Exserohilum turcicum is the causal agent of northern corn leaf blight, a destructive foliar disease of maize that results in yield losses worldwide. In South Africa, typical yield losses range from 15 to 30%. Previous studies found high haplotypic diversity with evidence for sexual recombination in E. turcicum populations from tropical climates such as Kenya. However, the population genetic structure and method of reproduction of E. turcicum in South Africa is unknown and, therefore, was investigated. Twelve polymorphic microsatellite markers were screened on 258 E. turcicum isolates from maize collected during 2012 and 2013 from three maize fields in South Africa. A multiplex polymerase chain reaction assay amplifying both mating type idiomorphs was applied to investigate the distribution of mating types. No distinct genetic clusters were observed. Shared haplotypes were identified between isolates separated by distances of up to 762 km, which provided evidence of migration. High haplotypic diversity indicated that sexual reproduction is occurring among E. turcicum isolates, although mating type ratios and linkage disequilibrium analyses did not support the hypothesis of random mating. The population genetic structure of E. turcicum in South Africa is likely due to the direct movement and spread of isolates undergoing a mixed reproductive lifecycle.

Nine maize inbred lines with excellent grey leaf spot resistance and good combining ability were evaluated for their reaction to infection by Exserohilum turcicum. The lines were compared to a differential set consisting of Oh43 and B73 (susceptible controls) and Oh43Ht1B, Oh43Ht2, Oh43Ht3, B37HtN (containing resistant genes) in both growth chamber and field trials. All nine lines possessed seedling resistance to northern corn leaf blight (NCLB). Two field trials were planted over two growing seasons (2007/08 and 2008/09) as randomized complete block designs with three replicates. Trials were inoculated twice (at the 4–5 and 8–12 leaf stages) with NCLB and disease assessments made at growth stages R1, R2, R3, R4 and R5. Disease progress curves were created and total severity (y_t ), diseased plant severity (y_dp ) and AUDPC were determined for each replicate. Linearised forms of the exponential, logistic and Gompertz models were fitted to the disease-progress data and the best model selected for each trial. Rate of disease increase (r) and level of disease at the onset of the epidemic (y₀ ; time=0) were obtained from which y₀ ^* (back-transformed from y₀ ) were determined. Canonical vari-ate analysis (CVA) indicated that y_t, y_dp , sAUDPC and y₀ ^* were responsible for 73.68% of the variation observed between the lines, with r explaining 15.44% of the variation. Ranking of lines were based on their y_t, y_dp , sAU-DPC and y₀ ^* performance. GLS resistant lines 185–2, 182–2, 72–2A and 182–1 demonstrated higher levels of resistance to NCLB than that of lines Oh43Ht1B, Oh43Ht2, Oh43Ht3 and B37HtN while 122–2, 578 and 72–3 showed higher levels of resistance than that of Oh43Ht2 and B37HtN.

Northern corn leaf blight (NCLB) caused by Exserohilum turcicum and Fusarium ear rot caused by Fusarium verticillioides, are economically important maize diseases in South Africa. The effect of induced plant stress by NCLB on F. verticillioides ear rot and fumonisin production is unknown. Four field trials were conducted during 2016/2017 and 2017/2018 (November and December planting dates) at the Agricultural Research Council – Grain Crops in Potchefstroom (South Africa). Three maize cultivars with varying resistance levels to NCLB were selected (IMP50-10B – susceptible, BG3292 – moderately susceptible, DKC 61-94BR – resistant). NCLB severities were created through eight treatments: TMT1 – maximum control (three fungicide applications); TMT2 – standard control (two fungicide applications) and TMT3 – natural control (not inoculated or sprayed). The remaining treatments were inoculated with a cocktail of five NCLB races (Race 3, 3N, 23, 23N and 13N): TMT4 (five weeks after planting / WAP); TMT5 (five and six WAP); TMT6 (five, six and seven WAP); TMT7 (six and seven WAP); and TMT8 (seven WAP). Maize ears were naturally infected with F. verticillioides. Fifteen random plants were labelled at dent stage and NCLB severity (%), area under the disease progress curve, ear rot diseased area, ear rot severity (%), ear rot incidence (%) and total fumonisins (FB1+FB2+FB3; ug/kg) were established. Low levels of cob rot severity and fumonisins were obtained in all four trials. NCLB severity did not affect ear rot related parameters measured. Mean fumonisin levels were below the South African tolerance levels. Fumonisin concentrations differed significantly between cultivars but was not affected by NCLB severity or the cultivar x treatment interaction. Significance: This is the first study to investigate the effect of NCLB severity as a predisposing factor of ear rot incidence and severity of maize. The study confirmed that ear rot incidence and severity are not impacted by secondary stressors induced by NCLB, and that the cultivation of NCLB-resistant varieties would not bring about lower ear rot incidences.

Although crop rotation reduces the incidence of soil-borne diseases, reduced tillage practices create the risk of increased disease incidence. Monoculture maize (MM) under both conventional tillage (CT) and conservation agriculture (CA) crop systems were evaluated against two-and three-year rotation CA systems of maize with cowpea and sunflower, respectively. Pearl millet was the third crop in the three-year rotation systems. Plant mass, root and crown rot severity and frequencies of 15 soil-borne fungi on maize roots and crowns were investigated at three sampling dates, i.e. 21, 70 and 100 days after planting (DAP), together with yield. The trial was initiated during 2008/09 on a sandy loam soil in the Highveld climatic area of South Africa and the current study focused on the 2011/12, 2012/13 and 2013/14 seasons. Lower yields and plant mass at 100 DAP were obtained for the MM-CT treatment compared with MM-CA and rotated maize during the drought-stricken 2012/13 season. Yields obtained for this season correlated with crown rot severity at 100 DAP. Stenocarpella maydis and Macrophomina phaseolina correlated negatively with crown rot and plant mass at 100 DAP as well as yield (2012/13). Both fungi were most prominent in the MM-CT system.