1 Engineering resistance to pathogenic fungi L. Sági, Katholieke Universiteit Leuven (KULeuven), Leuven, Belgium Introduction Fungi are far more complex organisms than viruses or bacteria and have developed numerous strategies to survive in nature, which include saprotrophy, necrotrophy, hemibiotrophy and biotrophy. Interactions between plant pathogenic fungi and their hosts are particularly complex and involve different mechanisms, such as the production of fungal toxins and enzymes that degrade the plant cell wall and the formation of occlusions in vascular tissue (Rodriguez and Redman, 1997). It is difficult, therefore, to design a simple defense strategy, such as the use of a single gene or a few genes from the genome of the pathogen, as in the case of viruses. The development of efficient antifungal strategies requires a detailed genetic, cytological and biochemical characterization of the particular host-fungal pathogen interaction. Higher plants and other organisms including fungi use a wide range of defense mechanisms to protect themselves against fungal invasion, some of which might be utilized in gene transfer technologies. Fungal diseases of bananas All major organs and tissues of bananas, including the foliage, the root system, the vascular tissues and the fruits are affected by some 50 fungal pathogens. Of these, the most important ones are those causing leaf spot diseases (black leaf streak disease caused by Mycosphaerella fijiensis and Sigatoka disease caused by Mycosphaerella musicola), Fusarium wilt, also known as Panama disease, (caused by Fusarium oxysporum f. sp. cubense) and fruit anthracnose (Colletotrichum musae) (Table 1). Mycosphaerella pathogens have consecutive asexual (conidial) and sexual (ascospores) cycles, whereas only the asexual state is known in Fusarium. Spores of Mycosphaerella can spread relatively far by wind and raindrop splashes (Parnell et al., 1998) whereas Fusarium commonly disseminates via infected rhizomes. The spores of Mycosphaerella infect via appressoria over the stomata and produce a mass of hyphae in the intercellular space. Symptoms appear after an incubation time of 2-4 weeks. These features indicate that the pathogen (at least initially) has the characteristics of a biotroph. Black leaf streak disease has been present in East Africa since the end of the 1980s and was first reported in Uganda in 1990 (Tushemereirwe and Waller, 1993). Many East African highland banana cultivars are susceptible and the disease commonly occurs in the region, although data on production losses and the economic impact of the disease are not well known. Chlamydospores of Fusarium mainly enter root tips, and occasionally wounded surfaces. After germination, microconidia colonize the xylem vessels using the transpiration flux and finally block the vascular system, which results in typical H. Atkinson, J. Dale, R. Harding, A. Kiggundu, K. Kunert, J.M. Muchwezi, L. Sagi, A. Viljoen (eds). 2003. Genetic transformation strategies to address the major constraints to banana and plantain production in Africa. INIBAP: Montpellier (FRA), 130 p.