First Report of Flower Anthracnose Caused by Colletotrichum karstii in White Phalaenopsis Orchids in the United States

Abstract

In October 2010, a Colletotrichum species was isolated from white Phalaenopsis flowers growing in a greenhouse in San Francisco, CA. This Phalaenopsis is a common commercial orchid hybrid generated mostly likely from Phalaenopsis amabilis and P. aphrodite. The white petals showed anthracnose-like lesions where necrotic tissue is surrounded by a ring of green tissue. The green halo tissues around the necrotic tissue contain functional chloroplasts. One-centimeter disks were cut around the necrotic sites and surface-sterilized with 95% ethanol and 0.6% sodium hypochlorite. The disks were placed on potato dextrose agar (PDA) medium to establish cultures. Pure cultures were obtained by subculturing hyphal tips onto fresh PDA plates. The generated colonies had white aerial mycelia and orange conidial mass. The color of the reverse colony varies between colorless and pale orange. Microscopic observations identified the conidia as cylindrical, straight, and rounded at both ends. In addition, the conidia were approximately 15.0 to 18.0 μm long and 5.0 to 6.5 μm in diameter. These observed morphological features suggested that these isolates possessed the same characteristics as previously described for Colletotrichum karstii, a species considered as part of the C. boninense species complex (1). Four putative independent Colletotrichum isolates were recovered (DED9596, DED9597, DED9598, and DED9599). To confirm the Colletotrichum isolates as the causative pathogen, healthy white Phalaenopsis flowers (five total) in a whole plant were sprayed with a conidial suspension (approximately 1.2 × 106 conidia/ml) of the isolates and incubated at 20°C and 100% relative humidity with cycles of 16 h light and 8 h of darkness. Approximately 1 ml of conidial suspension solution was used for each flower. The plants were watered regularly and flowers were sprayed with sterile double-distilled water daily. As negative controls, five flowers in a whole plant were sprayed with water. Fifteen to twenty days after inoculation, lesions started to form on the petals sprayed with the putative Colletotrichum isolates. All controls remained healthy. The Colletotrichum-inoculated flowers remained alive and did not die as a result of the infection. This same experiment was repeated and the same results were obtained. DNA was extracted from the necrotic regions of the petals infected by the pure cultures of the four isolates and used to sequence the 18S rRNA ITS (internal transcribed spacer) region. All four isolates gave identical ITS sequences. Analysis of the obtained representative sequences (GenBank Accession No. JQ277352) suggested that the isolated pathogen as C. karstii. Using the published ITS data for the C. boninense species complex (1), a phylogenetic tree was generated via the maximum likelihood method. This created tree places the isolates in the same group as C. karstii. This type of C. karstii infection in Phalaenopsis orchid petals was not documented in the U.S. before, although it has been reported in China and Thailand (2). To our knowledge, this is the first report of infection and green island formation caused by C. karstii on orchid flower in the United States. References: (1) Damm et al. Studies in Mycology 73:1, 2012. (2) Yang et al. Cryptogamie Mycologie 32:229, 2011.

Identification, Pathogenicity, and Spore Trapping of Colletotrichum karstii Associated with Twig and Shoot Dieback in California

“…Although C. karstii has been reported from citrus in China, Italy, and Portugal (Aiello et al 2015;Guarnaccia et al 2017;Huang et al 2013;Peng et al 2012;Ramos et al 2016), in the United States, C. karstii has only been reported from Leucospermum (Crous et al 2013) and Phalaenopsis spp. (Jadrane et al 2012). In North America, C. karstii has been reported in Mexico from two additional hosts, Annona cherimola Musa sp.…”

Revisiting the Taxonomy of <I>Colletotrichum</I> Using Multi-Locus Gene Phylogeny

“…A few studies have been undertaken to decipher the etiology of the disease. Colletotrichum cymbidiicola on Cymbidium, C. oncidii on Oncidium and a species of C. boninense complex on Dendrobium (Damm et al, 2012a), C. gloeosporioides on Cymbidium sinense in China (Huang et al, 2012) and C. karstii on Phalaenopsis in USA (Jadrane et al, 2012) have been associated with the disease. Anthracnose caused by Glomerella cingulata was first reported on Vanda coerulia in India as early as 1952 (Ramakrisnan et al, 1952).…”

Endophytic Colletotrichum species from Dendrobium spp. in China and Northern Thailand

“…Colletotrichum species has been listed as one of the top ten fungal pathogenic genera in molecular plant pathology based on scientific/economic importance (Dean et al 2012). Anthracnose caused by Colletotrichum species can be a devastating disease in many economically important crops, including fruit crops, vegetables, cassava, sorghum, as well as ornamental plant such as orchids (Prusky and Plumbley 1992; Hyde et al 2009a, b; Cannon et al 2012; Dean et al 2012; Jadrane et al 2012; Jayawardena et al 2016a; Diao et al 2017). Many pathogenic Colletotrichum species that adopt biotrophic life strategies are present as symptomless endophytes in living plant tissues (Photita et al 2004), although a large number of non-pathogenic species also occur as endophytes (e.g.…”