First Report of Powdery Mildew Caused by Golovinomyces latisporus on Helianthus annuus in the Arizona.

Abstract

Helianthus annuus, known as the common sunflower, is an annual plant indigenous to the United States. The crop is grown for its edible oil, seeds, and as an ornamental. In November 2021, powdery mildew-like signs and symptoms were observed on sunflower in a house garden located at Fortuna Foothills, Yuma County, AZ (32.6725°N, 114.4329°W). Signs of powdery mildew included white blotches of amphigenous and caulicolous mycelia. Initially, signs appeared as circular spots that expanded over the entire leaf and were also observed later on petioles and stems. Fungal hyphae were branched, septate, and with nipple-shaped appressoria. Foot cells of conidiophores were erect, cylindrical, and followed by one to three short cells bearing conidia. The conidiophores were hyaline, straight, cylindrical and produced short chains of up to four immature conidia. Conidia were ovoid to ellipsoid, seldom cylindrical, without-fibrosin bodies and measured 25 to 40 μm in length (mean= 34 μm) and 16 to 24 μm in width (mean= 18.6 μm) (n = 10), with an average length: width ratio of 1.8. The germination of conidia was Euoidium type with short germ tube. Chasmothecium formation was not detected. As the disease progressed, the infected leaves became wilted and senesced. The morphological characteristics of this fungus were in accordance with those described for G. latisporus (Braun and Cook, 2012; Qiu et al., 2020). For molecular identification, genomic DNA was isolated from fungal colonies using the DNeasy Plant kit (Qiagen). The internal transcribed spacer (ITS), large ribosomal subunit (28S), intergenic spacer (IGS), beta- tubulin (TUB2) and chitin synthase 1 (CHS1) regions were amplified using the primer sets, ITS5/ITS4 (White et al., 1990), LSU1/LSU2 (Scholin et al., 1994), IGS-12a/NS1R (Carbone and Kohn, 1999), TubF1/TubR1 and gCS1a1/gCS1b (Qiu et al., 2020). Sequences were submitted in the GenBank under accession numbers OP160535 (ITS), OP153874 (28S rDNA), OP160534 (IGS), OP168959 (TUB2) and OP168960 (CHS1). A BLAST search revealed 98.7 to 100-% identity to G. latisporus sequences (MK452603, NG_068877, MK452520, MK452476, and MK452428). Phylogenetic analysis based on the combined sequence datasets of ITS+28S rDNA+IGS+TUB2+CHS1 also grouped Yuma isolate in a monophyletic clade with G. latisporus accessions from Qiu et al. (2020). A pathogenicity test was carried out by inoculating four sunflower varieties (Velvet queen, Evening sun, Skyscraper and Mammoth). Conidia from infected leaves were dusted on the leaves of 10 individually potted, five-week-old seedlings of each variety. Four seedlings of each variety were not inoculated and served as controls. The inoculated and control plants were then kept in two separate greenhouses at 25-28°C at the Yuma Agricultural Center, Yuma, AZ. Inoculated plants started showing symptoms of powdery mildew within 7 days of inoculation, while the control plants stayed healthy and disease-free. The morphological characteristics of powdery mildew fungus on the inoculated plants were identical to the isolate collected from the garden, with the same sequence following PCR as above, thus fulfilling Koch's postulates. G. latisporus has been previously reported on common sunflower from Washington and California states (Qiu et al., 2020), however, this is a first report from Arizona. Although sunflower is not a major crop in Arizona, the wild sunflower population could serve as reservoir for the spread of the disease.