Factors Influencing Infection of Soybean Seedlings by SouthernDiaporthe phaseolorum

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Abstract
The influences of tissue type, genotype, temperature, and inoculum density on infection of soybean seedlings by ascospores and .alpha.-conidia (spores) of the fungus causing soybean stem canker in the southeastern United States (southern Diaporthe phaseolorum) were studied under high moisture conditions (free water maintained on plant surfaces for 48 hr). Leaf laminae were the least frequently infected tissues of those assayed for infection. Significantly higher levels of infection were observed for petioles, petiole bases, and stem tissue (P < 0.01). No relationship was found between the field susceptibility or resistance of 12 genotypes tested and the frequency with which a genotype was infected. Events responsible for resistance in soybean to southern stem canker apparently occur after infection has taken place. Maximum levels of infection occurred at 28 and 34 C; lower levels of infection occurred at 10, 16, and 22 C. Infection did not occur at 40 C. Frequency of infection and inoculum density (log10) were positively correlated (P < 0.001; r2 = 0.97) within a range of 1 .times. 103 to 1 .times. 106 spores per milliliter.