Doctoral Degrees (Plant Pathology)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/563

Browse

Browsing Doctoral Degrees (Plant Pathology) by browse.metadata.advisor "Mahuku, George"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Evaluation of East African bananas for resistance to Fusarium Oxysporum f. Sp. cubense race 1
    (Stellenbosch : Stellenbosch University, 2020-03) Ndayihanzamaso, Privat; Viljoen, Altus; Mostert, Diane; Mahuku, George; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Banana is a staple food and source of income for millions of smallholder farmers in East and Central Africa (ECA). Consumption per capita in countries such as Burundi, Rwanda and Uganda ranges from 120 kg to more than 400 kg per year, which is six to 20 times the global average consumption per capita. Bananas cultivated in ECA consist of cooking varieties, such as East African Highland bananas (EAHB), Bluggoe, and juice/sweet dessert varieties such as Pisang Awak, Sukari Ndizi, Gros Michel and Cavendish bananas. EAHB include diploid bananas such Mchare, Muraru, Mlali and Paka (mostly cooking types), whereas EAHB triploids include Matooke (a cooking type) and Mbidde (a juice/beer type). Fusarium wilt of banana, caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc), is present in most banana-growing regions of ECA. Foc comprises three races based on their pathogenicity to a group of differential cultivars, with Foc race 1, race 2 and race 4 causing disease to Gros Michel, Bluggoe and Cavendish bananas, respectively. All three races are present in Africa, but only Foc races 1 and 2 occur in ECA. Foc races 1 and 2 strains in ECA group consist of six vegetative compatibility groups, which cluster together as Foc Lineage VI. In this study, molecular markers specific to Foc Lineage VI were developed from the DNA-directed RNA polymerase III subunit (RPC2) gene region. The primer set was combined in a multiplex PCR assay with the primer set FocLin6bF/R, which was developed from the translation elongation factor-1 alpha (TEF-1α) gene. The multiplex PCR assay was validated on a worldwide population of 623 known Foc isolates, other formae speciales and non-pathogenic isolates of Fusarium oxysporum. The multiplex PCR can be used as an accurate diagnostic tool for Foc Lineage VI strains. Effective management of banana Fusarium wilt can be achieved by planting banana varieties resistant to Foc. Resistant bananas, however, require many years of breeding and field-testing under multiple geographical conditions. Field evaluation is reliable but time consuming and expensive, and not feasible for quarantine strains. Small plant screening methods are, therefore, needed to speed up the evaluation of banana varieties for Foc resistance. To this end, a small plant screening method for resistance to banana Fusarium wilt was optimized by investigating the effect of inoculum concentration, inoculation method and plant age on disease development, and the value of phenolic compounds and Foc DNA as indicators of disease resistance. The method, which consisted of planting 2- to 3-month-old banana plants in soil amended with 2-10 g Foc-colonised millet seeds per kg of potting soil, was reliable, and qPCR and rhizome discoloration were suitable for evaluating and ranking the disease response of banana varieties. Phenolic compounds were, however, not consistent in differentiating cultivars’ resistance when the same genotypes were inoculated with Foc race 1 and subtropical race 4 (STR4), and cannot be considered a reliable indicator of resistance. The optimized millet seed technique is useful in mass screening of newly developed genotypes for resistance to Foc, and can be used in the screening for Fusarium wilt resistance against quarantine variants of Foc in quarantine facilities. EAHB triploid banana cultivars are resistant to Foc race 1 and 2 in ECA, but dessert varieties in the region are susceptible. Resistance of diploid Mchare, Muraru and Mlali bananas, as well as newly developed diploid and triploid EAHB hybrids, is largely unknown. Therefore, in this study eight Mchare cultivars and 19 NARITA hybrids were evaluated for resistance to Foc race 1 in the field and screen house in Tanzania and in Uganda. Eight Muraru cultivars, 23 Mchare hybrids and 60 Matooke hybrids were also screened in pot trials in a screen house. Mchare and Muraru cultivars were all susceptible to Foc race 1, whereas the response of Mchare, NARITA and Matooke hybrids ranged from susceptible to resistant. Triploid hybrids were not expected to be susceptible as their parents were resistant to Foc race 1. This suggest that resistance in banana is multi-gene controlled and heterozygous, and that the genes segregated during meiosis into gamete cells leading to a loss of resistance. This study generated valuable information towards the management of Fusarium wilt in the ECA region. Molecular markers that were developed are reliable and affordable to research centres and extension services in the region, and can speed up the diagnosis of Foc Lineage VI strains. The screening method developed in this study will improve the reliability of small plant testing, and will reduce time and cost associated with field evaluation of new varieties.