RESUMEN
Melothria indica Lour. is a wild ornamental plant widely distributed in South China. In November 2020, powdery mildew symptoms with 100% (60 plants) incidence were observed on M. indica climbing on a fence in Zhanjiang (21.17N,110.18E), Guangdong, China. The symptoms were typical for powdery mildew with white colonies on leaf surfaces and stems. Conidiophores appeared in all symptomatic tissues. Chasmothecia were observed only during the late stage of disease. Hyphae were hyaline, branched, and septate. Conidiophores were erect, hyaline, smooth, and had a dimension of 61.5 to 185.6 µm × 8.5 to 14.5 µm (n=20) and a cylindrical, flexuous foot cell, followed by 1 to 5 (-6) shorter cells. Conidia were ellipsoid to ovoid and had a dimension of 24.5 to 38.5 µm×15.5 to 21.8 µm (n=50) with well-developed fibrosin bodies. Germ tubes were in the lateral position. Chasmothecia were gregarious or scattered, subglobose, (64.8-) 65.5 µm to 115.5 (-120.5) µm (n=20) in diameter. The appendages were few, and hyphoid. Ascus one per ascomas, clavate, or subglobose, 56.5 to 78.3 (-90) µm×52.5 to 60.5 (-72) (n=20) µm. Each ascus had eight ascospores that were broadly ellipsoid and sized 13.8 to 18.6 µm×12.5 to 16.5 µm (n=30). The morphological characteristics were consistent with the previous description of Podosphaera xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). Three voucher specimens, Ms-1, Ms-2, and Ms-3, were deposited in the fungus collection at Aquatic Organisms Museum of Guangdong Ocean University, and were used for molecular analysis. Their internal transcribed spacer (ITS) regions were amplified using primers ITS1/ITS4. Amplicons were sequenced and submitted to GenBank (accession no. MW512919, MW512920, and MW512921). The sequences were identical to each other and 100% similar to two of P. xanthii (Accession No. MT472035 and MN818563). On the basis of the morphological and molecular characteristics, the fungus was identified as P. xanthii. Pathogenicity was examined through inoculation by gently pressing the naturally infected leaves onto healthy ones of three potted M. indica plants with three leaves. Healthy leaves were leaves of three further plants which served as the control. White powdery mildew colonies with an incidence of 100% were similarly observed after 7 days at 28 °C and 80% relative humidity in a greenhouse. The fungal colonies on diseased leaves were morphologically identical to the specimen, and the control plants developed no symptoms. The Koch's postulates have completed. Golovinomyces cichoracearum is known to cause powdery mildew on M. indica in China (Liu et al. 2015). P. xanthii (synonym:P. fusca p.p.) is the cause of powdery mildew on cucurbits worldwide (Braun and Cook 2000), including M. indica (synonym:M. japonica) in Korea (Kwon et al. 2015) and Japan (Takamatsu et al. 2005), but hitherto not for China. While, the teleomorph of the fungus on cucurbits is seldom found worldwide and in China only in the north (Liu et al. 2011), chasmothecia are recorded for here southern China (21.17N,110.18E).
RESUMEN
Oryza rufipogon Griff is a wild rice germplasm that might contain genes valuable for rice breeding. In May to June 2019, a leaf disease on wild rice (O. rufipogon cv. 'Haihong-12') was observed in a 3.3 ha field in Zhanjiang (20.93° N, 109.79° E), Guangdong, China. Early symptoms were yellow spots from the tip of leaves. Later, the spots gradually expanded downward the entire leaf to turn brown in turn. Symptoms were found in the tillering to the grain-filling stages (Supplementary Figure 1). The disease incidence on plants was between 10% and 40%. Twenty diseased leaves were collected from the field. The margin of the diseased tissues was cut into 2 mm × 2 mm pieces, surface-disinfected with 75% ethanol and 2% sodium hypochlorite for 30 s and 60 s, respectively, and rinsed three times with sterile water before isolation. The tissues were plated onto potato dextrose agar (PDA) medium and incubated at 28 °C. After 5-day incubation, grayish fungal colonies appeared on PDA. Single-spore isolation method was used to recover pure cultures for three isolates (Aas-1, Aas-2, and Aas-3). The colonies first produced light-grayish aerial mycelia, which turned dark grayish upon maturity. Conidiophores were branched. Conidia were two to four in chains, dark brown, ovoid or ellipsoid, and mostly beakless; had one to four transverse and zero to three longitudinal septa; and measured within 7.0-18.5 (average = 12.5) × 3.0-8.8 (average = 4.5) µm (n = 30). Morphological characteristics of the isolates were consistent with the description of Alternaria alternata (Fr.) Keissler (Simmons 2007). The internal transcribed spacer (ITS) region, partial RNA polymerase II largest subunit (RPB2) gene, translation elongation factor, and glyceraldehyde-3-phosphate dehydrogenase were amplified with primers ITS1/ITS4, RPB2-6F/RPB2-7R, EF-1α-F/EF-1α-R, and GDF1/GDR1, respectively (Woudenberg et al. 2015). Amplicons were sequenced and submitted to GenBank (accession nos. MW042179 to MW042181, MW090034 to MW090036, MW090046 to MW090048, and MW091450 to MW091452, respectively). The sequences of the three isolates were 100% identical (ITS, 570/570 bp; RPB2, 1006/1006 bp; TEF, 254/254 bp and GADPH, 587/587 bp) with those of CBS 479.90 (accession nos. KP124319, KP124787, KP125095, and KP124174) through BLAST analysis. The sequences were also concatenated for phylogenetic analysis by maximum likelihood. The isolates clustered with A. alternata CBS 479.90 (Supplementary Figure 2). The fungus associated with brown leaf spot on wild rice was thus identified as A. alternata. Pathogenicity tests were done in a greenhouse at 24 °C-30 °C with 80% relative humidity. Individual rice plants (cv. 'Haihong-12') with three leaves were grown in 10 pots, with around 50 plants per pot. Five pots were inoculated by spraying a spore suspension (105 spores/mL) onto leaves until runoff occurred, and another five pots were sprayed with sterile water to serve as controls. The test was done three times. Disease symptoms were found on the leaves after 7 days. The tips of the leaves turned yellow and spread downward. Then, the whole leaf turned brown and dried out, but the controls stayed healthy. The pathogen was re-isolated from infected leaves and phenotypically identical to the original isolate Aas-1 to fulfill Koch's postulates. To our knowledge, this report is the first one on A. alternata causing brown leaf spot on wild rice (O. rufipogon). The pathogen has the potential to reduce wild rice yields and future breeding should consider resistance to this pathogen.