A procedure for isolating Pyrenochaeta terrestris from onion roots

Abstract

Surface sterilised stelar tissues from onion roots exhibiting typical pink root symptoms were plated on water agar and incubated under cool white fluorescent light (CWFL) for 12 h day 1 at 20°C. After 4-5 days, the plates were examined with the 10 x objective of a compound microscope and tissues producing conidia of Fusarium spp. were discarded. Sub-cultures of isolates suspected to be Pyrenochaeta rerreslris, grown from Fusarium-negative tissues, were transferred to chloroamphenicol-amended (500 ppm) corn-meal (CCMA) agar plates and mcubated at 24°C under CWFL for a further 10-14 days. Cultures with setose rycnid1a were identified asP. lerrestris. Plates were again scanned for conidia of fusarium spp., and cultures negative for Fusarwm spp., but not producmg rycn1dia, were suspected to be P. terrestris. These were compared with known isolates on CCMA. On this medium growth of P. rerrestris was slow, producing appressed, pinkish colonies which were circular with a smooth margin. All isolates of P. terrestris isolated by the procedure caused pink root of onion when tested, whereas none of the Fusarium spp. isolated produced the disease. However, a mixed inoculum of P. rerresrris and Fusanum so/ani produced typical pink root symptoms