Downy mildew of coleus (Solenostemon scutellarioides) caused by Peronospora sp. in Florida

Abstract

In Florida, coleus (Solenostemon scutellarioides [= Coleus blumei, Plectranthus scutellarioides]) is an important and very popular commercial foliage and bedding plant. In 2006, a downy mildew disease affected coleus at wholesale container and retail nurseries in Florida (Alachua, Bradford, Marion, and Miami-Dade counties). The disease first occurred in November and continued to affect new foliage throughout the year during cool, foggy weather. The disease primarily affected newly expanded young leaves. Infected foliage was chlorotic, distorted, and stunted, usually turning brown and necrotic, resulting in defoliation and death of young seedlings. The abaxial sides of affected leaves were heavily colonized by extensive violet brown growth morphologically resembling Peronospora lamii (Francis, 1981). The pathogen was tentatively identified based on the following characteristics: hyaline conidiophores ranging from 343·5 to 561 by 9·5 to 15 µm emerged from stomata, branched dichotomously; branches ended in slender, curved branchlets with the angle between ends usually at a right angle; conidia slightly brown, ovoid to ellipsoid, mostly nonpapillate, and ranged from 17·0 to 24·5 by 16·5 to 25·5 µm. The oospores were not observed in plant tissue. The ITS regions of an isolate of downy mildew on coleus from Alachua Co., Florida were sequenced bidirectionally (GenBank Accession DQ980194). Sequencing distinguished this Peronospora sp. from P. lamii. Results confirm downy mildew on coleus in Florida and agree with other results published on downy mildew caused by an undescribed Peronospora species on coleus in New York and Louisiana and basil in Switzerland (Belbahri et al., 2005; Daughtrey et al., 2006). However, this is the first report of downy mildew on coleus in Florida.