The Response of Peronospora Parasitica to a Liver Medium

Abstract

Peronospora parasitica Pers. ex Fr. is an obligate parasite on crucifers; attempts at culture have failed. In water the conidia produce whip-like germ tubes, which swell slightly only where they touch the dish. The medium (2) used to culture species of Puccinia (rusts) that are parasites on higher plants reduces the length of the P. parasitica germ tubes unless it is a 1/10 dilution. In the diluted medium, germtube length is no greater than in the control. Since Syncephalis nodosa van Tieghem, a mucoraceous parasite on other Mucorales, has been grown on a medium containing cubes of beef liver (1), the development of P. parasitica on this medium was studied. Washed, autoclaved 2-mm cubes of liver were placed in a 9-cm Petri dish and covered with 20 ml of a mixture of 0.01% tryptone (Difco: pancreatic digest of casein), 0.04% K2HPO4 and 2% agar: a 1/10 dilution of the K2HPO4 and tryptone concentrations used by Ellis (1). Ten ,ug/ml streptomycin (Calbiochem) were added to this medium to retard bacterial growth. After the agar had solidified, drops of a suspension of conidia in water were put on the surface of the agar over each piece of liver. The plates were incubated at 18 C. After 4 da the germ tubes in the control without liver had disintegrated but in a 5-mm-diam zone around the liver pieces, 90% of the germ tubes were growing toward the liver. They grew from the surface toward the bottom of the medium and formed large swellings or lobes within the agar. The closer the germinating spore was to the liver, the larger were the lobes (FIGS. 1 and 2, A-B). The length of the lobed germ tubes was no more on the average than those without liver, but the width was many times greater and the wide germ tubes did not disintegrate even though they were kept at 18 C for several wk