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First Report of Twig Blight Disease of Citrus Caused by Haematonectria haematococca in the Philippines

Citrus has recently become one of the most important crops in the Philippines with approximately 151,000 t of production in 2005. A new disease affected citrus (Citrus reticulata Blanco cv. Satsuma) twigs and trunks in 2005. The problem has caused significant concerns to the industry because of its destruction and high severity of infection. Symptoms include twig drying, bark hardening, twig girdling, leaf chlorosis, and defoliation. Infection occurs at all stages of the plant where severe symptoms were observed from January to April. Average temperature of these months ranged from 20 to 28°C. Affected twigs were often covered with pink or salmon-colored fungal propagules (1). In April 2009, citrus twigs displaying the above-described symptoms and signs were collected from Malabing Valley, Kasibu, Nueva Vizcaya, the Philippines (16°20′4.6912″N, 121°16′41.0742″E). Fungal perithecia were surface sterilized and observed with a stereomicroscope. Single ascospore was picked and inoculated into the peptone-pentachloronitrobenzene medium (2) and the plates were incubated at 22 ± 2°C for 7 days with a 12-h light/dark regimen. Periodically, the plates were screened for the growth of mycelia or discrete colonies on the medium. The cultured fungus produced microconidia and multiple canoe-shaped macroconidia. With a hand-held sprayer, approximately 20 ml of distilled water containing fungal conidia (2 × 10⁵ spores/ml) was inoculated each onto 1-year-old cv. Satsuma citrus (30 plants) bearing young twigs. Control plants were sprayed with sterile distilled water. After inoculation, plants were maintained at 20 to 28°C and 75 to 85% relative humidity, enclosed in clear plastic bags, placed under 70% woven shade cloth, and watered regularly. The inoculated plants started showing the initial twig blight symptoms in all inoculated branches at 37 days post inoculation (dpi). The occurrence of pink-to-orange perithecia of Haematonectria haematococca was observed at 45 dpi, which has similar morphological appearance to the perithecia collected from the field. Symptoms were not observed on branches treated with sterile water. H. haematococca was reisolated from the symptomatic twigs and the recovered isolates were morphologically similar to H. haematococca, thus completing Koch's postulates. Control plants remained healthy. DNA was extracted from all isolates, and the nuclear ribosomal internal transcribed spacer (ITS) region was amplified with primers ITS1 and ITS4 and sequenced. A portion of the ITS sequence has been deposited in the NCBI database (GenBank Accession No. HQ696788). A BLAST search of the NCBI database with the ITS sequence revealed H. haematococca (Berk. & Broome) Samuels & Nirenberg as the closest match with 100% sequence similarity. To our knowledge, this is the first report of H. haematococca causing citrus twig blight in the Philippines. To maintain the economic profitability of the citrus industry in the Philippines, control measures must be implemented to minimize tree loss.

References: (1) P. J. Chester. Phil. Agri. Rev. 11:69, 1919. (2) S. M. Nash and W. C. Snyder. Phytopathology 52:567, 1962.