Susceptibility of Maize Inbreds and Incidence of Symptomless Infection by the Head Smut Pathogen, Sphacelotheca reiliana
A real-time PCR assay was used to compare seedling infection by Sphacelotheca reiliana, the causal agent of head smut, among five inbred genotypes representing low, moderate, and high susceptibility to the disease. Seeds were coated with teliospores and planted in autoclaved field soil in a growth chamber. Incidence of seedling infection at growth stage V3 differed between an inbred genotype of low susceptibility and those of moderate and high susceptibility, but did not differ between the high and moderately susceptible groups (P < 0.05). The real-time PCR assay was also used to compare infection status at early and late vegetative stages with observable symptoms in the field. We detected infection via real-time PCR in maize at both growth stages during field trials conducted in Texas and California but observed no disease symptoms (smutted ears or tassels). Notably, the fungus was present in up to 31% of the ear shoots in plots without disease symptoms. The real-time assay can be a useful tool for screening seedling-stage host resistance, and for better understanding the progress of infection in different maize genotypes. The field data suggest that asymptomatic infection is much more common than previously thought, and may have important implications for the epidemiology of this fungus under diverse plant resistance and growing conditions. Accepted for publication 11 December 2015. Published 5 January 2016.