First Report of Phytophthora palmivora Causing Foliar Blight of Pachira aquatica in Florida
Authors: Palmateer A. J. , Cating R. A. , Lopez P. , Maguire I. 
Published: May 2012

Pachira aquatica Aubl. is a member of the Bombaceae, indigenous to Central America and northern South America. Known as the money tree within the ornamental plant industry, this tropical species is well adapted to landscapes in south Florida, Hawaii, and milder areas in southern California. Recently, it's become more popular as a potted plant for use in the interiorscape. During August 2011, several local nurseries submitted P. aquatica samples to the Florida Extension Plant Diagnostic Clinic in Homestead, FL. The foliage exhibited dark brown to black water soaked spots that became papery as the disease progressed, and rapidly enlarged and coalesced, resulting in severe leaf blight. Both young and mature leaves were affected. Phytophthora was initially confirmed by serological testing with a commercially available ImmunoStrip test (Agdia, Elkhart, IN). On closer examination, the pathogen was further identified as Phytophthora palmivora by the presence of numerous papillate, deciduous, ellipsoidal to ovoid sporangia with short pedicels. The sporangia averaged 53 × 32 μm with ranges of 48 to 59 × 29 to 35 μm (1). Phytophthora species-specific primers (pal1s and pal2a) targeting part of the 18S rRNA gene, the ITS 1, the 5.8S rRNA gene, and the ITS 2 resulted in a PCR product of 648 bp, testing positive for P. palmivora (2). The PCR product was cleaned (Qiagen Purification Kit) and sequenced (GenBank Accession No. JQ354937). The sequence from our isolate was nearly identical (exhibited 99% nucleotide identity) to an isolate of P. palmivora (GenBank Accession No. HE580280) collected from diseased cassava in China. To further support identification, phylogenetic analysis by the maximum likelihood method (Tamura-Nei model) was performed using the obtained sequence and several other Phytophthora and Pythium species from GenBank (MEGA 5.05). Our isolate grouped with other P. palmivora isolates with high support (100% bootstrap values, 1,000 replicates). Pathogenicity of the sequenced isolate was evaluated in shade house experiments. Six-month-old Pachira aquatica plants were inoculated with sporangial suspensions (1 × 106 conidia/ml) of P. palmivora. Inoculum or autoclaved water was sprayed over the foliage until runoff. Six plants were sprayed per treatment, and the experiment was repeated twice. Inoculated plants were placed in a shade house (70% shade) when temperatures ranged from 25 to 32°C with 78 to 98% relative humidity. Plants were observed for disease development, which occurred within 7 days of inoculation. No symptoms developed on the control plants. Foliar lesions closely resembled those observed in the affected nurseries and P. palmivora was reisolated from symptomatic leaf tissue. To our knowledge, on the basis of serological testing, molecular analysis, and distinguishing morphological characters, this is the first report of P. palmivora causing foliar blight of Pachira aquatica in Florida. The high incidence and severity of this disease may seriously influence local tropical foliage producers in the future.

References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. American Phytopathological Society, St Paul, MN, 1996. (2) Tsai et al. Botanical Studies 47:379, 2006.