A New Leaf Blotch Disease of Sudangrass Caused by Pantoea ananas and Pantoea stewartii

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Abstract
An unreported disease of sudangrass (Sorghum sudanense) was observed in commercial fields in Imperial Valley of California. Symptoms included light-colored necrotic streaks, and white or tan irregular blotches, often associated with reddish purple to dark brown margins. Pantoea ananas was consistently isolated from the blotches with reddish margins, while Pantoea stewartii or mixtures of both species were isolated from necrotic streaks without reddish margins. Fourteen seed samples harvested in different locations were assayed and found to be 0.0 to 3.6% infested with P. ananas. Seed transmission may be a means by which the pathogen is introduced. Symptoms in inoculated plants appeared as early as 2 and as late as 20 days after inoculation, depending on the inoculum level, methods of inoculation, temperature, and available moisture. The initial symptoms caused by inoculations with both bacteria were similar, but as symptoms progressed, P. ananas was associated with white streaks or irregular necrotic blotches often surrounded by a reddish or purplish hue. P. stewartii was associated with light-colored necrotic streaks. A synergistic or antagonistic relationship was not observed between the two pathogens in co-inoculations. In host range studies, both bacteria caused disease on sorghum and sudangrass at similar levels of severity. P. ananas was also pathogenic on corn and oat. P. stewartii from sudangrass was pathogenic on corn but did not cause wilting that was observed with Stewart's wilt strains of P. stewartii from corn. The sudangrass strains of P. stewartii also infected oat and triticale, while the Stewart's wilt strains did not. Both P. ananas and P. stewartii from sudangrass grew at relatively high temperatures (43 and 37°C, respectively) and caused disease at elevated temperatures and conditions of relative humidity similar to those in the Imperial Valley during late summer when epidemics of the disease were common.