RESUMEN
In August 2022, powdery mildew symptoms were detected on lettuce (Lactuca sativa) in a commercial field located in Quecholac, Puebla, Mexico. Signs appeared as whitish powdery masses on leaves. Disease incidence was about 100% and signs covered up to 40% of leaf surface. Mycelium was amphigenous forming white patches. Hyphal appressoria were indistinct or nipple-shaped and solitary. Conidiophores (n= 30) were hyaline, erect, arising from the upper surface of hyphal mother cells or lateral, and of 90 to 201 µm long. Foot cells were cylindrical, of 49 to 92 × 10-15 µm, followed by 1-3 shorter cells, and forming conidia in chains. Conidia (n= 100) were hyaline, ellipsoid-ovoid, doliiform-subcylindrical, 27 to 40 × 14 to 20 µm. Conidial germination belonging to the Euoidium type. Chasmothecia were not observed. The morphological characters were consistent with those of Golovinomyces bolayi (Braun et al. 2019). A voucher specimen was deposited in the Herbarium of the Department of Agricultural Parasitology at the Chapingo Autonomous University under accession number UACH451. To confirm the identification of the fungus, genomic DNA was extracted from conidia and mycelium following the CTAB method (Doyle and Doyle 1990), and the internal transcribed spacer (ITS) region was amplified by PCR using the primers ITS5/ITS4 (White et al. 1990) and sequenced. The resulting 506 bp sequence had 100% identity to those of G. bolayi (LC417109 and LC417106). Phylogenetic analyses using the Maximum Likelihood and Maximum Parsimony methods were performed and confirmed the results obtained in the morphological analysis. The isolate UACH451 grouped in a clade with isolates of G. bolayi. The ITS sequence was deposited in GenBank under accession number OR467546. Pathogenicity was confirmed by gently dusting conidia onto ten leaves of healthy lettuce plants. Five non-inoculated leaves served as controls. The plants were maintained in a greenhouse at 25 to 30 ºC, and relative humidity of 70%. All inoculated leaves developed similar symptoms to the original observation after 10 days, whereas control leaves remained disease free. Microscopic examination of the fungus on inoculated leaves showed that it was morphologically identical to that originally observed. Based on morphological data and phylogenetic analysis, the fungus was identified as G. bolayi. This pathogen has been previously reported causing powdery mildew on lettuce in Argentina, Canada, Chile, Ecuador, Peru, USA and Venezuela (Braun et al. 2019; Mieslerová et al. 2020). To our knowledge, this is the first report of G. bolayi causing powdery mildew on lettuce in Mexico.
RESUMEN
The Brussels sprout (Brassica oleracea var. gemmifera) is a cruciferous vegetable with high health-promoting value and Mexico is one of the most valuable exporters worldwide (Data Mexico 2023). From September to November 2021, white mold symptoms (Rimmer et al. 2007) were observed in Brussels sprouts (cv. Confidant) fields in Tonatico, Estado de México, Mexico. Irregular, necrotic lesions were observed on leaves, whereas abundant white mycelium, and later black sclerotia were produced outside and inside of stems. Disease incidence ranged from 20 to 40% in five fields. For fungal isolation, symptomatic stem pieces were surface sterilized with 2% sodium hypochlorite for 2 min, rinsed in sterilized distilled water twice, placed on PDA medium, and incubated at 25°C in darkness for 3 days. Sclerotinia-like colonies were consistently obtained and six isolates were purified by the hyphal-tip method. Fungal colonies were white and fluffy. Irregular, black, and small (3 to 6 mm diameter) sclerotia were produced at the edge of colonies after 5 days of incubation. The morphological characters were consistent with those of Sclerotinia sclerotiorum (Saharan and Mehta 2008). Two representative isolates were selected for molecular analysis and pathogenicity tests. The isolates were deposited in the Culture Collection of Phytopathogenic Fungi at the Colegio Superior Agropecuario del Estado de Guerrero under the accession numbers CSAEG50 and CSAEG51. For molecular identification, genomic DNA was extracted, and the internal transcribed spacer (ITS) region was amplified by PCR and sequenced using the primer pair ITS5/ITS4 (White et al. 1990). The sequences were deposited in GenBank (accession nos. OQ878510 and OQ878511). BLASTn searches in GenBank showed 100% identity with the available sequences of Sclerotinia sclerotiorum (accession nos. OQ891471, OQ891472, HQ833448, and MT177216). A phylogenetic analysis using the Maximum Likelihood method placed isolates CSAEG50 and CSAEG51 in the same clade as S. sclerotiorum. Pathogenicity tests were performed by inoculating 10 healthy Brussels sprout seedlings (cv. Confidant) grown in pots. A mycelial plug was directly placed on the stem of each plant. Five uninoculated Brussels sprout seedlings were used as control. All plants were placed in a moist chamber at 25°C with a 12-h photoperiod for 2 days. White mold symptoms appeared on inoculated plants after 3 days, whereas control plants remained symptomless. The fungi were reisolated from the infected plants and found to be morphologically identical to the isolates used for inoculation, fulfilling Koch's postulates. Pathogenicity test was performed twice with similar results. Sclerotinia sclerotiorum has been previously reported to infect Brussels sprouts in the USA (Campbell 1947). To our knowledge, this is the first report of Sclerotinia sclerotiorum causing white mold of Brussels sprouts in Mexico. The disease is widely distributed in Brussels sprouts fields in the central region of Mexico, therefore additional studies are needed to develop effective disease-management strategies.
