ABSTRACT
Bacterial heart rot caused by a yet undetermined species of Dickeya was first observed in December 2003 in Oahu, HI, on a pineapple cultivar (Ananas comosus 'PRI 73-114') recently imported from Central America. Identical symptoms were later seen in the same plantation in fields that had been planted with propagules from the Philippines. Dickeya strains isolated from symptomatic plants and irrigation water collected over subsequent years were identified using bacteriological tests and partial 16S ribosomal DNA sequencing and characterized using repetitive sequence-based polymerase chain reaction (rep-PCR) with the BOXA1R primer (BOX-PCR), pathogenicity on pineapple leaves, and reactivity with two monoclonal antibodies (MAbs). Strains exhibited variability in pathogenicity and in reactivity with MAbs. BOX-PCR separated the plant-isolated Dickeya strains into seven haplotypes that were placed into four fingerprint groups (A to D). Strains from the A and B groups were isolated from the Central American stocks, whereas strains in the D group were identified from Philippine material. Strains from the C group were isolated from both planting materials. Most strains from water sources were placed into three haplotypes that loosely formed group E. BOX-PCR polymorphisms between the Dickeya strains isolated from foreign pineapple plants, a local collection of Dickeya strains, and strains from the plantation's water sources support the possibility that at least two separate introductions of genetically distinct strains occurred via imported planting stocks.
ABSTRACT
Bacterial black spot of mango (Mangifera indica) caused by Xanthomonas citri pv. mangiferaeindicae (Xcm), is an economically important disease in tropical and subtropical areas (3). Xcm can infect a wide range of mango cultivars and induces raised, angular, black leaf lesions, sometimes with a chlorotic halo. Fruit symptoms appear as small, water-soaked spots on the lenticels that become star-shaped, erumpent, and exude an infectious gum (3). The bacterium can also cause latent infections (2). Immature mango fruit with black spots on the epidermis were collected in August 2012 from mango trees of the cvs. Raposa and Pirie at a residence in Pukalani, Hawai'i, on the island of Maui. Similar symptoms were seen on a tree of the mango cv. Common (also known as 'Spanish' or 'Lahaina') at a nearby golf course. Mango fruit with black lesions, and leaves showing black lesions with yellow halos, were collected in August 2012 from mango trees of the cv. Haden at a residence in Kaimuki, Hawai'i, on the island of O'ahu. Xanthomonas-like bacterial colonies were isolated on TZC agar. Suspect colonies were non-pigmented on YDC agar. A fruit strain of the bacterium from Maui (A6081A) and a strain from each of a fruit (A6081B) and a leaf (A6082) from O'ahu were each gram-negative, oxidative, positive for both starch and esculin hydrolysis, and negative for nitrate reduction, resulting in presumptive identification as a Xanthomonas sp. The three strains were further characterized by Microlog (Biolog, Inc. Hayward, CA), which showed the closest match with X. campestris. In addition, 16S rDNA PCR assays showed the closest match (99% similarity) with X. citri strains, and RIF marker analysis of dnaA (4) grouped the three strains with Xcm strain LMG 941 (Accession No. CAHO01000002.1). Hypersensitivity responses typical of xanthomonads were observed when these strains were infiltrated into tobacco leaves, whereas no response was observed using sterile water. Leaves of 3-week-old mango seedlings were infiltrated using 10 µl (~108 CFU/ml) of each strain suspended in sterilized water (six to eight inoculations per leaf, four leaves per plant, and three replicate plants per strain). The negative control treatments consisted of inoculation with sterile water, as well as an incompatible pathogen, X. hortorum pv. vitians (A6076), isolated from lettuce. Typical symptoms of bacterial black spot were observed for all strains assayed approximately 2 weeks after inoculation. No lesions were observed on the negative control plants. Koch's postulates were satisfied following reisolation and identification of the Xanthomonas strains from the infected plant tissues, using the biochemical and PCR methods described above. Results for strains from the two islands confirmed published descriptions of the pathogen, indicating that the pathogen causing symptoms on these mango trees is Xcm (1). Cultures and infected plant samples were sent to USDA APHIS and CPHST NPGLB facilities where this identification was confirmed. To our knowledge, this is the first report of bacterial black spot of mango in Hawai'i or anywhere in the United States. It is unknown whether this disease is a new occurrence or has not been reported previously. The origin of the primary inoculum is unknown. References: (1) B. Manicom and F. Wallis. Int. J. Syst. Bacteriol. 34:77, 1984. (2) O. Pruvost et al. Microbial Ecol. 58:928. (3) O. Pruvost et al. Plant Dis. 95:774, 2011. (4) K. Schneider et al. PLoS 6:e18496, 2011.
