Occurrence of Bacterial Stripe of Pearl Millet in Georgia

Bacterial stripe, caused by Acidovorax avenae subsp. avenae, was observed on breeding lines of pearl millet (Pennisetum glaucum (L.) R. Br.) in Georgia in 1999 and 2001. A gram-negative, oxidase-positive, rod-shaped bacterium that produced circular, cream-colored, nonfluorescent, butyrous colonies with entire margins on King's medium B was consistently isolated from leaf lesions. The bacterium was identified as A. avenae subsp. avenae by gas-chromatography of extracted, whole-cell, fatty acid methyl esters using the Sherlock Microbial Identification System (MIDI, Newark, DE) and by substrate utilization patterns using the Biolog Identification System (Biolog Inc., Hayward, CA). Isolates from pearl millet produced amplicons of expected size (360 bp) from 16S rDNA after conducting polymerase chain reaction (PCR) with primers WFB1 and WFB2, which are specific for A. avenae. When bacterial suspensions of 1 × 10⁸ CFU/ml were infiltrated into the intercellular spaces of leaves of pearl millet seedlings in the greenhouse, typical water-soaked, reddish-brown stripes developed and were identical to those observed in the field. In contrast to previous reports (1), the pearl millet strains produced atypical symptoms on sweet corn (cvs. Merit and Primetime). Necroses were restricted, lacked customary water-soaking, and were similar to symptoms produced by the watermelon pathogen, A. avenae subsp. citrulli, which was used as a negative control. In contrast, three strains of A. avenae subsp. avenae previously isolated from corn in Georgia produced typical water-soaked stripes in both millet and the sweet corn 'Merit'. However, like the millet strains, A. avenae subsp. avenae strains from corn produced atypical symptoms on the sweet corn 'Primetime'. Using immunomagnetic separation and PCR (2), A. avenae subsp. avenae was detected in remaining samples of pearl millet seed planted in Georgia in 2001, as well as in remnant samples of seed sent to Puerto Rico for increase in 2000. The A. avenae subsp. avenae strain recovered from seed was identified by the methods listed above, and in the greenhouse it was identified by the production of typical water-soaked stripes after inoculation of pearl millet. This is the first report of A. avenae subsp. avenae infecting pearl millet in the United States. The detection and distribution of seedborne inoculum in breeding lines is significant since the program at Tifton represents a major effort by the U.S. Department of Agriculture to develop higher-yielding, disease-resistant pearl millet hybrids. Furthermore, the strains from pearl millet appear to be different from previous A. avenae subsp. avenae strains isolated from corn in Georgia, because they did not produce typical disease symptoms when infiltrated in corn leaves. References: (1) L. E. Claflin et al. Plant Dis. 73:1010, 1989. (2) R. R. Walcott and R. D. Gitaitis. Plant Dis. 84:470, 2000.