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Maintaining Bremia lactucae on Washed Seedlings of Lactuca sativa in Deep Petri Dishes. J. E. Yuen, Graduate research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; J. W. Lorbeer, professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 71:1232-1234. Accepted for publication 10 March 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-1232.

Attempts to culture Bremia lactucae, an obligate parasite, on living host tissue in closed dishes were not successful because of contamination by other microorganisms during either germination or inoculation. Contamination was avoided with a new technique similar to growing bean sprouts. About 60 lettuce seeds were sown on moist filter paper in a 10-cm petri dish and incubated at room temperature. When the cotyledons had emerged but were not unfolded (5–7 days later), the seedlings were transferred to a 10-cm deep petri dish, washed several times with tap water to remove seed coats, and incubated at 14 C in an illuminated incubator (3,000 lux, 12-hr photoperiod). The seedlings were then washed daily with two to three changes of tap water. When the cotyledons had expanded, they were sprayed to runoff with a suspension of sporangia of B. lactucae in distilled water (1 × 10⁵ sporangia per milliliter), and the daily washings were resumed the next day. The fungus sporulated on both surfaces of the cotyledons in 10–14 days. This technique was also used with detached cotyledons: the two cotyledons and about 1 cm of hypocotyl were removed from seedlings, placed in 125-ml Erlenmeyer flasks, inoculated, incubated, and washed as for seedlings until the fungus sporulated.