Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Author "Ncube, Edson"

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    Interactive effect of busseola fusca and fusarium verticillioides on ear rot and fumonisin production in maize
    (Stellenbosch : Stellenbosch University, 2017-03) Ncube, Edson; Viljoen, Altus; Flett, B. C.; Van den Berg, J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Maize is a crop of great economic importance in southern Africa, and is widely consumed as a staple food and animal feed. Production of maize, however, is hampered by pathogens and pests such as Fusarium verticillioides and the African stem borer Busseola fusca, respectively. Fusarium verticillioides infection results in Fusarium ear rot (FER) and contamination of maize kernels with fumonisin mycotoxins, while B. fusca, causes significant damage to maize tissues during larval feeding. Despite attempts to control F. verticillioides, fungal infection and fumonisin production remains a threat to maize production due to a lack of resistant maize cultivars and the inability to target the pathogen with fungicides and biocontrol products. Planting Bt maize hybrids have become an important mechanism for the management of stem borers of maize. However, the recent discovery of B. fusca resistance to Bt maize with a single crystal protein MON810 gene, indicates that care should be taken not to solely rely on this technology for the management of B. fusca. The interactive effect of B. fusca and F. verticillioides on FER and fumonisin production in maize was investigated in this study. Maize ears were inoculated with F. verticillioides alone, with both F. verticillioides and B. fusca, and with B. fusca alone. Fusarium verticillioides isolate MRC826 was inoculated by injecting a spore suspension of the fungus into the silk channel of each primary ear at the blister stage. For B. fusca infestation, aliquots of 10-15 neonate larvae were deposited into the whorl of each plant at the 12-13th leaf stage before tasselling using a mechanical applicator. Maize ears were also mechanically wounded at the blister stage with a cork borer (different sizes and number of wounds) to mimic hail damage, and half of the wounds infected with F. verticillioides. Results from this study indicated that the impact of B. fusca infestation on FER varied seasonally, possibly due to its sporadic damage to maize ears. Busseola fusca, however, did not result in a significant increase in fumonisin production. The severity of wounding of maize ears was an important contributor to FER development and fumonisin production. The effect of host plant genetic modification and pesticide application on FER and fumonisin production in maize was investigated by studying the response of a Bt hybrid and its non-Bt isohybrid to F. verticillioides infection and B. fusca infestation; and by treating plants with Beta-cyfluthrin (non-systemic) and Benfuracarb (systemic) insecticides. The field trials were conducted over three seasons using a randomised complete block design with six replicates per treatment. Uninoculated, uninfested and undamaged control treatments were included. All ears were harvested at physiological maturity and FER, total fumonisin concentration, stem borer cumulative tunnel length (B. fusca damage) and target DNA of fumonisin-producing Fusarium spp. were quantified. Busseola fusca infestation had no effect on fungal colonisation and fumonisin production in maize. Bt and non-Bt kernels were equally contaminated with fungal DNA, but FER and fumonisin production were reduced in the Bt hybrid under natural farming conditions. Despite the evidence found in this study and others that Bt maize indirectly reduces FER and fumonisin production, this was also inconsistent over seasons. Benfuracarb controlled stem borers, and therewith indirectly reduced FER and fumonisin production. FER development and fumonisin production by F. verticillioides varied over seasons, indicating the importance of environmental conditions on FER and fumonisin production. A survey was also conducted at two sites in the North West province and one site in the Free State province of South Africa to analyse mycoflora in B. fusca frass. The exposure of B. fusca larvae to F. verticillioides in stem borer frass was also evaluated in both greenhouse and field trials. Maize whorls were inoculated with a spore suspension of F. verticillioides MRC826 4 weeks after plant emergence and infested with aliquots of 5-10 neonate B. fusca larvae 2 days later. The control treatment consisted of B. fusca infestation only. Several fungal species were associated with stem borer frass, including Acremonium zeae, Aspergillus flavus, A. niger, F. chlamydosporum, F. incarnatum-equiseti species complex, F. oxysporum, F. subglutinans, F. verticillioides, Mucor circinelloides, Rhizopus oryzae and Talaromyces flavus. The occurrence of A. niger in the frass suggests that further studies need to be conducted to determine the effect of A. niger infection on fumonisin production in maize in South Africa. DNA quantity of fumonisin-producing Fusarium spp. was significantly more in frass collected from greenhouse plants inoculated with F. verticillioides and infested with B. fusca larvae than in frass collected from the uninoculated and infested control, whilst the field trial showed no significant differences in quantity of target DNA in frass from inoculated and uninoculated plants infested with B. fusca larvae. This indicates that plants in the field were naturally infected with F. verticillioides. This study showed that Bt maize had no effect on infection of maize ears by fumonisinproducing Fusarium spp. and the subsequent production of fumonisin in F. verticillioidesinoculated maize ears, indicating that the effect of Bt maize on fumonisin production in maize ears is indirectly associated with its control of severe stem borer damage. Busseola fusca frass was a reservoir of different fungal species; some pathogenic to maize, and others antagonistic to maize pathogens. Moreover, B. fusca infestation of maize stems was associated with higher levels of fumonisin-producing Fusarium spp. in larval frass when F. verticillioides was present on the plant. Multiple large wounds created by cork borers resulted in significantly more FER symptoms and fumonisin production, irrespective of artificial F. verticillioides inoculation of maize ears whereas B. fusca infestation resulted in a significant increase in FER in only one of the three seasons, moreover, it had no effect on fumonisin production in all three seasons. This indicates that severe wounds that opens up husk coverage and exposes maize kernels; caused by factors such as insects, hail and bird damage, and damage by implements; are important entry points for F. verticillioides that may lead to the transition from symptomless infection to necrotrophic pathogenicity resulting in FER and concomitant fumonisin production in maize kernels. However, climatic conditions are also important in FER and fumonisin production in maize. Moreover, Acremonium zeae endophytes occurring in frass can be used for the biological control of F. verticillioides resulting in the management of FER and subsequent fumonisin production.