Pathogenicity and Cultural Characteristics of Sclerotinia minor Causing Lettuce Drop in Greenhouse Production Systems

Abstract

Lettuce drop disease caused by Sclerotinia minor is a serious constraint in greenhouse lettuce production systems, leading to rapid plant collapse, crown rot, and significant yield losses under favorable environmental conditions. Diseased lettuce plants showing wilting, white mycelial growth at the crown, and sclerotial formation were collected for pathogen isolation and characterization. Fungal cultures consistently produced fast-growing white cottony mycelia with abundant small, black, irregular sclerotia typical of S. minor. Considerable variation in colony growth rate, sclerotial size, and density was observed among isolates obtained from different greenhouse locations. Pathogenicity tests conducted on healthy lettuce plants reproduced characteristic drop symptoms, including crown decay, leaf wilting, and plant collapse, whereas control plants remained symptom-free. Re-isolation of the fungus from inoculated tissues fulfilled Koch’s postulates, confirming its causal role in disease development. Cultural studies revealed that growth and sclerotial formation were strongly influenced by temperature, moisture, and nutrient composition of growth media. Optimal mycelial growth and sclerotial production were observed under cool and moist conditions typical of greenhouse environments. Molecular identification using ITS region sequencing confirmed the identity of isolates as Sclerotinia minor, showing high similarity with reference sequences in global databases. Phylogenetic analysis clustered field isolates within the S. minor clade with strong bootstrap support, indicating genetic consistency among populations. Disease development was favored by high humidity, dense canopy conditions, and prolonged soil moisture, which enhanced pathogen survival and infection efficiency. Severe infections resulted in substantial reduction in plant stand, biomass, and marketable yield. The integration of pathogenicity assays, cultural characterization, and molecular identification provided a comprehensive understanding of the pathogen.