Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Author "Kimunye, Janet Njeri"

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    Distribution and genetic diversity of Pseudocercospora SPP. associated with banana Sigatoka diseases in East Africa
    (Stellenbosch : Stellenbosch University, 2020-12) Kimunye, Janet Njeri; Viljoen, Altus; Mahuku, George; Stellenbosch University. Faculty of Agrisciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Sigatoka leaf diseases are a major constraint to banana production worldwide. They are caused by phylogenetically related pathogens belonging to the genus Pseudocercospora. Pseudocercospora fijiensis, the cause of black Sigatoka, is the most widespread and damaging species, causing yield losses of 20-50%. Pseudocercospora fijiensis is heterothallic, and produces infective conidia and ascospores that are dispersed by wind and rain splash. In commercial farms, black Sigatoka is managed by spraying fungicides weekly. This method is not suitable for smallholder farmers who represent most banana producers in Africa. Banana varieties with resistance to black Sigatoka is the most feasible control method for resource poor farmers. An understanding of pathogen distribution, genetic diversity and population dynamics is a prequisite for developing effective and sustainable disease management strategies. A survey was conducted in five banana-growing regions in Tanzania and Uganda to identify the Pseudocercospora spp. associated with Sigatoka leaf spots and determine disease severity. Sigatoka-like symptoms were present in all localities and on all cultivars. Species-specific primers revealed that P. fijiensis was the predominant species in all areas except Kilimanjaro, where Mycosphaerella musae was associated with Sigatoka- like leaf spots. Black Sigatoka was more severe in Uganda, with a mean disease severity index (DSI) of 37.5%, than in Tanzania (DSI=19.9%). Pseudocercospora fijiensis was detected at altitudes of up to 1877 m above sea level, which suggests a habitat range expansion from the previous threshold of <1350 m above sea level in East Africa. This expansion threatens sustainability of banana production in the region. Genetic diversity, population structure and mating type idiomorph distribution was assessed on 319 P. fijiensis single-spore isolates from seven regions, using 16 simple sequence repeat markers and mating type (MAT)-specific primers. The populations were characterised by a high genotypic diversity (296 multi-locus genotypes) and low clonality (7%), with MAT 1 and 2 occurring at a 1:1 ratio in Uganda, while MAT 1 was over- represented at a ratio of 4:1 in Tanzania. The index of association revealed that all populations were at linkage equilibrium (P>0.05), supporting the hypothesis of a random association of alleles. Sub-populations had a moderate level of genetic diversity (Hexp = 0.12-0.31; mean 0.29). These findings are consistent with a pathogen that reproduces both clonally and sexually. Isolates collected at the different locations did not show geographical differentiation, with 90% of the variation occurring among isolates within a subpopulation. This finding suggests a common origin for the isolates and supports the hypothesis of frequent recombination of genotypes. Multi-location evaluation of 21 newly developed East African Highland banana hybrids (NARITA) for resistance to P. fijiensis was conducted in five sites in Uganda and Tanzania. Significant differences in disease severity was observed between the hybrids, test locations, and their interaction (GEI). The environment had the greatest influence (39.1%) on genotypes’ response to P. fijiensis, with GEI accounting for 23.4%. Most of the hybrids exhibited broad adaptability in their response to black Sigatoka. Hybrids with low disease development and a stable response across locations were NARITA hybrids 2, 7, 8, 21 and 23. These can be provided to farmers for managing black Sigatoka in the region. NARITA hybrids 10 and 18 were identified as susceptible, and could be used as susceptible checks in future evaluations. The Mitarula site in Tanzania was identified as a representative test location to evaluate banana hybrids for their response to the black Sigatoka pathogen. To identify potential sources of P. fijiensis resistance, a collection of 95 banana accessions, including selected breeding parents, were evaluated in the field at Sendusu in Uganda. Out of these, 33% of the accessions; belonging to 22 subspecies of Musa acuminata ssp. malaccensis, M. acuminata ssp. zebrina and M. acuminata ssp. Burmannica; were either resistant or partially resistant to P. fijiensis. Symptom progression in these accessions stopped at early lesion development (Stages 2, 3, and 4). Symptom development in Long Tavoy, Pahang, Pisang KRA, 0074 Malaccencis, M.A Truncata, Tani and Balbisiana stopped at Stage 2, like that in the resistant Musa acuminata ssp. burmannicoides, var. Calcutta 4, and these varieties can thus be considered as potential sources of resistance.