First Report of a Ceratocystis manginecans Causing Quick Decline of Psidium guajava in Pakistan.

Abstract

Psidium guajava is a widely grown fruit tree of Asia for food and medicinal purposes. Also being reported to have anti-inflammatory, antimicrobial, antioxidant, antidiarrheal, antimutagenic properties (Somu, 2012). In April 2018, quick decline disease of guava was observed in orchards of Sheikhupura, Lahore, Faisalabad, Kasur and Chiniot districts of Punjab, Pakistan. Approximately 68% of the trees were found declined with mummified fruits. Initial infection symptoms appeared as wilting of leaves, bark discoloration, followed by the leaf drooping, crown area discoloration, bark splitting, mummified fruits, dying of branches and lately whole tree death in weeks to months. The fungus formed a dark brown to black discoloration (3 to 5 cm wide and 7 to 9 cm long) in vascular bundles of P. guajava tree. Sixty-five samples of discolored wood from the main stem were collected, and pathogen was isolated using carrot bait method (Moller and DeVay, 1968). Isolation and purification were done on 2% Malt extract agar (MEA) plates incubated at 25 ± 2 °C in 12 h light and dark period. After 6 days of incubation, fungal hyphae, fruiting structures, sexual & asexual spores were observed on MEA plates. Black globose to subglobose ascomata with bases (151-) 200 (-278) µm in diameter with long neck (511-) 535 to 600 (-671) µm long, (23-) 28 to 39 (-47) µm wide at base, (13-) 13- 19 (-25) µm wide at tip and light brown to hyaline divergent ostiolar hyphae (50µm) were developed and produces hat-shaped hyaline ascospores 3 to 5 µm long and 6-7 µm (with sheath) and 4 µm (without sheath) wide. After 7 days, initially white mycelium turned into olivaceous green and produced primary phialidic conidiophore with emerging primary cylindrical hyaline conidia (7 to 12 × 4 to 6 µm), secondary conidiophore with emerging chain of secondary barrel-shaped hyaline conidia (9-) 10 to 12 (-13) µm long × (5-) 5 to 9 (-11) µm wide and dark brown dematiaceous chlamydospores conidia (12 ×10 µm) were observed. All morphological characteristics were consistent to the description of Ceratocystis manginecans (Van Wyk, et al., 2007). For further confirmation, from a purified isolate GWD10, genomic DNA was extracted. The internal transcribed spacer (ITS) and translation elongation factor 1-alpha (TEF 1-α) region were amplified with primer pairs ITS1/ITS4 and EF1/EF2 (Jacobs et al., 2004; White et al., 1990) respectively. Generated sequences (Accession Nos. MN 365128 & MT952139) on BLAST analysis showed 100% homology for ITS and TEF with Ceratocystis manginecans (Accession No., KC261852 CMW 13582 Voucher, NR-119532.1 type material, MH863135; EF433317, respectively) reported from Oman and Pakistan (Van Wyk et al., 2007 & Vu et al., 2019). For pathogenicity test, one-year-old healthy P. guajava plants were inoculated by making a T-shaped slit of 5 × 7.5 mm in the bark. Two weeks old cultures of GWD10, 5-mm mycelial discs were aseptically transferred and covered with moistened sterilized cotton swab followed parafilm to maintain humidity. Fifteen plants were inoculated with fungal cultures and five plants were inoculated with MEA plugs as controls. All plants were maintained at 25 ± 2 °C with 80 ± 5% relative humidity (RH) in greenhouse Initial bark discoloration developed after 14 days of inoculation. After 40 days of inoculation plants started wilting and dying, similar to the symptoms were observed in naturally infected trees. Control plants remained asymptomatic. To fulfill Koch's pustulates, the same pathogen was re-isolated from the test plants and identified on morphological features to GWD10. The pathogen has been associated with mango decline in Oman and Pakistan (Van Wyk et al., 2007), acacia wilt in Indonesia (Harrington et al., 2015) and siris wilt in Pakistan (Razzaq et al., 2020). P guajava is an important fruit and medicinal plant, and the infection of C. manginecans is a great concern to the producers of P. guajava (Harrington et al., 2015; Huang et al., 2003). To our knowledge, this is the first report of Ceratocystis manginecans causing quick decline of P. guajava worldwide.