Floral parts of infected plants are replaced with a mass of black powdery spores during head development. Infected tillers usual head before healthy tillers. Spore mass is disseminated by wind and/or rain, spreading spores to normal heads of surrounding plants. Germinating spores infect developing kernels but permit normal grain development. Fungus remains dormant in seed until planted and germination occurs. Infection progressively moves up the tiller during normal stem development. Infection of the developing head completes the cycle.