A multi-gene phylogeny for species of Mycosphaerella occurring on Eucalyptus leaves

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MYCOSPHAERELLA LEAF DISEASE (MLD)

DISEASE EPIDEMIOLOGY

An understanding of the epidemiology of MLD provides valuable information for the management of this disease. Epidemiological knowledge, such as the infection process and disease development of Mycosphaerella spp. occurring on Eucalyptus spp., is based largely on studies of Mycosphaerella cryptica (Cooke) Hansf. and Mycosphaerella nubilosa (Cooke) Hansf., two of the most important Eucalyptus leaf pathogens (Park & Keane 1982a, b, Park 1988a, b). The infection process and disease development of these Mycosphaerella spp. on Eucalyptus can be considered in terms of three phases namely, spore deposition and infection, fungal growth and formation of reproductive structures and finally spore liberation.

Spore deposition and infection

Both the ascospores and conidia of Mycosphaerella spp. can be involved in disease development on Eucalyptus leaves. However, ascospores act as the primary source of inoculum in many species (Beresford 1978, Park & Keane 1987). The primary inoculum source is from attached infected leaves or from fallen overwintered leaf litter, which is the general trend for many ascomycete leaf-inhabiting fungi (Luley & McNabb 1989, Patton & Spear 1983, Park & Keane 1987). Ascogonia or conidiomata of Mycosphaerella spp. have been shown to remain viable for a period of several months, providing sufficient inoculum for successive infection cycles (Cheah & Hartill 1987, Park & Keane 1987, Park 1988b). In certain Mycosphaerella spp. such as Mycosphaerella citri Whiteside, the cause of citrus greasy spot, it is known that ascospore production can occur throughout the entire year ensuring a continual inoculum source (Mondal & Timmer 2002).
Infection of Eucalyptus leaves by Mycosphaerella spp. predominantly occurs during the vegetative period of the host during the summer and autumn months (Ganapathi 1979, Cheah & Hartill 1987). Park (1988a), showed that the young expanding leaves, less than 46 days old, of E. globulus, were particularly susceptible to M. nubilosa, while Ganapathi (1979), showed that leaves of Eucalyptus delegatensis were most susceptible to infection during the first 21 days after they unfold. However, as the leaves of Eucalyptus spp. age they become more resistant to infection as a result of the deposition of resistant compounds such as lignin (Park 1988a).
Infection of the leaf surface by the spores may either be direct or indirect. Germ tubes of M. cryptica ascospores are able to penetrate both directly, through the cuticle, or indirectly through stomata (Park & Keane 1982b, Park 1988a). During direct penetration, a protoappresorium is formed alongside the ascospore or at the end of the germ tube (Ganapathi 1979, Park & Keane 1982b). The protoappresorium forms an infection peg that penetrates the cuticle allowing the spore to form invasive hyphae that grow between the cuticle and epidermal cells. Branching hyphae are subsequently formed that grow intercellularly throughout the epidermal layer (Park 1988a). In the case of indirect penetration, infection occurs through the stomata where the germ tubes produce hyphal swellings within the stomatal pores and substomatal cavities (Park & Keane 1982b, Niyo et al. 1986). Conidia of Mycosphaerella lateralis Crous & M.J. Wingf. have been shown to germinate on both adaxial and abaxial leaf surfaces, but only penetrate the leaf through stomata on the abaxial leaf surface and do not produce hyphal swellings or appresoria (Jackson et al. 2004). Crous et al. (1989c), showed from growth room inoculations how conidia of Phaeophleospora epicoccoides (Cooke & Massee) Crous, F.A. Ferreira & B. Sutton (as Phaeoseptoria eucalypti Hansf.) were able to penetrate leaves of Eucalyptus spp., from the subgenus Symphyomyrtus, through leaf stomata. Due to the larger number of stomata on abaxial leaf surfaces there is an increase in infection and subsequent pseudothecial development on abaxial leaf surfaces (Niyo et al. 1986).
Levels of moisture in the environment affect the ability of the fungal spores to infect host material. Higher levels of infection, and consequently disease development, have been noted for spores of Mycosphaerella populorum G.E. Thomps., on Populus, after periods of rainfall and leaf wetting (Luley & McNabb 1989). Conidial germination of Mycosphaerella fijiensis var. difformis J.L. Mulder & R.H. Stover decreased as levels of relative humidity decreased and maximum germ tube development was observed in the presence of free water (Jacome et al. 1991). Temperature also affects the ability of spores to infect the leaf surface. It has been shown that ascospores and conidia of M. fijiensis var. difformis germinate at temperatures ranging from 20−35 °C with maximum germination occurring at 25 °C (Jacome et al. 1991).
Once ascospores or conidia are deposited onto the leaf surface they germinate and form germ tubes. Germination of ascospores and conidia generally occurs in surface moisture, although it has been shown that spores of some Mycosphaerella spp. occurring on Eucalyptus, such as M. nubilosa and M. cryptica, can survive periods of desiccation on the leaf surface and still remain viable and infective (Beresford 1978, Park 1988a).

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Fungal growth and formation of reproductive structures

Upon entry into the leaf, fungal hyphae grow along the vascular bundles and colonize the leaf tissue, becoming established throughout the leaf. Following chlorosis, hyphae grow intercellularly throughout the spongy and palisade mesophyll and eventually aggregate in the substomatal cavities as has been shown in M. cryptica and M. nubilosa occurring on Eucalyptus (Park & Keane 1982b). These hyphal aggregates then develop into immature ascomata with trichogynes (Park & Keane 1982b).
Ganapathi (1979) described the development of pseudothecial ascomata of M. cryptica (as M. nubilosa) in detail. His studies showed that the ascocarp initials comprise a group of cells. This developing ascoma has the appearance of a stroma with the presence of developing trichogynes, which grow toward the stromatal apex. During ascogonial development, the stroma matures and breaks through the host surface. The developing trichogynes grow through the top of the stroma and are fertilised by spermatia of the genus Asteromella Pass. & Thüm. Spermatia are formed in a gelatinous matrix that seaps from the ostiole onto the leaf surface (Ganapathi & Corbin 1979). After fertilisation, ascogonia mature and through successive developmental steps form asci and ascospores. Mature ascogonia of Mycosphaerella spp. generally have large thick, elongated cells impregnated with melanin that form the outer layers of the ascocarp wall (Niyo et al. 1986). Although cells making up the inner ascocarp walls generally contain lower melanin levels than those of the outer ascogonial walls, similar cellular organelles are observed in both cell types (Niyo et al. 1986).

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Spore liberation

Liberation of ascospores is dependant on moisture. For example, in M. cryptica and M. nubilosa, ascospores are discharged when the relative humidity is greater than 95 % and are not discharged when the relative humidity is below 90 % (Beresford 1978, Park & Keane 1982b, Cheah & Hartill 1987). Cheah & Hartill (1987) found that ascospores of M. cryptica are discharged after rainfall and that discharge continues for up to two hours after rainfall has ceased. They also suggest that longer periods of light rainfall will lead to more ascospores being discharged because this allows for ascospore maturation in the asci. Discharge of ascospores in this case will continue in the presence of sufficient moisture and relative humidity until the asci within the pseudothecia are exhausted of ascospores (Cheah & Hartill 1987). Mondal et al. (2003), observed that ascospore release of M. citri under field conditions occurred 1 to 2 hours after rainfall had begun, with peak ascospore release occurring 6 to 8 hours after the onset of rainfall. Furthermore, ascospore release continued for up to 16 hours after rainfall. Dew may also serve as a stimulus for ascospore release, but fewer ascospores are found to be released under these conditions as has been shown in M. populorum on mixed hybrids of Populus and suggested for M. citri on Citrus (Luley & McNabb 1989, Mondal et al. 2003).
Park & Keane (1982b), found that the optimum temperature for ascospore discharge in M. nubilosa and M. cryptica was 25 °C and 20 °C, respectively, and that they may be ejected up to a distance of 12−15 mm above the pseudothecia. This would allow the spores to be wind dispersed. Ascospores are likely to be dispersed by wind for considerable distances as has been suggested for ascospores of M. citri that may be wind dispersed for a distance of up to 80 meters (Mondal et al. 2003). However, conidia of Mycosphaerella spp. would not be dispersed for long distances, for example, conidia of M. cryptica are usually produced in a gelatinous matrix on the leaf surface and as such would are splash-dispersed over short distances within the same tree (Beresford 1978, Cheah & Hartill 1987).

Acknowledgements
Preface
Chapter One: The taxonomy, phylogeny and population biology of Mycosphaerella spp. occurring on Eucalyptus: A literature review
1.0 Introduction
2.0 Mycosphaerella Leaf Disease (MLD)
3.0 Symptomatology
4.0 Teleomorph concepts
5.0 Anamorph associations of Mycosphaerella spp. occurring on Eucalyptus leaves
6.0 Identification techniques
7.0 Phylogeny
8.0 Species complexes
9.0 Population biology of Mycosphaerella species
10.0 Spread and control of Mycosphaerella species
11.0 Conclusion
12.0 References
Chapter Two: A multi-gene phylogeny for species of Mycosphaerella occurring on Eucalyptus leaves
Abstract
Introduction
Materials and Methods
Results
Discussion
References
Chapter Three: Pseudocercospora flavomarginata sp. nov., from Eucalyptus leaves  in Thailand
Abstract
Introduction
Materials and Methods
Results
Discussion
References
Chapter Four: Development of polymorphic microsatellite markers for the Eucalyptus leaf pathogen Mycosphaerella nubilosa
Abstract
Introduction
References
Chapter Five: Global movement and population biology of the Eucalyptus leaf pathogen Mycosphaerella nubilosa
Abstract
Introduction
Materials and Methods
Results
Discussion
References
Chapter Six: Intra-specific variation in Mycosphaerella nubilosa sensu lato
Abstract
Introduction
Materials and methods
Results
Discussion
References
Summary
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