THE PROPORTION OF LEAF SPOTS CAUSED BY MYCOSPHAERELLA CRYPTICA AND M. NUBILOSA ON EUCALYPTUS GLOBULUS, E. NITENS AND THEIR F1 HYBRIDS IN A FAMILY TRIAL IN TASMANIA, AUSTRALIA Angus J. Carnegie 1 & Peter K. Ades Institute of Land and Food Resources, The University of Melbourne, Parkville, Victoria 3052, Australia. Abstract Disease caused by M. cryptica and M. nubilosa was assessed on leaves collected from E. globulus, E. nitens and their Fi hybrids to determine the proportion of disease caused by these two pathogens on the three hosts and to examine the variation in symptoms caused by each species on these hosts. The two pathogens could be distinguished on each host mainly by pseudothecial distribution, and by size and shape of lesions. There were slight variations in symptoms of the two fungi on the three hosts. Mycosphaerella cryptica varied in lesion shape and colour, in whether the margin was callused or not, and in pseudothecial size. Mycosphaerella nubilosa varied mainly in whether pseudothecia were hypophyllous or amphigenous. Symptoms of both M. cryptica and M. nubilosa - did not vary amongst families within a species (or hybrid). On E. globulus, both M. cryptica and M. nubilosa were equally important in causing leaf necrosis on juvenile leaves. Damage to juvenile leaves of E. nitens was caused exclusively by M. cryptica. Mycosphaerella cryptica caused the majority of damage on the hybrid, although M. nubilosa was present on the majority of samples. Only M. cryptica was observed on adult foliage of all three hosts. There was significant variation in proportion of the two pathogens on families within the hybrids but not with either parent species. A.J. Carnegie & P.K. Ades (2002). The proportion of leaf spots caused by Mycosphaerella cryptica and M. nubilosa on Eucalyptus globulus, E. nitens and their Fl hybrids in a family trial in Tasmania, Australia. Australasian Mycologist 21 (2): 53-63. Introduction Species of Mycosphaerella have been associated with severe necrosis, defoliation and loss of growth in Eucalyptus globulus Labill. and E. nitens (Deane & Maiden) Maiden plantations in Australia (Carnegie et al. 1994, 1998, Dungey et al. 1997, Park & Keane 1982b) and South Africa (Lundquist 1987, Lundquist & Purnell 1987). Eucalyptus globulus and E. nitens are members of Eucalyptus subgenus Symphyomyrtus (Schouer) Brooker, series Globulares Blakely (Pryor & Johnson 1971). They are two of the most important species used in short rotation forest plantations in southern Australia and in other temperate regions of the world. Symptoms caused by M. cryptica (Cooke) Hansf. and M. nubilosa (Cooke) Hansf. vary with host species, leaf type (juvenile or adult), and leaf age at time of infection (Carnegie 1991, Carnegie et al. 1997, Crous et al. 1989, Park 1988, Park & Keane 1982b). Symptoms caused by Mycosphaerella spp. also vary on different eucalypt species in South Africa (Crous et al. 1991). There is wide variation in susceptibility of Eucalyptus spp. to infection by M. cryptica and M. nubilosa (Carnegie et al. 1998, Stone et al. 1998a, b, Wilcox 1982, Zandvoort 1977). Mycosphaerella nubilosa has a restricted host range, being recorded mainly on E. globulus, but also on E. cypellocarpa L. Johnson, E. bridgesiana R.T. Baker and E. quadrangulata Dean & Maiden (Carnegie & Keane 1994), and very rarely on E. botryoides Smith and E. grandis Hill ex Maiden (Crous et al. 1995). It has not been recorded from E. nitens. In contrast, M. cryptica has a known host range of over 50 eucalypt species (Carnegie 2000, Carnegie & Keane 1994, Crous et al. 1995, Park & Keane 1982¾). Hybrid eucalypts are of interest for plantation forestry as they may combine desirable characteristics of both parents. For example, E. nitens has a higher frost tolerance than E. globulus and as a consequence is becoming more important as a plantation species in Tasmania (Tibbits & Reid 1987a, b, Volker et al. 1994). Eucalyptus globulus, on the other hand, has generally superior growth and pulping qualities (Jordan et al. 1993). These two 1 Current address: Research and Development Division, State Forests of NSW, PO Box 100, Beecroft, NSW, 2119, Australia, E-mail: angusc@sf.nsw.gov.au