Evaluation of arbuscular mycorrhiza and other biocontrol agents against Phytophthora parasitica var. nicotianae infecting papaya ( Carica papaya cv. Surya) and enumeration of pathogen population using immunotechniques

Abstract

Root rot of papaya, caused by Phytophthora parasitica var . nicotianae, is the most widespread and important disease of papaya and is particularly damaging to many papaya varieties popular in southern India, such as Coorg Honey Dew and Surya. The objective of this study was to evaluate biocontrol agents (BCAs) under controlled and field conditions for their efficacy against Phytophthora infecting papaya cv. Surya and to detect and quantify the reduction in the pathogen population by immunological techniques. Glomus mosseae, Trichoderma harzianum and Pseudomonas fluorescens were inoculated at the time of planting in the nursery and at the time of transplanting in single, dual and tripartite combinations allowing colonization up to 90 days. Plants were challenged thereafter with Phytophthora inoculum multiplied on specialized Phytophthora medium. Uninoculated plants and those inoculated with pathogen only were controls. All the BCAs in general improved plant growth and reduced severity of disease compared to uninoculated control in both pot experiments and under field conditions. Plants preinoculated with G. mosseae + T. harzianum, provided the best results when challenged with Phytophthora, with increased plant height, girth and yield and also reduced disease severity over plants not inoculated with BCAs. Studies with Phytophthora antibodies conjugated with fluorescein isothiocyanate (FITC) and enzyme-linked immunosorbent assay (ELISA) further substantiated the reduction in infection and population of Phytophthora in BCA pretreated plants. Beneficial effect of BCAs may be due to the resistance acquired by the plant and reduction in the population of the pathogen caused by their colonization.