A toothpick inoculation method was applied for evaluating the disease severity and infection frequency of Pythium root rot, which seriously restricts forage corn production in Japan. Seven corn hybrids were inoculated with toothpicks infested with Pythium arrhenomanes, the causal pathogen of the disease. Between 2011 and 2013, toothpicks were inserted into holes of the bottom stalks of corn plants just above the ground at the silking stage. At the inoculation plots throughout the experimental years, the infection frequencies of highly susceptible hybrids were stable even under suboptimal climate conditions. Therefore, we have concluded that the toothpick method is useful especially for discarding highly susceptible hybrids based on the data obtained from a single year. Further experiments will be required to overcome such problems as the low correlation between hybrids and their parental inbreds, and the development of more accurate screening systems for selecting resistant genotypes.