First report of powdery mildew caused by oidiopsis haplophylli on garden nasturtium (Tropaeolum majus L.) in Florida

Garden nasturtium (Tropaeolum majus L.) is an herbaceous annual plant that is used as an ornamental in gardens and as a medicinal plant. Flowers, flower buds, and leaves are edible and are considered a valuable source of glucosinolates and other health-promoting compounds (Ghedira and Goetz, 2013; Ribeiro et al. 2007). In May 2015, leaves showing symptoms typical of powdery mildew were collected in a noncommercial, tunnel greenhouse located in Immokalee, FL, dedicated to the conventional production of transplants and vegetables. Chlorotic leaf lesions were first observed, which later became angular and necrotic, and in advanced stages the foliage could be nearly completely covered. The whitish fungal structures were apparent on both surfaces of the leaves, with a higher abundance on the abaxial surface. All ten garden nasturtium plants in the greenhouse were symptomatic but tomato, herbs, and other edible flower crops in the same structure were asymptomatic. Microscopic examination identified conidiophores that were hyaline, septate, and cylindrical (4.5 to 6 × 260 to 330 µm). Conidia were dimorphic: primary conidia were navicular (12 to 18 × 61 to 90 µm) whereas secondary conidia were ellipsoid-cylindrical (15 to 20 × 64 to 72 µm). The description matches previous reports of this pathogen on garden nasturtium (Ribeiro et al. 2007; Park et al. 2010). To further confirm the identity of this pathogen, DNA was obtained by using a microwave extraction method. The amplification and sequencing were carried out using the conserved primers ITS1/ITS4. The PCR product was 670 bp long, and had 100% identity to Leveillula taurica (Accession No. AY912077) with a maximum score of 1210 and 97% query coverage. Powdery mildew caused by Oidiopsis haplophylli (anamorph of Leveillula taurica) is reported on garden nasturtium in Brazil and Korea (Ribeiro et al. 2007; Park et al. 2010). In the United States, this pathogen has been reported on other crops including spinach (Koike et al. 2015) and squash (Braun and Mohan, 2013). To our knowledge, this is the first report of powdery mildew caused by O. haplophylli on garden nasturtium in the United States. In the United States, the market value of garden nasturtium as a commercial crop is limited to local production distributed in specialty stores and farmers markets, but this disease may be a limiting factor for the commercial cultivation of this crop in greenhouse-type structures in Florida.