Characterization of fungal pathogen associated with leaf rot disease of coconut (Cocos nucifere L.) and in vitro evaluation of phylloplane microflora as biocontrol agents

Abstract

Leaf rot disease (LRD) is a major foliar disease affecting coconut plantations of Southern Kerala especially in root (wilt) affected areas. In this context, the study entitled ‘Characterization of fungal pathogen associated with leaf rot disease of coconut (Cocos nucifera L.) and in vitro evaluation of phylloplane microflora as biocontrol agents’ was conducted in the Department of Plant Pathology, College of Agriculture, Vellayani during the year 2018-2020, with the objective to identify and characterize the major fungal pathogens associated with the LRD of coconut and in vitro evaluation of phylloplane microflora of coconut against the pathogens. The isolation of LRD pathogens was carried out from six taluks of Thiruvananthpuram district such as Thiruvananthapuram, Neyyattinkara, Nedumangad, Chirayinkeezhu, Kattakada and Varkala. Three locations were selected from each taluk and a total of eighteen samples were collected during the study. The results revealed that the disease in Thiruvananthapuram district was caused by a spectrum of pathogens such as Colletotrichum gloeosporioides, Fusarium spp., Gliocladium sp., and Scytalidium sp. The LRD was caused either by a single pathogen or by combinations of pathogens. C. gloeosporioides and Fusarium spp. were found as the major pathogens of LRD based on the frequency of isolation. Each and every isolate of the same pathogen differed from one another in cultural characters and virulence. All the pathogens produced water soaked brown lesion on artificial inoculation on detached spindle leaves; though the days taken for symptom initiation and size of the lesion developed varied. The isolate C3 (Isolate from Anayara, Thiruvananthapuram taluk) was found to be more virulent among the C. gloeosporioides isolates; and among the Fusarium spp. isolates, the isolate F5 (Isolate from Alamkode, Chirayinkeezhu taluk) was found to be more virulent. By observing the spore characters of the isolates, it was found that the spore size and pigmentation of the culture haven’t any significance to the virulence of the pathogen. Dual inoculation of the major pathogens on detached spindle leaves caused severe incidence of the disease compared to the individual inoculation of the pathogens. This result indicated that the LRD caused by fungal complex is more severe than that caused by individual fungal isolates. There are phylloplane fungi existing on healthy leaves of the infected palm with enough inhibition potential to LRD. The phylloplane fungal isolate PF5 showed more per cent inhibition to mycelial growth of C. gloeosporioides (54.44%) followed by the isolate PF4 (43.33%); and the isolate PF4 showed more inhibition to Fusarium sp. (64.44%) followed by the isolate PF5 (45.55%) in the dual culture assay. The detached spindle leaf assay also supported the same fact that the isolate PF5 was observed to be more suppressive to the disease caused by C. gloeosporioides (28.77%) and the isolate PF4 was reported to have more suppression to the disease caused by Fusarium sp. (34.56%). These pre-treatment effects are more promising than Pseudomonas fluorescens PN026, but inferior to copper oxy chloride (0.2%). Thus, the present study revealed that the LRD of coconut in Thiruvananthapuram district is caused by a combination of pathogenic fungi viz., C. gloeosporioides, Fusarium spp., Gliocladium sp., and Scytalidium sp. Prophylactic application of the phylloplane fungal isolates PF4 and PF5 could reduce the LRD severity in vitro to a promising level and these isolate can be further tested for in vivo biocontrol potential before going for the development of a formulated product.