Sphaerella nubilosa Cooke (1891); Mycosphaerella nubilosa (Cooke) Hansf. (1956); Mycosphaerella juvenis Crous & M.J. Wingf. (1996)
Leaf spots predominantly occur on young leaves, but are also found on mature leaves. They occur on both surfaces of the leaves, varying in size from pin spots or flecks to small, round or irregular spots, up to 15 mm diam. These spots coalesce to form larger blotches over the leaf surface up to 3 cm diam on older leaves. Initial lesions appear as pale-green spots surrounded by purple margins and, once mature, generally yellow to pale brown with dark-brown raised borders (Crous & Wingfield 1996; Hunter et al. 2009).
Ascomata (sexual fruiting structures) are generally on the under surface of leaves but are rarely observed on both surfaces of the leaves. They are black, globose, immersed with a papillate ostiole, becoming erumpent with age, 70–90 µm wide, 65–90 µm high. Asci are bitunicate, ellipsoidal to obclavate, subsessile, and 30–60 × 9–18 µm. Ascospores are ellipsoidal to obovoid, colourless, smooth, not or slightly constricted at the median septum, widest in the middle of the upper cells, tapering more strongly towards the lower end, 10–15 × 3–4 µm, and surrounded by a non-persistent mucous sheath. Ascospores germinate from both ends and become prominently constricted at median septum. Primary ascospore cell becomes distorted during germination. Germ tubes grow parallel to the long axis of the spore. Colonies on malt extract agar reach 16–29 mm diam after 1 month at 25˚C in the dark, olivaceous black to grey olivaceous (Crous & Wingfield 1996; Hunter et al. 2009).
Disease cycle: The disease typically completes several generations during an epidemic cycle. Both ascospores and conidia can initiate disease on leaves. Ascospores, however, act as the primary source of inoculum for the majority, whereas some infect primarily by means of conidia (Park 1988b).
Infection mechanism: Ascospores start to germinate a few hours after landing on leaf surface. A germ tube penetrates through stomata, producing hyphal swellings within the stomatal pores. The germ tube expands along the vascular bundles, grows between cells throughout the spongy mesophyll, colonizes the leaf surface tissue causing chlorosis, and eventually aggregates in the substomatal cavities (Park & Keane 1982). The infection predominantly occurs during the vegetative period of the host during the summer and autumn months. Young leaves are particularly susceptible and mature leaves become more resistant due to the deposition of resistant compounds (Park 1988a).
Favourable conditions: The ability of T. nubilosa to infect Eucalyptus leaves is dependent on the level of moisture in the environment, which also affects the rate of the ascospore discharge. The longer leaves are exposed to wetness, the higher the severity of the disease (Park 1988b). Rainfall is the main stimulus for the release of ascospores from mature ascomata (Park 1988b). Optimal infection occurs between 5 to 7 days of wetness at 15–20˚C (Park 1988a; Park & Keane 1982). Ascospores can be ejected up to a distance of 12–15 mm above the ascomata, which allows the spores to be dispersed by wind (Park & Keane 1982).
Crous PW, Groenewald JZ, Mansilla JP, Hunter GC, Wingfield MJ. 2004. Phylogenetic reassessment of Mycosphaerella spp. and their anamorphs occurring on Eucalyptus. Studies in Mycology 50: 195-214.
Crous PW, Wingfield MJ. 1996. Species of Mycosphaerella and their anamorphs associated with leaf blotch disease of Eucalyptus in South Africa. Mycologia 88: 441-458.
Hunter GC, Crous PW, Carnegie AJ, Wingfield MJ. 2009. Teratosphaeria nubilosa, a serious leaf disease pathogen of Eucalyptus spp. in native and introduced areas. Molecular Plant Pathology 19(1): 1-14.
Park RF. 1988a. Effect of certain host, inoculum, and environmental factors on infection of Eucalyptus species by two Mycosphaerella species. Transactions of British Mycological Society 90(2): 221-228.
Park RF. 1988b. Epidemiology of Mycosphaerella nubilosa and M. cryptica on Eucalyptus spp. in south-eastern Australia. Transactions of British Mycological Society 91(2): 261-266.
Park RF, Keane PJ. 1982. Three Mycosphaerella species from leaf diseases of Eucalyptus. Transactions of the British Mycological Society 79(1): 95-100.
