Assessment of Regional Site-Specific Sorghum Ergot Severity Potential Using Radar-Rainfall Measurement

The United States produces approximately 25% (14) of the world grain sorghum on an estimated 3 to 3.6 million hectares annually. A large proportion of grain sorghum produced in the country is used for livestock feeding, and most of the production is concentrated in the Great Plains and south-central states. Kansas is the leading state in production, followed by Texas, and in 2004, these two states accounted for 72.2% (42.7 and 29.5%, respectively) of the total grain sorghum planted acreage in the country (1). In addition to grain sorghum production, about 95% of the United States’ hybrid sorghum seed supply is produced in the Texas Panhandle (approximately 40,000 ha), primarily because of the absence of major pest problems (partly attributed to the desertlike climate), which provided ideal conditions for seed production. However, this changed in 1997 with the introduction of the fungus Claviceps africana Freder., Mantle, & De Milliano, the causal agent of sorghum ergot (8). Since its first detection in 1997, sorghum ergot has been consistently present in the Texas Panhandle and periodically has caused widespread damage (Fig. 1). During the first year of its introduction, nearly 45% of the seed production fields in the region were affected. Ergot prevalence in the region declined during the subsequent 3 years, and the threat of the disease appeared to have diminished. However, cross-spectral analysis of historic temperature and rainfall data for the Panhandle predicted that occurrences of ergotfavorable conditions in the region are periodic rather than a regular year-to-year phenomenon (20). As predicted, in 2002, sorghum ergot affected 87% of the seed production fields in the region, some so severely that harvesting was difficult because of the sticky honeydew. There was another widespread ergot epidemic in 2004 that affected over 90% of the fields in the Panhandle. In collaboration with various seed companies in the region, we have been monitoring the prevalence of sorghum ergot in the Texas Panhandle every year since 1997. To determine the magnitude of the impact of rain on the epidemiology of sorghum ergot in the region, we collected rainfall data from three North Plain Evapotranspiration (NPET) network weather stations representing the southern (Earth), central (Bushland), and the northern (Etter) Panhandle. Total rainfall data for the months of August and September (most of the flowering occurs during these months) were collected for the past 8 years (1997 to 2004). The scatter diagram of the relationship between rainfall amount and the percentage of fields with ergot showed a moderately high correlation (r = 0.72, P < 0.0001) (Fig. 2; the scatter plot and the confidence ellipse were generated using SAS software, SAS Institute Inc., Cary, NC). Rainfall frequency also correlated significantly with prevalence of the disease, but to a lesser degree than rainfall amount (r = 0.54, P = 0.0059, data not shown). C. africana generally requires cool weather and humid conditions for optimum infection (5). Rainfall reportedly is not required for infection as long as relative humidity is high (2). However, the Texas Panhandle is characterized by a desert-like climate with hot, dry weather during the summer. Maximum temperatures ranging between 35 and 40°C, accompanied by low relative humidity, are not uncommon during sorghum flowering season. Prolonged occurrences of high humidity during the summer are usually associated with rain showers. In this region, rain also plays a significant role in suppression of maximum temperature such that it remains within ergot favorable range (20). A sudden drop of several degrees in temperature is not an uncommon event during an afternoon rain shower. Furthermore, rainy and damp weather that favors ergot development is suboptimal for pollination and can lead to a reduction in pollination efficiency, exposing the ovaries to infection. Rainfall also may be instrumental in washing spores of the pathogen from the air and may also play a major role in spore release from the honeydew matrix and subsequent dispersal in the form of wind-driven aerosol. Therefore, it stands to reason that rainfall has to be a major component of any regional ergot risk-assessment system in the Texas Panhandle.