Interactive effect of busseola fusca and fusarium verticillioides on ear rot and fumonisin production in maize

[摘要] ENGLISH ABSTRACT: Maize is a crop of great economic importance in southern Africa, and is widely consumed asa staple food and animal feed. Production of maize, however, is hampered by pathogens andpests such as Fusarium verticillioides and the African stem borer Busseola fusca, respectively.Fusarium verticillioides infection results in Fusarium ear rot (FER) and contamination of maizekernels with fumonisin mycotoxins, while B. fusca, causes significant damage to maize tissuesduring larval feeding. Despite attempts to control F. verticillioides, fungal infection andfumonisin production remains a threat to maize production due to a lack of resistant maizecultivars 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 stemborers of maize. However, the recent discovery of B. fusca resistance to Bt maize with a singlecrystal protein MON810 gene, indicates that care should be taken not to solely rely on thistechnology for the management of B. fusca.The interactive effect of B. fusca and F. verticillioides on FER and fumonisin productionin 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 isolateMRC826 was inoculated by injecting a spore suspension of the fungus into the silk channel ofeach primary ear at the blister stage. For B. fusca infestation, aliquots of 10-15 neonate larvaewere deposited into the whorl of each plant at the 12-13th leaf stage before tasselling using amechanical applicator. Maize ears were also mechanically wounded at the blister stage witha cork borer (different sizes and number of wounds) to mimic hail damage, and half of thewounds 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 maizeears. 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 andfumonisin production.The effect of host plant genetic modification and pesticide application on FER andfumonisin production in maize was investigated by studying the response of a Bt hybrid andits non-Bt isohybrid to F. verticillioides infection and B. fusca infestation; and by treating plantswith Beta-cyfluthrin (non-systemic) and Benfuracarb (systemic) insecticides. The field trialswere conducted over three seasons using a randomised complete block design with sixreplicates per treatment. Uninoculated, uninfested and undamaged control treatments wereincluded. All ears were harvested at physiological maturity and FER, total fumonisinconcentration, stem borer cumulative tunnel length (B. fusca damage) and target DNA offumonisin-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 equallycontaminated with fungal DNA, but FER and fumonisin production were reduced in the Bthybrid under natural farming conditions. Despite the evidence found in this study and othersthat Bt maize indirectly reduces FER and fumonisin production, this was also inconsistent overseasons. Benfuracarb controlled stem borers, and therewith indirectly reduced FER andfumonisin production. FER development and fumonisin production by F. verticillioides variedover seasons, indicating the importance of environmental conditions on FER and fumonisinproduction.A survey was also conducted at two sites in the North West province and one site in theFree 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 greenhouseand field trials. Maize whorls were inoculated with a spore suspension of F. verticillioidesMRC826 4 weeks after plant emergence and infested with aliquots of 5-10 neonate B. fuscalarvae 2 days later. The control treatment consisted of B. fusca infestation only. Several fungalspecies were associated with stem borer frass, including Acremonium zeae, Aspergillusflavus, A. niger, F. chlamydosporum, F. incarnatum-equiseti species complex, F. oxysporum,F. subglutinans, F. verticillioides, Mucor circinelloides, Rhizopus oryzae and Talaromycesflavus. The occurrence of A. niger in the frass suggests that further studies need to beconducted to determine the effect of A. niger infection on fumonisin production in maize inSouth Africa. DNA quantity of fumonisin-producing Fusarium spp. was significantly more infrass 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 fieldtrial showed no significant differences in quantity of target DNA in frass from inoculated anduninoculated plants infested with B. fusca larvae. This indicates that plants in the field werenaturally infected with F. verticillioides.This study showed that Bt maize had no effect on infection of maize ears by fumonisinproducingFusarium spp. and the subsequent production of fumonisin in F. verticillioidesinoculatedmaize ears, indicating that the effect of Bt maize on fumonisin production in maizeears is indirectly associated with its control of severe stem borer damage. Busseola fuscafrass was a reservoir of different fungal species; some pathogenic to maize, and othersantagonistic to maize pathogens. Moreover, B. fusca infestation of maize stems wasassociated 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 resultedin significantly more FER symptoms and fumonisin production, irrespective of artificial F.verticillioides inoculation of maize ears whereas B. fusca infestation resulted in a significantincrease in FER in only one of the three seasons, moreover, it had no effect on fumonisinproduction 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 birddamage, and damage by implements; are important entry points for F. verticillioides that maylead to the transition from symptomless infection to necrotrophic pathogenicity resulting inFER and concomitant fumonisin production in maize kernels. However, climatic conditions arealso important in FER and fumonisin production in maize. Moreover, Acremonium zeaeendophytes occurring in frass can be used for the biological control of F. verticillioides resultingin the management of FER and subsequent fumonisin production.

[发布机构] Stellenbosch University