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© 2008 Plant Management Network.
Accepted for publication 19 September 2008. Published 19 December 2008.


Long-term Survival and Seed Transmission of Acidovorax avenae subsp. citrulli in Melon and Watermelon Seed


C. C. Block, USDA-ARS North Central Plant Introduction Station, Ames, IA 50011; and L. M. Shepherd, Seed Science Center, Iowa State University, Ames, IA 50011


Corresponding author: C. C. Block.  charles.block@ars.usda.gov


Block, C. C., and Shepherd, L. M. 2008. Long-term survival and seed transmission of Acidovorax avenae subsp. citrulli in melon and watermelon seed. Online. Plant Health Progress doi:10.1094/PHP-2008-1219-01-BR.


Seed transmission of Acidovorax avenae subsp. citrulli is a key factor in the dissemination of bacterial fruit blotch of cucurbits. In this study, we report on the survival and seed transmission of A. avenae subsp. citrulli from watermelon (Citrullus lanatus) and melon (Cucumis melo) seed lots that were more than 30 years old. The longest previously-reported survival of A. avenae subsp. citrulli was from 19-year-old watermelon seed (4) and the longest reported survival for any seedborne bacterial plant pathogen was 24 years for Curtobacterium flaccumfaciens pv. flaccumfaciens from common bean seed (2).

Seedling grow-outs were conducted on 500 melon seed lots and 125 watermelon seed lots from collections in Ames, IA, and Griffin, GA. All seed lots were produced in the United States between 20 and 40 years ago and were conserved in cold storage at 5°C or below. A few lots were known to be infected by A. avenae subsp. citrulli, but little information was available for the majority. The grow-out assay was based on one recognized as the USDA National Seed Health System standard (www.seedhealth.org). Fifty to 70 seeds from each lot were planted into 20 × 30 × 6-cm bleach-sanitized plastic trays filled with soilless potting mix (SunGro Horticulture, Bellevue, WA). Each tray was enclosed in a clear plastic bag which raised the humidity to near 100% and prevented cross-contamination between trays. Greenhouse temperatures were maintained at 24 to 28°C. Germinated seedlings were observed through the plastic and the bags were only opened if symptoms of water-soaking or necrosis were observed on the cotyledons (Fig. 1). Bags were opened in isolation and latex gloves were used to collect samples for preliminary testing by ELISA immunostrips (Agdia Inc., Elkhart, IN). Isolations were made from immunostrip-positive plants by dilution plating onto King’s B agar, and suspect colonies were selected for further testing. Pathogenicity was tested by toothpick inoculations of known susceptible varieties ‘Edisto’ melon and ‘Crimson Sweet’ watermelon. Isolates were further confirmed as A. avenae subsp. citrulli by conventional PCR (3).


 

Fig. 1. Watermelon seedling showing water-soaked necrotic spots from seed transmission of Acidovorax avenae ssp. citrulli.

 

Seed transmission was detected from 10 of the 500 melon seed lots, seven of which were 35 to 40 years old. All positive seed lots were grown a second time for confirmation. Seed transmission was detected from 10 of the 125 watermelon seed lots, the oldest being 34 years old. Due to limited seed supplies, a second grow-out was done with two watermelon lots and both were confirmed as positive. Generally, one to two infected seedlings were observed per lot.

Attempts to isolate the pathogen by dilution plating from seed washes of a 37-year-old and a 38-year-old melon seed lot were unsuccessful, perhaps due to low bacterial numbers, bacterial dormancy, or competition from saprophytes. Problems with isolation from old seed lots have been noted previously (1). However, A. avenae subsp. citrulli was detectable in the seed washes by immunostrips and, when seeds were planted into a sterile potting mix, positive plants (Fig. 2) were identified in each of the two seed lots. This demonstrated that viable, pathogenic bacteria were indeed present and multiplied on the host tissue during germination.


 

Fig. 2. Cucumis melo plants from 38-year-old seed showing seed transmission of Acidovorax avenae ssp. citrulli.

 

The fact that A. avenae subsp. citrulli can survive over 35 years on dried seed suggests that the pathogen is highly resistant to aging and may survive as long as the seed is viable. Refrigerator or freezer storage improves seed longevity and probably improves chances for pathogen survival as well. While the pathogen may not survive as long under different storage conditions, this study demonstrates that it does have the capacity to survive for decades on seed.


Literature Cited

1. Bashan, Y., Okon, Y., and Henis, Y. 1982. Long term survival of Pseudomonas syringae pv. tomato and Xanthomonas campestris pv. vesicatoria in tomato and pepper seeds. Phytopathology 72:1143-1144.

2. Burkholder, W. H. 1945. The longevity of the pathogen causing the wilt of the common bean. Phytopathology 55:743-744.

3. Walcott, R. R., Gitaitis, R. D., and Castro, A. C. 2003. Role of blossoms in watermelon seed infestation by Acidovorax avenae subsp. citrulli. Phytopathology 93:528-534.

4. Sowell, G., Jr., and Schaad, N. W. 1979. Pseudomonas pseudoalcaligenes subsp. citrulli on watermelon: Seed transmission and resistance of plant introductions. Plant Dis. Reptr. 63:437-441.