Screening for resistance to Fusarium stalk rot under natural infection has been difficult because of irregularities in disease development. The toothpick inoculation technique was introduced to overcome this problem, but it lacked a mechanism to monitor inoculum dose. In this study, we introduce an improved procedure where inoculum doses are monitored and disease incubation periods are optimized. Five inoculum doses of Fusarium verticillioides (0, 1 × 103, 1 × 104, 1 × 105 and 1 × 106 conidia ml-1) and five disease incubation periods (14, 21, 28, 35 and 42 days) were studied using sorghum genotypes of variable stalk rot reaction. Plants were inoculated on day 14 after flowering. Disease reaction was scored as length (cm) of necrotic lesion and number of nodes crossed. The effects of both inoculum dose and incubation period were significant. Inoculum doses of 1 × 104 conidia ml-1 and greater and incubation periods of 21 to 42 days clearly resolved differences among genotypes. Noting the burden of producing a highly concentrated inoculum and the lack of need to keep plants after physiological maturity, incubation for 28 days following inoculation with 1 × 104 to 1 × 105 conidia ml-1 is recommended for germplasm screening.
Key words: Stalk rot, Fusarium, Sorghum bicolor, inoculum dose (ID), incubation period (IP).
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