First Report of Powdery Mildew Caused by Erysiphe heraclei on Fennel (Foeniculum vulgare) in Mexico

Abstract

Fennel is a plant widely used as a fresh or dried herbal tea and herbal medicine in Mexico that is cultivated principally in backyards. In March 2018, fennel exhibited symptoms of powdery mildew with 95% disease incidence and 75% severity in plants growing in backyards in Axapusco County (19.856605° N; –98.749448° W) in Mexico State. Symptoms first appeared as white colonies on the basal leaves, which exhibited abundant mycelial growth covering the whole surface. Infected plants were unsuitable for consumption mainly by the white fungal growths on leaves and stems. All stages of development were susceptible to powdery mildew from seedlings to flowering plants. Infection caused leaf withering and premature senescence; infected plants may remain stunted, causing in some cases plant death. Mycelium on leaves, stems, and inflorescences, amphigenous; hyphae were septate, branched, and 3 to 8 μm wide. Appressoria were lobed and solitary. Conidiophores were cylindrical, erect from top of the mother cell, 73 to 106 × 5 to 8 μm, and composed of three cells (foot cell and two shorter cells). Foot cells were straight, cylindrical, and 18 to 45 µm long. Hyaline conidia formed singly and were cylindrical, 26 to 38 × 10 to 12 μm, without fibrosin bodies. Germ tubes arose from one end of the conidia. Chasmothecia were not observed. These morphological characteristics were consistent with those of Erysiphe heraclei (Braun and Cook 2012; Choi et al. 2015; Tovar-Pedraza et al. 2016). For molecular confirmation, DNA was extracted from scraped conidia of infected tissue. The complete internal transcribed spacer (ITS) region of rDNA was amplified with primers ITS1/ITS4 (White et al. 1990) and sequenced. The resulting 558-bp sequence was deposited in GenBank (accession MN207135). The obtained ITS sequence shared >98% similarity with those of E. heraclei accessions KP717418 and KP729443, and phylogenetic analysis with the neighbor-joining method (bootstrap test equal to 1,000 replicates) grouped the sequence as similar to E. heraclei (Choi et al. 2015; Koike et al. 2015). Pathogenicity testing was carried out in March 2019, under natural conditions (22 ± 4°C and 52 ± 5% relative humidity). Potted plants were obtained from seed collected the previous year and kept confined in a greenhouse for 55 days previous to inoculation. A spore solution was prepared in sterile distilled water. The risk of natural infection was reduced by placing the seedlings in an area away from fennel plants, and only one branch of each plant was selected and inoculated by depositing the spore solution with a brush; the control was treated with sterile distilled water. Only inoculated branches developed symptoms after 8 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was identical in morphology to those observed in the field. Powdery mildew caused by E. heraclei has been reported in Mexico on Ammi majus, Coriandrum sativum, and Daucus carota (Rodriguez-Alvarado et al. 2010; Tovar-Pedraza et al. 2016; Yanez-Morales et al. 2009). However, this is the first report of fennel powdery mildew caused by E. heraclei in Mexico. In backyards, fennel is mostly grown without using chemical products, especially those used for pest control; also, there are no fungicides authorized against powdery mildews. For this reason, E. heraclei represents a serious threat to quality and marketability.