Sporisorium reilianum, the fungus that causes sorghum head smut, was g
rown with sucrose, lactose, trehalose or raffinose in liquid suspensio
n or on a solid medium. Liquid culture media were analyzed for hydroly
sis products of these carbohydrates to determine extracellular enzyme
activity of the fungus. Increased amounts of glucose and fructose in t
he culture medium of S. reilianum grown with sucrose or raffinose indi
cated that invertase (beta-fructofuranosidase, 3.2.1.26) activity was
present. No evidence of extracellular galactosidase or trehalase activ
ity was found. Enhanced sporidial colony formation on carbohydrates th
at can be hydrolyzed to hexoses, and specific forms of mycelial growth
on lactose, trehalose or on a carbohydrate-deficient medium might sug
gest that mycelial growth is a way of foraging for food sources. Howev
er, the rapid and profuse mycelial growth on the host cell wall glycop
rotein appears to be in response to abundant food supply (probably of
a different type). Therefore availability of different kinds of carbon
sources in the environment of the growing fungus might determine dimo
rphism and associated pathogenesis by S. reilianum.