Studies on mango soilborne diseases with special reference to Phytophtora root rot

Abstract

Mango (Mangifera indica L.) is an important fruit crop in many tropical and subtropical countries. Recently, mango has been introduced into Italy mainly in some provinces of Sicily. However, its future as approaching commodity in Sicilian agriculture is threatened by diverse biotic and abiotic threats. This study aimed to assess the occurrence of the fungal soilborne diseases and their causal agents in the island. Special reference was provided to Phytophthora species, oomycetes -like fungi that cause Phytophthora root and crown rot on mango. Surveys were conducted over summer and spring (2010- 2011) in different mango orchards located in five provinces (Palermo, Messina, Catania, Agrigento and Ragusa) in Sicily. Several diseases induced by soilborne pathogens were reported in all the investigated orchards. Typical symptoms of damping off, root rot, crown rot, wilt, Armillaria root rot and wood decays diseases were observed. Morphological and molecular identification of the isolated fungi and oomycetes showed that they belong to different genera: Rhizoctonia, Fusarium, Pythium and Armillaria. The percentage of disease incidence and fungal frequency were recorded. Verticillium wilt, a vascular disease caused by Verticillium dahliae, was reported for the first time in a new mango grove in Catania province. Typical symptoms of the disease were observed. The pathogen identity was initially made based on colony morphology and formation of microsclerotia and further confirmed by molecular method. Greenhouse inoculation trial, performed on young Kensington Pride cv. mango plants, fulfilled its pathogenicity. Phytophthora root and crown rot disease was reported in provinces of Messina and Palermo. P. cryptogea was consistently isolated from diseased tissues taken from the crown and necrotic roots of mango. The fungus was identified on the basis of colony morphology, characterization of the sexual and asexual reproductive structures, and temperature range. In addition, DNA sequence data of ITS, COI, LSU and 60S loci were used for phylogenetic inferences. Pathogenicity tests conducted to assess its ability to cause disease revealed that the fungus is a possible pathogen of this crop. The diversity of P. cryptogea, the causal pathogen of mango root rot, within Phytophthora worldwide populations was assessed. In this study, re-evaluation of global collection of 140 isolates assigned to P. cryptogea, P. drechsleri and P. erythroseptica was carried out. Single and multiple gene phylogenetic analyses were performed on DNA sequences of nuclear (Internal Transcribed Spacers, ITS) and mitochondrial (Cytochrome c Oxidase subunit I, COI) genes. Both markers provided an acceptable resolution for these species. High levels of intraspecific variation were found within P. cryptogea population The possibility to set up a molecular approach to provide an accurate detection of P. cryptogea was investigated. Species-specific primer pairs for P. cryptogea, and two other species (P. megasperma and P. citrophthora), were designed from the most variable fraction of IGS regions. P. cryptogea specific primer (Cry5F/Cry5R) amplified 79 bp short fragment, while the primers Cit3F/Cit3R and Mega10F/Mega10R amplified 144 and 121bp fragments in P. citrophthora and P. megasperma, respectively. The above three sets of species specific primers pair were chosen to develop specific probes for the detection of the three Phytophthora species in the Real-time PCR (TaqMan) assay.