J.A.G. IRWIN¹, R. DILL-MACKY² and R.A. BRAY³

¹Botany Department, University of Queensland, St. Lucia 4067.
²Formerly Botany Department:, University' of Queensland, St Lucia 4067
³CSIRO Division of Tropical Crops- and Pastures, 306 Carmody Road, St. Lucia, Queensland 4067.

Abstract

Isolates of a Stemphylium sp., collected from 12 lucerne fields in southern Queensland, showed a wide range of relative virulence level on cloned material of a lucerne genotype from cv. Baron. There was significant variation both between collections, and between monoconidial isolates from a single collection.
Disease severity was much greater at temperatures of 18–20°C than at 24–26°C following inoculation with isolates from southern and northern Queensland and California. Based on these temperature responses and symptomatology, it is concluded that the disease in Australia is caused by the cool temperature (C-T) biotype of Stemphylium, first reported in California.
Two Californian experimental lines of lucerne with known resisitance to the C-T biotype, namely UC 1249 and UC 1250, were also highly resistant to the Queensland isolates. Although the recently bred Queensland cultivars Trifecta and Sequel were susceptible, resistant plants existed in each line, indicating that recurrent selection for resistance should be possible.