AN IMPROVED SELECTIVE MEDIUM FOR THE ASSAY OF SEPTORIA-NODORUM FROM WHEAT SEED

An agar medium (designated SNAW), developed to improve the recovery of Septoria nodorum from wheat seed, was compared with oxgall agar, the most selective agar medium currently used. Metabolites produced by S. nodorum on oxgall agar fluoresce under near ultraviolet light, but the fungus does not sporulate. Growth of other seedborne fungi is only partially suppressed. On SNAW, S. nodorum fluoresces and sporulates within 7 days. Growth of most other seedborne fungi was reduced by greater than 95%, but Fusarium spp. were reduced only 74%. SNAW contained 10 g Difco potato-dextrose agar, 15 g agar, 1.5 g oxgall, and and 1 g peptone per liter of deionized distilled water. After autoclaving, chloroneb (5 mg/L), cupric hydroxide (5 mg/L), dicloran (5 mg/L), chloramphenicol (3.13 mg/L), erythromycin (3.13 mg/L), tetracycline hydrochloride (12.5 mg/L), and neomycin sulfate (10 mg/L) were added to inhibit bacteria and seedborne fungi other than S. nodorum. SNAW and oxgall agar were compared by assaying wheat seed lots differing in degree of colonization by S. nodorum and the population of other fungi. The number of fluorescing colonies 4 days after plating on SNAW increased 38%, and the total number of seeds from which S. nodorum was recovered increased by 48% on SNAW, compared with oxgall agar. The percentage of "clean colonies" (number of seeds with more than 50% of the area around the seeds colonized by S. nodorum/total number of seeds with S. nodorum) was also 40% higher on SNAW than on oxgall agar. SNAW offers significant improvement over oxgall agar in recovery of S. nodorum from wheat seed, but maximum recovery is not achieved because of competition from other seedborne fungi, especially Fusarium spp.