First report of powdery mildew caused by Podosphaera xanthii on the invasive Verbena brasiliensis in Texas

Brazilian verbena (Verbena brasiliensis, family Lamiaceae) is a highly invasive plant found throughout the southern portion of the United States. As suggested by its name, it originates in South America but has prospered in the US due to its highly ruderal growing pattern and its ability to tolerate drought and disturbance. During the summer of 2019, sixty-four V. brasiliensis plants were growing in the University of Houston's temperature-controlled greenhouse (Houston, TX). Eight plants exhibited symptoms of powdery mildew infection on the stems and adaxial surface of the leaves. White circular powdery colonies with high numbers of aerially dispersing spores were observed on leaves, seemingly uncorrelated with leaf age. Upon examination, chlorosis of leaf tissue was detected in areas of infection. Conidiophores (n=25) were on average 210 μm in length and produced 6 to 9 conidia in true chains. Base-cells of conidiophores branched from hyphae forming right angles and averaged 35 μm long at the base. Conidia were hyaline, ovate, and measured 28-31 × 19-21μm. These structures are typical of the powdery mildew oidium anamorph of the genus Podosphaera. No chasmothecia were observed within colonies.The morphological characteristics and measurements were consistent with those of Podosphaera xanthii (Braun and Takamatsu 2000). Pathogenicity was confirmed by gently rubbing symptomatic V. brasiliensis leaves onto healthy leaves of V. brasiliensis plants (Lee 2013). Fifteen plants were inoculated, and five remained uninoculated to serve as negative controls. Inoculated plants developed powdery mildew symptoms between 6 to 12 days (averaging 10 days), whereas all controls remained disease-free. DNA was extracted from fungal tissue from the original plant, inocula sample, and the newly infected leaves. The internal transcribed region was amplified using the ITS1f and ITS4 primers (White 1990). Three samples, one from the original plant, the inocula, and the reisolated fungi were sequenced, identified using NCBI BLAST, and the resulting sequences were deposited in GenBank (MN818562, (inocula), MN818563 (re-isolate), MN818564 (original)). All three sequences had 98.7% similarity to the P. xanthii on Brazilian verbena reported in South Korea assession number KJ472787 (Cho et al 2014). All three samples were amplified using the mating type primer sets and PCR protocal described in Brewer et al 2011. All three samples were determined to be MAT1-1-1 based on positive and ne