DETECTION BY ENZYME-LINKED-IMMUNOSORBENT-ASSAY OF RHIZOCTONIA SPECIES ON POINSETTIA STEM CUTTINGS

A multiwell enzyme-linked immunosorbent assay(ELISA) and rapid assay ELISA were evaluated for detection of binucleate and multinucleate isolates of Rhizoctonia spp. on poinsettia cuttings in propagation. Rice grain inoculum of each test isolate was positioned 2 cm on either side of the cutting. At 2, 3, and 7 days, poinsettia stem samples were rated visually for Rhizoctonia stem rot and then split in half for ELISA or for culture on acidified potato-dextrose agar. Rhizoctonia spp. were detected by ELISA and by culture within 2 days of inoculum placement for multinucleate isolates of R. solani and 3 days for binucleate isolates. Generally, multinucleate isolates which caused severe stem rot in 5-7 days gave absorbance readings between 1.0 and 2.0, and the same isolates gave rapid assay readings between 40 and 80, Poinsettia stems exposed to binucleate isolates generally gave much lower, but positive, test readings than multinucleate isolates. One of three binucleate isolates induced lesions of Rhizoctonia stem rot on poinsettia stems. A multinucleate isolate was detected in a stem lesion as small as 1 mm long (8.6 mm2) with either ELISA format. In a survey of poinsettia cuttings with symptoms of stem rot from commercial greenhouses, the multiwell kit was reliable in detection of Rhizoctonia spp. compared to visual assessment and culture results. The two ELISA formats proved useful for detection of Rhizoctonia-like fungi and R. solani from poinsettia.