Several Alternaria spp. are known to cause diseases of cotton throughout the world, however, these species are usually considered weak pathogens of Upland cotton (Gossypium hirsutum L.). Alternaria macrospora Zimm. is the predominant species experienced in the Southwestern United States, and causes two different disease symptoms. Leaf spot is the most widely recognizable symptom, and is characterized by brown, circular lesions on the foliage. A second symptom, stem blight, is also associated with A. macrospora. Infected areas in a field exhibiting stem blight are small and have a circular appearance, resembling a lightning strike. Plants within these areas appear wilted and desiccation of leaves is apparent. As the disease progresses down the stem, the terminals of infected plants die and may exhibit a Shepherd's crook. Alternaria stem blight was first observed during the 1999 growing season and was restricted to Dawson and Gaines Counties. Over the past six years, there has been a steady increase in the number of fields exhibiting the stem blight, and the range of infected fields has increased substantially. In addition, the disease has been observed on conventional and transgenic cultivars with both stripper and picker backgrounds. During the 2006 season, localized outbreaks of Alternaria stem blight occurred in cotton production areas of West Texas. Severity of stem blight was variable with infected areas ranging from 5m in diameter to approximately 0.4 ha. Isolations were made on water agar after surface sterilization of infected stems with 1.5% sodium hypochlorite for 1 min. Plates were incubated at room temperature for 48 hr and transferred to potato dextrose agar. Following 1wk incubation, isolated colonies were olive brown to black, which is consistent with Alternaria spp. Morphological characteristics of conidia fit the description of A. macrospora. In addition, severe outbreaks of Alternaria leaf spot occurred throughout the region. Leaf spot severity was variable and appeared to be somewhat cultivar-dependant. In general, leaf spot was more severe on early maturing cultivars, which had a heavy boll load, or were experiencing pre-mature defoliation. Diseased leaves were handled as previously described, and the cultural characteristics of the isolates obtained were similar to those for A. macrospora. However, the conidial characteristics were not consistent for A. macrospora in all cases, indicating that additional Alternaria spp. may be associated with leaf spot. Studies are currently being conducted to identify the Alternaria spp. associated with these diseases, and to quantify the impact of these outbreaks on cotton yield and/or grade. Results from these studies will be presented.