Variability in Morphology, Oxalic Acid Production and Mycelial Compatibility Grouping Among Sclerotium rolfsii Isolates Causing Southern Blight Disease

ABSTRACT This study characterised the variability among 15 Sclerotium rolfsii isolates collected from different host plants by analysing their morphological characteristics, oxalic acid production, mycelial compatibility, and pathogenicity. The isolates exhibited diverse colony morphologies, with radial growth rates ranging from 22.7 to 30.0 mm/day. Colonies were white to off‐white in colour and displayed either compact or fluffy growth patterns. Sclerotial production varied significantly, with the majority of isolates producing between 400 and 700, whereas a few formed 100–200 sclerotia per plate. Sclerotia were predominantly spherical, measuring 0.50–2.57 mm in diameter, although isolates SR1 and SR4 produced irregular shapes. Mycelial compatibility testing of 105 pairings revealed 85 compatible interactions and 20 pairings showing antagonism. Based on mycelial interactions, all isolates were grouped into five mycelial compatibility groups (MCGs A–E). Most isolates produced oxalic acid, indicated by a colour shift from blue to yellow in bromophenol blue‐amended medium. Oxalic acid production levels correlated positively with both increased mycelial biomass and a reduction in medium pH. The highest oxalic acid concentration (25.55 mM) was recorded for isolate SR15 from Arachis hypogaea . Pathogenicity assays confirmed that all isolates were virulent. Detached leaf assays revealed differences in lesion size and infection timing, whereas pot trials on potato plants showed that isolates from A. hypogaea were the most pathogenic, causing the greatest reduction in tuber weight. A strong positive correlation was observed between oxalic acid production and pathogenicity across isolates.