First Report of Sclerotinia Boll Rot and Stem Blight of Cotton in Arizona

Abstract

Upland (Gossypium hirsutum L.) and Pima (G. barbadense L.) cotton are economically important throughout the southwestern United States. In August 2017, canopy wilting and boll rot symptoms were observed on upland cultivar Dyna-Gro 3109B2XF planted in Cochise County, Arizona. The affected field area was 8 ha with approximately 10% incidence of the disease. Limbs in proximity to the soil appeared to die first, and the infection rapidly progressed toward the upper canopy. Infected limb tips remained green and healthy for several days before wilting became evident. When the canopy was parted, bleached stems and the white cottony growth of a fungus were visible. Infections were observed on stems, bolls, and leaves. Diseased bolls appeared soft and rotted, and they were covered with white mycelium and large black irregularly shaped sclerotia. The bleached stems were surface sterilized, placed on potato dextrose agar amended with streptomycin (0.25 mg/liter). The inoculated dishes were incubated at 27°C for 3 days. The colony appearance of four isolates was identical, with white fluffy mycelium that turned gray after 7 days. Black irregularly shaped sclerotia were formed along the edge of the Petri dish, measuring 3 to 5.8 mm in length, 2.3 to 10.7 mm in width, and 0.9 to 5.3 mm in thickness. Based on these morphological characteristics, the fungus was identified as Sclerotinia sclerotiorum (Kohn 1979). Genomic DNA was extracted from the mycelia of four isolates. The internal transcribed spacer (ITS) region of rDNA was amplified with primers ITS5/ITS4, and the resulting four sequences of 586 bp were deposited in GenBank (MG249966 to MG249969). A BLASTn search revealed a 100% match with several sequences of S. sclerotiorum with a query coverage at 100% (e.g., KY750530.1, HQ833447.1, and JQ618848.1). To fulfill Koch’s postulates, pathogenicity tests were conducted using detached limbs with three bolls. Both limbs and bolls were first wounded with a sterile needle and inoculated by placing a mycelial plug (8 mm in diameter) from an actively growing region. Four limbs with 12 bolls were inoculated for each isolate. Twelve limbs with bolls were inoculated with plain agar plugs as a control. All inoculated limbs were incubated in a plastic container with over 90% relative humidity at room temperature. Symptoms similar to those observed in the field started to appear 3 days after inoculation. Sclerotia with similar size and shape were observed by the end of the second week. No symptoms were observed on control limbs or bolls. S. sclerotiorum was consistently reisolated from symptomatic plants. The pathogenicity test was repeated twice with similar results. To our knowledge, this is the first report of Sclerotinia boll rot and stem blight of cotton in Arizona. S. sclerotiorum has a wide host range, including many economically important plant species (Bolton et al. 2006). In Arizona, S. sclerotiorum can cause yield losses on lettuce and pinto bean. Due to wet and cool weather conditions during the “monsoon” season, cotton fields with a history of pinto bean production will have an increased risk of cotton boll rot and stem blight.