Evaluation and characterisation of effective fungal pathogens associated with the coconut eriophyid mite (Aceria guerreronis Keifer)

Abstract

Outbreak of coconut eriophyid mite has become a serious menace by inflicting heavy damage to coconut plantation both in terms of yield reduction and economic return. The increased awareness of the general public about the repercussions of extensive use of chemical pesticides and also because of the apprehension being expressed by scientists and policy makers on various side effects of chemicals, biocontrol is getting more attention in the management of mite. The present study on “Evaluation and characterisation of effective fungal pathogens associated with coconut eriophyid mite (Aceria guerreronis Keifer)” was carried out in the Department of Agricultural Entomology, College of Horticulture, Vellanikkara. The main objectives of the study were to isolate, identify and confirm the pathogenicity of fungi associated with the eriophyid mite, identification of the potential acarifungal pathogens and molecular characterisation of the selected isolates. Survey conducted at three different locations each from four panchayaths of Thrissur district for one year revealed that there was no significant difference in mite population (live mite, dead mite and egg) among the four panchayaths, but it varied significantly within the three locations of the four panchayaths. A uniform distribution of mite population was followed in the outer perianth, inner perianth and nut surface with maximum live mites and egg count on the nut surface and dead mites on the inner perianth. Live mite and egg count contributed to about 36 per cent while the dead mite formed 28 per cent of the total mite population. Predatory mite population varied significantly both among the four panchayaths as well as among the locations within the four panchayaths. Two types of predatory mites, Amblyseius spp. and Bdella spp. were mainly observed during the study with an average population range from 4.469 to 15.719 per nut. Seasonal variation showed three peaks of live mite population and egg count during the late monsoon (August), winter (January/February) and summer (April/May) months; where as two peaks during winter (January to February) and summer (March to April) for the dead mite population. Predatory mite population followed a normal range which increased with increase in mite population to a certain degree within the limited range of predatory mite population. Correlation analysis showed a positive relationship between population parameters of mite (live mite, dead mite, egg count) and predatory mite population and maximum temperature. A non-uniform correlation was observed between mite population and other weather parameters. Natural occurrence of the mite specific pathogen, Hirsutella thompsonii isolates at monthly intervals for 12 months contributed to 4.90 per cent. Maximum number of Hirsutella isolates was obtained during winter season followed by the monsoon season. Isolates belonging to the genus Fusarium spp., Acremonium spp. and Paecilomyces spp. were mostly isolated during the monsoon season while the isolates of actinomycetes during summer period over the entire four panchayaths. The fungal species which proved pathogenicity to coconut eriophyid mite were two species coming under the genus Hirsutella, viz., H. thompsonii and H. kirchnerii and the other fungal species coming under the genus Acremonium viz., A. strictum and A. incoloratum, Fusarium viz., F. lateritium and F. verticillioides, Paecilomyces, P. fumosoroseus and P. lilacinus. Based on the presence or absence of synnemata, two different varieties of Hirsutella were found to occur, namely, H. thompsonii var. thompsonii and H. thompsonii var. synnematosa. Cultural and morphological observations revealed that among the eight isolates coming under the two varieties H. thompsonii var. synnematosa isolate, Madakkathara-I recorded maximal fungal growth (4.167 cm) and biomass (2.660 g) while the H. thompsonii var. thompsonii isolate, Chirakkekodu-I possessed maximum mean sporulation (3.33 x 106 spores ml-1). Among the other fungal pathogens, maximum growth and sporulation was observed on Acremonium spp., A. strictum (7.99 cm) and A. incoloratum with a mean value of 43.11 x 106 spores ml-1. Mortality assessment of the Hirsutella isolates along with other fungi at their respective doses of sporulation revealed that the fungal pathogen, P. fumosoroseus recorded the maximum mortality of 83.65 per cent which was closely followed by H. thompsonii var. thompsonii (Chirakkekodu-I) with a mortality of 80.63 per cent. Molecular characterisation of the seven isolates of Hirsutella using the five primers comprising of the OPE and OPAH series exhibited a maximum genetic similarity of 74 per cent between the Hirsutella isolates, Madakkathara-I and Konnakuzhy-I. Fifty per cent genetic similarity was obtained between two clusters with the Hirsutella isolate, Marakkal-I in one cluster and all the other Hirsutella isolates and H. kirchnerii in the other cluster. The isolates coming under two varieties of H. thompsonii, H. t. var. thompsonii, Chirakkekodu–I with maximum sporulation and H. t. var. synnematosa Madakkathara-I with maximum growth rate and biomass came under the same subcluster.