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Title: Long-term survival and seed transmission of Acidovorax avenae ssp. citrulli in melon and watermelon seed

Author
item Block, Charles
item SHEPHERD, LISA - IOWA STATE UNIVERSITY

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2008
Publication Date: 12/19/2008
Citation: Block, C.C., Shepherd, L.M. 2008. Long-term survival and seed transmission of Acidovorax avenae ssp. citrulli in melon and watermelon seed. Plant Health Progress. Available: http://www.plantmanagementnetwork.org/sub/php/brief/2008/melon/.

Interpretive Summary: Seed transmission of bacterial fruit blotch of cucurbits is a key factor in the spread of the disease. In this study, we report that the causal agent, Acidovorax avenae subspecies citrulli (Aac), was able to survive and cause disease after lying dormant on infected seed for up to 40 years. The Aac bacterium survived and was seed-transmitted from 34-year-old watermelon seed (Citrullus lanatus) and from 40-year-old melon seed (Cucumis melo). Many plant pathogens die out in storage. The fact that this bacterium survived indicates that it is highly tolerant of drying and aging. Seed transmission was demonstrated by planting seeds from each seed lot in a plastic tray, covered with a clear plastic bag supported by a wire frame. The plastic bags helped raise the humidity to near 100% and prevented cross-contamination between trays. Plants that showed disease symptoms were tested with ELISA immunostrips and bacteria were isolated into pure culture. The cultures were used to inoculate 'Edisto' melon and 'Crimson Sweet' watermelon seedlings to confirm that they were pathogenic. The isolates were further confirmed as Aac-positive by PCR, using DNA from a known Aac isolate as a positive control. The longest previously-reported survival for a seedborne bacterial pathogen was 24 years, for the bacterial wilt pathogen of common bean, Curtobacterium flaccumfaciens pv. flaccumfaciens. These findings suggest that Acidovorax avenae ssp. citrulli is likely to survive as long as the seed is viable and that there is a risk of seed transmission from even the oldest seed lots.

Technical Abstract: Seed transmission of Acidovorax avenae ssp. citrulli (Aac) is a key factor in the dissemination of bacterial fruit blotch of cucurbits. In this study, we report seed transmission of Aac from 34-year-old watermelon seed (Citrullus lanatus) and from 40-year-old melon seed (Cucumis melo). The seed lots used for this work were held in refrigerated storage at 4C to 5C for 31 and 33 years, respectively, before being moved to freezer storage at -18C. Each seed lot was planted in a plastic tray and covered with a clear plastic bag supported by a wire frame. The plastic bags helped raise the humidity to near 100% and prevented cross-contamination between trays. Germinated seedlings were observed through the plastic and bags were only opened if symptoms of water-soaking and necrosis were observed. Symptomatic plant tissue samples were tested by ELISA immunostrips and bacterial isolations were made from positive plants by dilution plating onto King's B agar medium. Individual bacterial colonies were streaked onto fresh agar media to obtain pure cultures. Pathogenicity testing was done by toothpick inoculations of both 'Edisto' melon and 'Crimson Sweet' watermelon seedlings. Isolates were further confirmed as Aac-positive by PCR, using DNA from Aac isolate WFB9421 as a positive control. The longest reported survival for any seedborne bacterial pathogen was 24 years, for Curtobacterium flaccumfaciens pv. flaccumfaciens from common bean seed. The fact that Acidovorax avenae ssp. citrulli can survive for at least 40 years on seeds indicates that the Aac bacterium is highly tolerant of desiccation and aging. These findings suggest that Aac is likely to survive as long as the seed is viable.