Identification and Characterization of Isolates of Pythium and Phytophthora spp. from Snap Beans with Cottony Leak

Cottony leak is an important disease of snap bean in Oklahoma and nearby states. Oomycete pathogens isolated from diseased pods collected from commercial fields and research plots consisted of both Pythium spp. (n = 131) and Phytophthora spp. (n = 46). Isolates were identified to species by morphological characteristics and by sequencing a portion of the internal transcribed spacer region of representative isolates. The most common Pythium spp. were Pythium ultimum var. ultimum; Pythium 'group HS', a self-sterile form of P. ultimum that produces hyphal swellings in lieu of sporangia (n = 74); and P. aphanidermatum (n = 50). Phytophthora spp. included Phytophthora drechsleri (n = 41) and P. nicotianae (n = 5). Nearly all of the isolates (95%) and all species were pathogenic on detached pods but Pythium ultimum var. ultimum and Pythium 'group HS' were most aggressive. Phytophthora drechsleri was most aggressive on seedlings, causing preemergence damping off and seed rot. Pythium ultimum var. ultimum, Pythium 'group HS', and P. aphanidermatum were intermediate in virulence to seedlings, causing root rot, stunting, and limited postemergence damping off. Phytophthora nicotianae and Pythium diclinum (n = 4) were not pathogenic on seedlings. Most (87%) isolates were sensitive to metalaxyl-M (concentration that caused a 50% reduction in mycelial growth [EC₅₀] < 1 μg/ml) and the rest were intermediate in sensitivity (EC₅₀ > 1 to < 100 μg/ml). Phytophthora drechsleri was the most sensitive species (EC₅₀ = 0.06 μg/ml) compared with Pythium aphanidermatum, which was least sensitive (EC₅₀ = 1.3 μg/ml). Cottony leak is a disease complex caused by several oomycete species that should include Phytophthora drechsleri, a newly reported pathogen of snap bean in the United States.