on cucurbits. Acidovorax avenae subsp. citrulli (Aac) is a phytopathogenic bacterium responsible for the cucurbit disease bacterial fruit blotch. This seed-borne pathogen can infect plants at any growth stage but is typically seen at the seedling
W. Patrick Wechter, Amnon Levi, Kai-Shu Ling, Chandrasekar Kousik, and Charles C. Block
D.L. Hopkins, C.M. Thompson, and G.W. Elmstrom
Seedlings of 22 watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] cultivars and two plant introductions were screened in the greenhouse for resistance to the fruit blotch bacterium. There were significant differences in disease severity among cultivars, but no cultivar was immune to the bacterium. In field tests, fruit of 18 commercial cultivars were inoculated individually or became infected naturally from diseased foliage. Cultivars with relatively resistant fruit included `Sugar Baby', `Jubilation', `Mirage', `Calsweet', `Crimson Sweet', `Royal Sweet', and `Sangria'. The more susceptible cultivars generally had a light-colored rind. Cultivar level of resistance to bacterial fruit blotch may not be sufficient under conditions conducive to severe disease development.
D.L. Hopkins and C.M. Thompson
Prevention of the introduction of bacterial fruit blotch of watermelon, caused by Acidovorax avenae subsp. citrulli, into the transplant house or field is the most effective control strategy. Watermelon seedlots currently are screened for A. avenae subsp. citrulli, but other cucurbits, often grown in the same transplant house or field, generally are not as carefully monitored. In 1997 and 1999 field tests, cultivars of watermelon, muskmelon, honeydew melons, acorn squash, butternut squash, yellow squash, zucchini squash, cucumber, and pumpkin were evaluated for foliar and fruit susceptibility to bacterial fruit blotch and for seed transmission of A. avenae subsp. citrulli. The bacterium was introduced into the field on infected watermelon transplants or by misting a bacterial suspension onto fruit of the cucurbits. Foliar and fruit symptoms were more extensive in the watermelon, muskmelon, and honeydew melons than in the other cucurbits. In greenhouse grow-out assays, seed transmission of A. avenae subsp. citrulli was detected in every cucurbit in at least one of the two seasons, even though there were no fruit symptoms in some of them. Thus, any cucurbit crop plant should be considered a potential source for the introduction of A. avenae subsp. citrulli into the transplant house or field.
Branko R. Lovic and Donald L. Hopkins
Selecting production areas for low disease pressure, implementation of preventive spray programs, and continuous monitoring for disease symptoms are important steps to keep seed production fields free of potentially seedborne diseases, such as bacterial fruit blotch of cucurbits (Cucurbitaceae), caused by Acidovorax avenae ssp. citrulli. However, seeds of cucurbit crops and other fleshy vegetables typically remain remarkably free of pathogenic bacteria and fungi while in intact fruit. The most significant risk for seed contamination comes at harvest when the inoculum present in the field or in the seed harvesting area may contaminate the seeds. Properly executed fermentation and seed drying processes significantly reduce seed contamination. Application of a no-rinse disinfectant formulation to freshly harvested seed just before drying may be the single most efficacious procedure to reduce the seed contamination risk. However, the disinfection step should not be expected to be effective unless applied as part of a fully controlled seed harvest process.
Ron R. Walcott
Plant pathogens present a serious threat to seedling establishment and the potential for plant disease epidemics under greenhouse conditions is great. Hence, pathogen exclusion by detection and elimination of infested seedlots remains a requisite tactic for seedling production and disease management. Unfortunately, the numbers of contaminated seed within a lot may be low and infested seed may be asymptomatic making their detection difficult. To address these issues seed detection assays have been developed, but many of them have shortcomings that reduce their effectiveness. Examples of frequently used seed assays include visual examination, selective media, seedling grow-out and serological assays which, while appropriate for some pathogens, often display inadequate levels of sensitivity and specificity. Recently, the polymerase chain reaction (PCR) has emerged as a tool for detecting microorganisms in many diverse environments. Thus far, it is clear that DNA-based detection systems exhibit higher levels sensitivity than conventional techniques. Unfortunately, PCR-based seed tests require the extraction of PCR-quality DNA from target organisms in backgrounds of saprophytic organisms and inhibitory seed-derived compounds. The inability to efficiently extract PCR-quality DNA from seeds has restricted the acceptance and application of PCR for seed detection. To overcome these limitations several modified PCR protocols have been developed including selective target colony enrichment followed by PCR (BIO-PCR) and immunomagnetic separation and PCR. These techniques seek to selectively concentrate or increase target organism populations to enhance detection and have been successfully applied for detecting bacteria in seed. Other techniques with great potential for rapid detection of seedborne pathogens include magnetic capture hybridization and PCR, and DNA-chip technology. Ultimately, PCR will be available for the detection of all seedborne pathogens and may supersede conventional detection methods.
Donald N. Maynard and Donald L. Hopkins
Watermelon (Citrullus lanatus [Thunb.] Matsum & Nakai) fruit are affected by a number of preharvest disorders that may limit their marketability and thereby restrict economic returns to growers. Pathogenic diseases discussed include bacterial rind necrosis (Erwinia sp.), bacterial fruit blotch [Acidovorax avenae subsp. citrulli (Schaad et al.) Willems et al.], anthracnose [Colletotrichum orbiculare (Berk & Mont.) Arx. syn. C. legenarium (Pass.) Ellis & Halst], gummy stem blight/black rot [Didymella bryoniae (Auersw.) Rehm], and phytophthora fruit rot (Phytophthora capsici Leonian). One insect-mediated disorder, rindworm damage is discussed. Physiological disorders considered are blossom-end rot, bottleneck, and sunburn. Additionally, cross stitch, greasy spot, and target cluster, disorders of unknown origin are discussed. Each defect is shown in color for easy identification.
Karen R. Harris, W. Patrick Wechter, and Amnon Levi
bacterial diseases include fusarium wilt caused by the fungus Fusarium oxysporum f. sp. niveum ( Zhou and Everts, 2004 ) and bacterial fruit blotch caused by Acidovorax avenae ssp. citrulli ( Hopkins et al., 2003 ). In addition, watermelon cultivars
Chandrasekar S. Kousik, Scott Adkins, William W. Turechek, Craig G. Webster, and Pamela D. Roberts
moderately susceptible to gummy stem blight ( Didymella bryoniae ) and was rated 4.5 on a 0 to 9 scale ( Gusmini et al., 2005 ). It was found susceptible to bacterial fruit blotch ( Acidovorax avenae subsp. citrulli ) and was rated 8 on a 1 to 9 scale in
Howard F. Harrison Jr., W. Patrick Wechter, and Chandrasekar S. Kousik
–response curves procedure described by Seefeldt et al. (1995) were used to estimate the 70% methanol extract concentration required to cause a 50% reduction in radicle length (GR 50 ) for each genotype. Acidovorax avenae bioassay. The bacterial fruit blotch
George E. Boyhan, Suzanne O’Connell, Ryan McNeill, and Suzanne Stone
it was considered more susceptible to bacterial fruit blotch ( Acidovorax avenae ssp. citrulli ). This apparent greater susceptibility may have more to do with the light-green rind color, which made the infection more apparent than on watermelons