Acidovorax citrulli: generating basic and applied knowledge to tackle a global threat to the cucurbit industry

Acidovorax citrulli is the causal agent of bacterial fruit blotch (BFB) of cucurbit plants. In recent years, the disease has spread to many parts of the world, mainly via the inadvertent distribution of contaminated commercial seeds. Because of the costly lawsuits filed by growers against seed companies and the lack of efficient management methods, BFB represents a serious threat to the cucurbit industry, and primarily to watermelons and melons. Despite the economic importance of the disease, little is known about the basic aspects of A. citrulli pathogenesis. Nevertheless, the release of the genome of one A. citrulli strain, as well as the optimization of molecular manipulation and inoculation methods, has prompted basic studies and allowed advances towards an understanding of A. citrulli pathogenicity. In this article, we summarize the current knowledge about this important pathogen, with emphasis on its epidemiology and the factors involved in its pathogenicity and virulence.

Bacteria; Betaproteobacteria; order Burkholderiales; family C omamonadaceae; genus Acidovorax; species citrulli.

Gram-negative, strictly aerobic, rod-shaped; average dimensions of 0.5 μm × 1.7 μm; motile by means of an ~5.0-μm-long polar flagellum; colonies on King's medium B are round, smooth, transparent and nonpigmented; optimal temperatures for growth around 27-30 °C; induces a hypersensitive response on nonhost tobacco and tomato leaves.

Acidovorax citrulli strains are pathogenic to various species of the Cucurbitaceae family, including watermelon, melon, squash, pumpkin and cucumber. Significant economic losses have been reported in watermelon and melon.

Watermelon and melon seedlings and fruits are highly susceptible to A. citrulli. Typical seedling symptoms include water-soaked lesions on cotyledons that are often adjacent to the veins and later become necrotic, lesions on the hypocotyl, and seedling collapse and death. On watermelon fruits, symptoms begin as small, irregular, water-soaked lesions which later extend through the rind, turn brown and crack. On melon fruits, symptoms are characterized by small, often sunken rind lesions and internal fruit decay. Symptoms on the leaves of mature plants are difficult to diagnose because they are often inconspicuous or similar to those caused by other biotic or abiotic stresses. When they occur, leaf lesions can spread along the midrib and main veins. Lesions appear dark-brown to black on watermelon and light to reddish-brown on melon.

Bacterial fruit blotch of cucurbits at APSnet, http://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/BacterialBlotch.aspx; bacterial fruit blotch guide from ASTA, http://www.amseed.com/pdfs/DiseaseGuide-BFB-English.pdf; Acidovorax citrulli AAC00-1 genome at JGI, http://genome.jgi-psf.org/aciav/aciav.info.html.