Above-normal temperatures during the winter and spring of 2011-2012 influenced the development of wheat diseases in Nebraska. Temperature is a component of environment, one of three factors that must be present for disease to occur. These three factors form what in plant pathology is popularly known as the disease triangle: a susceptible host, a virulent pathogen (capable of causing disease), and a favorable environment. Of the three factors, environment is the most important determinant of disease development and spread. Environmental components include temperature, moisture, humidity, wind, and sunlight.

Over the last several years, environmental conditions unique to a given year have favored epidemics of specific diseases in winter wheat. Notable among these are the leaf rust and barley yellow dwarf epidemics of 2007 (Figures 1 and 2), the barley yellow dwarf epidemics of 2011 and 2012, the Fusarium head blight epidemics of 2007 and 2008 (Figure 3), the stripe rust epidemic of 2010 (Figure 4), and the wheat streak mosaic epidemics of 2011 and 2012 (Figure 5). Leaf spot diseases, mainly tan spot and Septoria leaf blotch, occur each year and therefore do not usually stand out as much as the less frequent diseases when epidemics occur.

Fungal Diseases

In Nebraska, propagules (e.g. spores, mycelia) of many fungal pathogens are killed during the winter due to cold temperatures. Unseasonably warm temperatures during the winter can enable these pathogens to survive and cause disease earlier than normal during the growing season. In addition, during warm winters the southern overwintering zone for the rust and powdery mildew pathogens extends further north, increasing the potential for spores blowing into the northern wheat-growing areas earlier than normal in the spring. When diseases develop early in the growing season, the potential for yield loss is increased because damage can occur over a longer period.

Warm winter temperatures also favor the survival of soilborne fungal pathogens which cause root and crown rot diseases. During warm winters, soil water may not freeze deep enough to kill these pathogens. This can lead to buildup of larger pathogen populations and, subsequently, increased disease levels during the 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 longer, resulting in more severe symptom expression and greater yield loss.

Virus Diseases

Warm fall, winter, and spring temperatures can increase the prevalence, incidence, and severity of wheat virus diseases by favoring the survival and reproduction of both the viruses and their vectors. They also lengthen the period during which the viruses can damage the wheat crop.

This was the case in 2012 when wheat streak mosaic and barley yellow dwarf were detected in most fields surveyed, with some fields showing very severe symptoms (Figure 5). Wheat soilborne mosaic (Figure 6) was also observed at higher than normal levels of severity due to the warm fall and winter temperatures.

2012 Conditions Lead to Increase in Wheat Diseases

In conclusion, the potential for more disease and therefore greater yield loss is increased during a growing season that follows a warm winter; however, the extent of disease prevalence, incidence, and severity largely depends on environmental conditions, especially moisture. In the 2012 growing season, dry conditions slowed down or prevented the development of some fungal diseases. Despite the dry conditions, the early onset of disease due to above normal winter and spring temperatures and intermittent periods of wet weather are likely to result in greater winter wheat yield loss in 2012 than in a normal year. Some farmers have baled their wheat into hay or plowed it under in order to plant an alternative crop.