active leaf rust pustules in the research plots at SCAL, but they were very few compared to those seen in November 2005. Leaf rust pustules were similarly very few in the grower’s field on March 6. Presence of powdery mildew and some leaf rust pustules on wheat in early March implies that if we do not get an extended period of subfreezing temperatures in March and April, disease pressure may build up earlier than normal during this growing season.
For foliar pathogens that normally overwinter in Nebraska such as those causing tan spot and Septoria leaf and glume blotches, warm winter temperatures can result in survival of larger than normal amounts of inoculum (mycelia and fruiting structures). Hence, the potential is also increased for these pathogens to cause more severe disease during a growing season preceded by a warm winter compared to growing seasons preceded by cold winters.
Warm fall and winter temperatures can increase severity of wheat streak mosaic during the growing season. This is because extended warm temperatures in the fall favor prolonged survival of volunteer wheat and some weedy or wild grasses that support wheat curl mite populations. Hence the wheat curl mite, the vector of wheat streak mosaic virus, is able to survive longer, resulting in increased transmission of the virus to fall-sown wheat. Normally, the virus stays dormant in the fall sown crop during cold winters and resumes activity when temperatures warm up in the spring; however, during prolonged warm fall and winter temperatures, the virus can remain active in the fall-sown crop. This increases the time when the virus damages the crop, resulting in more severe symptom expression during the growing season. Therefore, warm fall and winter temperatures can affect wheat streak mosaic in two ways:
Warm winter temperatures also favor survival of soilborne pathogens which cause root and crown rots. During warm winters, soil water may not freeze deep enough to kill these pathogens. This can result in larger pathogen populations and increased disease presence the following growing season. If winter temperatures are too warm, soilborne pathogens can infect the fall sown crop during the winter or early spring. Hence the period during which the crop is damaged is increased, resulting in more severe symptom expression in a growing season preceded by a warm winter compared to growing seasons preceded by cold winters.
In summary, the potential for more disease and therefore greater yield loss is increased during a growing season which follows a warm winter; however, the extent of disease incidence and severity will largely depend on environmental conditions. Wet and humid conditions usually favor diseases more than dry conditions; therefore, disease risk is greater in irrigated than in dryland wheat. Because of the warm winter we have had and the fact that we are finding disease in wheat fields in early March, routine scouting to detect disease and the extent of any disease is recommended.
Stephen N. Wegulo
Extension Plant Pathologist, Lincoln
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