Morphological Identification of Phytophthora A Causal Organism of Pineapple Heart Rot Disease in Uganda

Abstract

Pineapple (Ananas comosus L. Merr) is an economic horticultural crop in Uganda. However, pineapple production is currently being threatened by the latest outbreak of pineapple heart rot disease in Uganda. Yet, information on pineapple heart rot disease causal pathogen is unknown. Therefore, the objective of this study was to identify the pathogen causing pineapple heart rot disease (PHRD) in Uganda. Samples of pineapple leaves with symptoms of PHRD were collected from four districts of Masaka, Luwero, Mukono and Kayunga. Pathogen isolation was done using corn meal agar (CMA) amended with Pimaricin, Ampicillin, Rifampicin Pentachloronitrobenzene, Hymexazol and Benomyl (PARPHB). Macro- and micro- morphological characteristics of the isolates were assessed using Potato dextrose agar (PDA) and V8 media, respectively. Pathogenicity of the isolates was tested using healthy pineapple plants based on Koch’s postulate. The results showed a significant difference (P0.05) in the growth rates, hyphae diameter and asexual structure dimensions of the isolates. Isolates were characterized by a dense rosette and stoloniferous mycelial growth pattern in PDA media. Although, sporangia were non-caduceus, terminal, papillate and mostly ovoid, obpyriform and limoniform sporangia (32-56μm) with a mean length/breadth ratio of 1.32:1μm were observed. Chlamydospores were spherical and thick-walled (25-42µm diameter) while Sporangiophores branching was sympodial. Based on the morphological characteristics of isolates, it was concluded that Phytophthora nicotianae is the species causing pineapple heart rot disease in Uganda. This study therefore represents the first comprehensive information in Uganda. However, molecular identification of the pathogen is recommended to confirm the genetic identity of the species.