Morphologic, molecular, and pathogenic characterization of Phytophthora palmivora isolates causing flower rot on azalea

Abstract

The objective of this work was to characterize two Phytophthora palmivora isolates causing floral blight and rot in azalea plants and to evaluate the pathogenicity of this oomycete pathogen on several plant species. Azalea plants with symptoms of flower blight and rot were obtained in the municipality of Holambra-SP. After an attempt of isolation, colonies with Phytophthora characteristics grown only on selective V8 medium. Molecular identification of the isolates was done by amplification and sequencing of ITS and COX2 regions. In the phylogenetic analysis, the azalea isolates clustered with reference isolates of P. palmivora. Morphological characteristics were similar to those described for P. palmivora. Isolates were inoculated in healthy azalea plants and caused leaf blight and floral rot. The pathogen was re-isolated from symptomatic plants completing Koch's postulates. In a host range test, the azalea isolates were able to cause lesions on leaves of vinca, snapdragon, basil, and tomato, and affected both leaves and flowers of geranium. Fruit rot was observed on tomato, potato, sweet pepper, scarlet eggplant, zucchini, cucumber, maroon cucumber, onion, apple, papaya, guava, and carrot. This is the first report of the species P. palmivora causing flower blight and rot in azalea plants in Brazil and probably in the world.

Keywords: Etiology; Ornamental plant; Pathogen host range; Rhododendron simsii.

Fig. 1

Phylogenetic trees constructed by Bayesian inference using Mrbayes plugin in Geneious Prime 2019 with 2 million generations and 25% burnin. Model GTR + G + I chosen separately for each region in MEGA X using Akaike information criterion. a Phylogenetic tree of ITS region with accession AF281230 of Pythium undulatum as outgroup. b Phylogenetic tree of COX2 partial sequence with accession JQ439490 of Pythium undulatum as outgroup

Fig. 2

Azalea plant showing symptoms of floral rot after inoculation with Phytophthora palmivora zoospores (a) and noninoculated control (b)

Fig. 3

Illustration of the symptoms in some plant species inoculated with Phytophthora palmivora isolates: a Scarlet eggplant (Solanum aethiopicum), b Tomato (Solanum lycopersicon), c Sweet pepper (Capsicum annuum), d Guava (Psidium guajava), e Carrot (Daucus carota), Maroom cucumber (Cucumis anguria), g Geranium (Pelargonium × hortorum), h Snapdragon (Antirrhinum majus), and i Vinca (Vinca rosea)