First Report of Scab Disease Caused by Venturia effusa on Pecan in Anhui Province of China

Abstract

Pecan (Carya illinoinensis) is a world-famous nut tree which widely cultivated in China. Quanjiao County, located in Anhui province, is reputed to be the capital of pecan production in China. Since 2019, typical scab symptoms were observed on most pecan cultivars in orchards located in the regions of Quanjiao (32°5′7.08″ N, 118°16′2.91″ E). In April, dark brown to black lesions of scab could be observed on both the abaxial and adaxial surface of the lamina, and were often associated with the veins or midrib. In July, small, brownish, and circular lesions ranging from 1 to 2 mm in diameter were observed at the end of stems and shoulder of the fruit. In the surveyed orchards, disease incidence on the leaves reached more than 35%. While, according to the number of infected nut clusters, disease incidence ranged from 40 to 60% on the infected fruits. Using a sterilized scalpel, conidia were scraped from the surface of a single lesion from the infected leaves or fruits, and a dilute spore suspension was prepared in sterile distilled water, of which 100 microliters was spread on 1% water-agar plate (Bock et al. 2014). The conidia were incubated at 25°C for 48 h under fluorescent lights with a 12-hphotoperiod. Single germinated conidia were selected and transferred into potato dextrose agar (PDA) plate to obtain monospore isolates. From 2019 to 2020, more than 20 isolates were obtained from the infected leaves and fruits. Incubated at 24°C for 6 weeks in darkness on PDA, the colonies were gray-black with circular morphology and floccose texture, which were consistent with the characteristics of Venturia effusa described previously (Gottwald 1982). The conidia were pyriform to ellipsoid, zero to one septate, smooth, attenuated towards apex and base, base truncate, pale brown and 10.08 to 18.14 × 4.86 to 9.56 μm (n = 50) in size. To further identify the isolates, the regions of internal transcribed spacer (ITS), beta-tubulin 2 (TUB2) and translation elongation factor 1 alpha (EF1-a) were amplified and sequenced from genomic DNA for the three representative isolates (AH-81 and AH-82 from the infected leaves, and AH-41 from the infected fruits), respectively (White et al. 1990; Young et al. 2018; Bensch et al. 2006). Sequences of them were deposited in GenBank under nos. OP199056 to OP199058 (ITS), OP566581 to OP566583 (TUB2) and OP566578 to OP566580 (EF1-a). Multilocus phylogenetic analysis revealed that three isolates and V. effusa were clustered in the same clade, indicating high genetic similarity between these organisms. Their morphological and molecular characteristics were consistent with those for V. effusa. The pathogenicity of three isolates were tested on two-year-old container-grown pecan seedlings, which were grown in the nursery. The conidial suspension with a concentration of 5 × 105 conidia/ml was sprayed evenly on the surface of leaves of a healthy pecan seedling, and each isolate inoculated four pecan seedlings. The pathogenicity experiment was repeated three times. The plants inoculated with sterile water were used a negative control. The inoculated plants were enclosed in plastic bags for 2 days, and kept in the nursery greenhouse. Four weeks after inoculation, a similar symptom of scab was observed on leaves of cultivar Mahan, and V. effusa was isolated again from inoculated leaves with the frequency of 100% by the single-spore isolation, whereas no symptoms were observed on the control plants. To our knowledge, this is the first report of V. effusa as a scab pathogen on pecan in Anhui Province of China and underscores the need for monitoring this disease and developing disease control strategies to prevent severe reduction in the value of fruit. References: Bensch, K., et al. 2006. Studies in Mycology, 55(1): 299-305. Bock, C. H., et al. 2014. Forest Pathology, 44(4): 266-275. Gottwald, T. R. 1982. Taxonomy of the pecan scab fungus Cladosporium caryigenum. Mycologia. 74 (3), 382-390. White, T. T., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Application. Academic Press, San Diego, CA. Young, C. A., et al. 2018. Phytopathology, 108(7): 837-846. The author(s) declare no conflict of interest. Keywords: Venturia effusa, Scab, Pecan, Identification