Mycosphaerella leaf spot diseases of bananas - CBS

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Mycosphaerella leaf spot diseases of bananas: present status and outlook Other biotic factors e.g. genetic host resistance also affect parts of the infection cycle (see: management of genetic resistance). Host-pathogen interactions In the past, host resistance was evaluated in the field under natural, and hence variable, pathogen pressure, and in environmental conditions that differed considerably between locations. Environmental factors can have major effects on the expression of resistance, especially partial resistance. Variability was reduced by using tissue-cultured plants inoculated with different isolates of Mycosphaerella. The plants could be maintained in controlled environmental chambers but the system was criticised for such things as using host plants that were too young, which could give rise to resistant phenotypes that were not true-to-type and results that were not reproducible. A miniaturized technique was thus developed. Using leaf fragments kept under artificial survival conditions, made it possible to work with older leaves, to work under closely controlled environmental conditions, and to allow the different banana genotypes to express the type and level of resistance to M. fijiensis typical of those expressed in the field. Crucially, all stages of development in a monocyclic infection were represented in the miniaturised system. The method can be used to analyse the nature of host-parasite interactions and to study the variability in virulence and aggressiveness in different populations of the pathogens. Research over the last 10 years has made use of various models: banana plants in natural conditions, young plants or leaves under artificial survival conditions, providing complementary data about the nature of compatible and incompatible interactions. Host-pathogen interactions occur in nature as the following phenotypes: very or highly resistant banana cultivars and partially resistant bananas with resistance varying from very marked partial resistance to very susceptible. All banana germplasm can be classified by these types of behaviour (Fouré et al., 2000). Only a few wild, and cultivated diploid and triploid acuminata are resistant to BLSD. High resistance in the triploid acuminata is present only in cultivars of the Ibota subgroup. Phenotypes with different levels of partial resistance appear to be widely distributed among all diploid and triploid acuminata and balbisiana genotypes. Genome B seems to give higher levels of partial resistance. The idea that the relationships between Mycosphaerella and banana were based on compatible and incompatible interactions was proposed at the San José workshop. Compatible interactions refer to susceptible, or partially resistant bananas that display different degrees of partial resistance. M. fijiensis can complete its entire infection cycle under this type of interaction. It was also suggested that partial resistance might result from a constitutive polyphenolic compound. The various cytological, ultrastructural and biochemical studies, and genetic analyses of different varieties of banana with different levels of partial resistance, clearly show that proanthocyanidins play a role in partial resistance. It is also clear that the compounds are not essential for the expression of partial resistance. Proanthocyanidin is probably involved in the rate of lesion elongation, but other factors may be involved at other stages of the infection monocycle and should be identified (Beveraggi et al., 1995; Mourichon et al., 2000). 14
Introduction to workshop X. Mourichon Using a model under controlled conditions, it is fairly easy to dissect partial resistance and evaluate the importance of certain sequences of monocyclic infection, e.g. incubation period, rate of lesion development, effectiveness of infection, latent periods and different parameters of sexual and asexual sporulation. Recent studies suggest that the presence of several components of resistance act on different stages of the infection cycle. Depending on the banana cultivar, partial resistance may be the result of different mechanisms. Thus, similar expressions of partial resistance could depend on different genetic interactions, e.g. efficacy of infection or level of sexual reproduction. These possibilities should be taken into account by breeders, for whom partial resistance is a major objective. Partial resistance in banana is important because it is considered to be more durable. However, the great diversity of pathogen populations and their capacity to evolve should be taken into account. Specific interactions between the pathogen and the host plant must be established for certain sequences of the infectious monocycle (Abadie et al., 2001a, b). It is possible that some specific interactions may select for more aggressive pathotypes of Mycosphaerella. The result would be gradual erosion, rather than a sudden decrease, of partial resistance. Incompatible interactions. Banana cultivars that are very resistant rapidly block progress of the fungus in the early stages of disease. At the San José workshop, it was suggested that such behaviour could be governed by an active defence mechanism. Studies of host-parasite interactions using cytology, particularly at the ultrastructural scale, provided accurate images of the interactions that occur after inoculation. There is clear evidence of active mechanisms such as cell collapse occurring after penetration of stomata (Beveraggi et al., 1995; Mourichon et al., 2000). Similarly, hypersensitive reactions have been elicited, and necrosis induced experimentally by fungal compounds of high molecular weight. Other research reported at the San José workshop demonstrated that phytotoxic compounds or toxins were released by M. musicola and M. fijiensis. This raised the question of the role of these compounds in the infection process. Breeders were interested in the use of toxic compounds in schemes for the early selection of bananas resistant to M. fijiensis, in particular. During the last decade, a large number of phytotoxic compounds produced by M. fijiensis have been described in the literature, e.g. juglone which displays a high level of biological activity (Stierle et al., 1991). Several research projects have shown that such compounds are not primary determinants of the disease but are secondary determinants of pathogenicity (Harelimana et al., 1997). The role of these compounds as agents in the selection of resistance, as originally considered, deserves discussion, in particular for studies on partial resistance. Breeding for resistance to Mycosphaerella leaf spot diseases At the San José meeting, it was stated that “little is known of the genetics and inheritance of resistance to Sigatoka diseases”. Several strategies and programmes for genetic improvement were presented but these had been developed mainly for the control of Fusarium wilt. 15
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List of participants Catherine Abad
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