Bacterial fruit blotch (BFB), caused by Acidovorax avenae subsp. citrulli, is a serious disease of cucurbit plants. The first important occurrence of BFB in Israel was during 2000 to 2003 on watermelon and melon. Twelve bacterial isolates associated with these outbreaks were confirmed as A. avenae subsp. citrulli by pathogenicity assays, gas chromatography of fatty-acid methyl esters, and substrate-utilization profiles. The isolates were characterized in terms of their aggressiveness in different hosts by seed, seedling, and fruit inoculations, and according to their DNA fingerprinting profiles using pulse-field gel electrophoresis (PFGE) and repetitive-PCR approaches. Results from the present work agree with previous studies supporting the existence of two differentiated groups within A. avenae subsp. citrulli, one including strains that are more associated with watermelon (group II), the other consisting of strains that are usually associated with nonwatermelon cucurbits (group I). This study indicates that isolates from both groups have been introduced to Israel. PFGE analysis revealed that the 12 analyzed isolates can be divided into five different haplotypes, of which four were previously unreported. Additional differentiating features between group I and II strains are presented.

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Bacterial fruit blotch (BFB) of cucurbits, caused by Acidovorax avenae subsp. citrulli, is a serious threat to the watermelon and melon industries. To date, there are no commercial cultivars of cucurbit crops resistant to the disease. Here we assessed the level of tolerance to bacterial fruit blotch of various commercial cultivars as well as breeding and wild lines of melon, using seed-transmission assays and seedling-inoculation experiments. Selected cultivars were also tested in a greenhouse experiment with mature plants. All tested cultivars/lines were found to be susceptible to the pathogen, and most of them showed different responses (relative tolerance vs. susceptibility) in the different assays; however, some consistent trends were found: cv. ADIR339 was relatively tolerant in all tested assays, and cv. 6407 and wild lines BLB-B and EAD-B were relatively tolerant in seed-transmission assays. We also provide evidence supporting a strong correlation between the level of susceptibility of a cultivar/line and the ability of the pathogen to adhere to or penetrate the seed. To the best of our knowledge, this is the first attempt to assess melon cultivars/lines for bacterial fruit blotch response.

Several seed extraction procedures, used for detection of Clavibacter michiganensis ssp. michiganensis ( Cmm ) in naturally infected and artificially infested tomato seed lots were evaluated. Extraction methods that included grinding the seeds were significantly better at detecting the pathogen in three different seed lots than methods that used only soaking. The detection threshold of Cmm in relation to seed sample size was determined by adding naturally infected seeds into samples of three different sizes. Cmm was detected by agar plating assay, on three media (CNS, mSCM, D 2 ANX), and by direct PCR from seeds and Bio-PCR (bacteria cultured on agar media prior to PCR). In samples of 10 000 seeds containing one infected seed, Cmm could be detected only by Bio-PCR and in only one replicate out of five. In samples containing five or 10 infected seeds per 10 000 seeds, three of five and five of five replicates, respectively, were detected by the three detection methods. In samples of 5000 seeds, one infected seed could be detected in all five replicates only after adding a concentration step. A high correlation ( R 2 = 0·9448) between artificially infested seeds and the disease incidence was found. Seed lots infested with less than 58 colony-forming units (CFU) per g did not cause disease under glasshouse conditions, whereas lots with about 1000 CFU g − 1 caused disease in 78 plants out of 2000.

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