Black mildew disease on the Siwalik (Miocene) monocot leaves of Western Himalaya, India caused by Meliolinites

Abstract

We have recovered disease-symptomatic monocot leaves from the middle Siwalik (late Miocene; 12–8 Ma) sedimentary strata of Himachal Pradesh, western Himalaya, India. Information about disease symptoms linked to fossil monocot leaves, however, is lacking. The present study therefore aims to elucidate their identity through the analysis of morphological characteristics of the plant pathogenic fungi (causal agent) associated with these disease symptoms. Black mildew disease caused by foliicolous fungal fossil-genus Meliolinites Selkirk ex Janson. and Hills (fossil Meliolaceae) is detected on infected host Siwalik monocot leaves. In the study presented here, we provide formal descriptions and illustrations for the fossil-genus. The pathogen Meliolinites is recognized by the presence of appressoria, phialides, mycelial seta, black non-ostiolate ascomata, and four-septate, five-celled ascospores. This is the first report of melioloid fungus causing black mildew disease on fossil monocot leaves. Here, we also reconstruct a possible disease cycle of black mildew pathogen on Siwalik monocot leaves. The in-situ evidence of Meliolinites on the monocot leaf cuticles indicates the possible existence of a biotrophic relationship in Himachal sub-Himalaya's ancient warm and humid tropical forest during the time of deposition.