First Report of Phytophthora palmivora Causing Black Pod on a Herrania sp. in Brazil

Abstract

During inspections in a germplasm collection in the municipality of Barro Preto, Bahia State, Brazil, it was observed that about 30% of the pods of a Herrania sp. (Malvaceae) genotype showed black pod rot in 2017. Symptoms initially appeared as brown, circular lesions that quickly enlarged at high humidity, and a white mycelium covered the diseased pod surface. The putative pathogen was isolated from six symptomatic pods on 2% water agar containing ampicillin, rifampicin, nystatin, and pentachloronitrobenzene. A hyphal-tip culture from a morphologically representative isolate was obtained on carrot agar (CA) and deposited in the Colecao de Fungos Fitopatogenicos of the Universidade de Brasilia (accession no. CCUB1089). The colony on CA was white and slightly cottony with sparse aerial mycelium. Sporangia (n = 50) were ellipsoid, deciduous, 32.5 to 55.5 µm long, and 25.5 to 39 μm wide, and papillae were 6.0 to 7.0 µm thick. Globose, terminal chlamydospores measuring 16.5 to 39.5 µm in diameter were formed abundantly on CA. The specimen was identified as mating type A1 by pairing with A1 and A2 tester strains of Phytophthora capsici and P. nicotianae. Partial nucleotide sequences of the internal transcribed spacer (ITS), β-tubulin (βt), elongation factor (EF), heat shock protein 90 (HSP90), and cytochrome oxidase subunit 2 (coxII) genomic regions were obtained and deposited in GenBank (accession nos. MH807778 to MH807782). In the Phytophthora database (Park et al. 2008), the ITS, coxII, βt, EF, and HSP90 sequences matched P. palmivora (isolate PD_00022) with 100.0, 100.0, 99.5, 99.8, and 98.9% identities (accession nos. HQ261635, GU222099, EU080339, EU080340, and EU080342, respectively). Additionally, our isolate clustered with other P. palmivora isolates in a Bayesian phylogenetic tree based on concatenated sequences (ITS, coxII, βt, EF, and HSP90) (study S23112 deposited in TreeBASE). Thus, based on morphological and molecular data, the pathogen was identified as P. palmivora. To confirm pathogenicity, mycelial plugs (5 mm) from a 7-day-old culture (grown on CA at 25°C) were placed onto three nonwounded pods of Herrania sp., Theobroma cacao (clone CCN51), T. grandiflorum, T. bicolor, T. speciosum, and T. subincanum. On control pods, CA discs were used. Pods were kept for 5 days in a humid chamber at 25°C. Inoculated pods showed symptoms similar to those observed in the field, and P. palmivora was successfully reisolated from all symptomatic pods. Control pods were symptomless. P. palmivora has been previously reported on Herrania sp. only in West Africa (Erwin and Ribeiro 1996). In Brazil, this pathogen is infecting Annona squamosa, Carica papaya, Citrus sp., Cocos nucifera, Ficus carica, Hevea brasiliensis, Solanum lycopersicum, Piper nigrum, T. cacao, and T. grandiflorum (dos Santos et al. 2017; Farr and Rossman 2018; Zentmyer et al. 1973). The pathogenicity of P. palmivora on T. bicolor, T. speciosum, and T. subincanum (and the absence of reports on these hosts) reinforces that this pathogen can infect species closely related to cacao. This is the first report of P. palmivora causing black pod on a Herrania sp. in Brazil.