RESUMO
Leucophyllum frutescens (Scrophulariaceae family), commonly known as Texas sage or cenizo, is an evergreen shrub native to southwestern United States and northern Mexico. This plant is commercially sold as a native, drought-tolerant ornamental. During the spring of 2019 and 2020, typical symptoms of powdery mildew were found on cenizo plants growing as ornamentals in urban areas in the municipality of Ahome, Sinaloa, Mexico. Disease incidence was 95% from a sampled population of 120 plants. Initial symptoms of powdery mildew developed as irregular white colonies on upper leaf surfaces which expanded as infections progressed. In severe infections, leaves became distorted, exhibiting premature defoliation. Microscopic examination showed nipple-shaped appressoria. Conidiophores (n= 30) were hyaline, cylindrical, erect, 89.4 to 134.2 µm long, and forming catenescent conidia. Foot-cells were cylindrical, 35.7 to 65.3 × 10.2 to 13.5 µm, followed by 1-3 shorter cells. Conidia (n= 100) were hyaline, ellipsoid to ovoid, 27.9 to 40.5 × 13.8 to 18.9 µm, containing distinct fibrosin bodies. Germ tubes were simple to forked and laterally produced from the middle of conidia. Chasmothecia were not found during the sampling period on the infected leaves. Based on morphological characteristics, the fungus was identified as Podosphaera xanthii (Braun and Cook 2012). A voucher specimen (accession no. FAVF219) was deposited in the Herbarium of the Faculty of Agronomy of El Fuerte Valley at the Autonomous University of Sinaloa (Juan Jose Rios, Sinaloa, Mexico). To further confirm the identification, total DNA was extracted, and the internal transcribed spacer (ITS) region was amplified by PCR using the primers ITS5/ITS4 (White et al. 1990) and sequenced. The resulting 503 bp sequence (GenBank accession no. MT624793) had 100% coverage and 100% identity to those of P. xanthii (MT568609-MT568611, MT472035, MT309699, MT250855, MT242593). A phylogenetic tree using the maximum parsimony (MP) and maximum likelihood (ML) methods and including published ITS sequences for Podosphaera species was obtained. Phylogenetic analyses revealed that ITS sequence from FAVF219 isolate was grouped into a clade with P. xanthii. Pathogenicity was demonstrated by gently dusting conidia from infected leaves onto 50 leaves of five healthy plants. Five non-inoculated plants served as controls. All plants were covered with polyethylene bags for 48 h to maintain high humidity and were maintained in a greenhouse at temperatures ranging from 20 to 35ºC. All inoculated plants developed similar symptoms to the original observations after 19 days, whereas no symptoms of powdery mildew were observed on control plants. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants, fulfilling Koch's postulates. This fungus has been reported infecting members of the Cucurbitaceae in Mexico (Félix-Gastélum et al. 2017; Farr and Rossman 2020). However, to our knowledge, this is the first report of P. xanthii causing powdery mildew on a member of Scrophulariaceae, specifically L. frutescens in Mexico and worldwide. Further studies for monitoring and control strategies of powdery mildew on Texas sage are required.
RESUMO
Of the 17 genera of the Erysiphaceae, only four genera (viz. Leveillula, Phyllactinia, Pleochaeta and Queirozia) exhibit (partly) endoparasitism. To investigate early evolution of this endoparasitic nature, we performed molecular phylogenetic analyses of powdery mildews belonging to the tribe Phyllactinieae collected in North and South America. The most ancestral taxa in the tribe Phyllactinieae belong to the Pleochaeta/Queirozia group, from which the genus Phyllactinia was derived. Finally, the truly endoparasitic genus Leveillula emerged from a part of Phyllactinia The present study showed clear evolutional polarity in the powdery mildews concerned (that is, partly endoparasitic group evolved from ectoparasitic group) and then a truly endoparasitic group emerged from a partly endoparasitic group. In addition, a group with distinctly dimorphic conidia proved to be basal in the Phyllactinieae, and a group without distinctly dimorphic conidia was derived from that group. The present analyses clearly showed that Leveillula derived from a part of the "Basal Phyllactinia group". However, all sister taxa to Leveillula were distributed in North and South America. Because the putative geographic origin of Leveillula is assumed to be Central and Western Asia or the Mediterranean region, we postulate a missing link during the evolution of Leveillula from Phyllactinia Based on the present phylogenetic studies and the new rules of the International Code of Nomenclature for algae, fungi, and plants (McNeill et al. 2012), the following new species and taxonomic re-allocations are proposed: Phyllactinia bougainvilleae sp. nov., Ph. caricae comb. nov., Ph. caricicola comb. nov., Ph. durantae comb. nov., Ph. leveilluloides sp. nov., Ph. obclavata comb. nov., and Ph. papayae comb. nov.