Bacterial Fruit Blotch Of Watermelon Research Articles

Acidovorax citrulli Type IV Pili PilR Interacts with PilS and Regulates the Expression of the pilA Gene

Acidovorax citrulli can cause bacterial fruit blotch of watermelon, melon, and other cucurbits, and has the potential to cause severe economic losses to growers throughout the world. This article investigated the functions and interactions of the pilR and pilS genes, two important genes in bacterial type IV pili systems, in A. citrulli. For each gene, deletion mutants and complementary strains were constructed via homologous recombination, and their phenotypes were determined. The results showed that the absence of pilR and pilS could significantly reduce the pathogenicity and twitching motility of A. citrulli while increasing the swimming motility, biofilm formation, and in vitro growth. Conversely, complementary strains were no different than the wild-type strain. Using quantitative reverse transcription PCR and promoter activity assays, we confirmed that the deletion of pilR and pilS genes leads to a significant decrease in the transcription level of pilA. Meanwhile, three methods including yeast two-hybrid, glutathione S-transferase pull-down, and luciferase complementation imaging assays were used to verify the direct interaction between the PilR and PilS proteins. These findings revealed the biological function of the pilR and pilS and confirms their regulatory role on pilA.

Acidovorax citrulli type III effector AopU interferes with plant immune responses and interacts with a watermelon E3 ubiquitin ligase.

Acidovorax citrulli is a seed-borne bacterium that causes bacterial fruit blotch of watermelon and other cucurbit plants worldwide. It uses a type III secretion system to inject type III effectors (T3Es) into plant cells, which affect the host immune responses and facilitate pathogen colonization. However, the current understanding of the specific molecular mechanisms and targets of these effectors in A. citrulli is limited. In this study, we characterized a novel T3E called AopU in A. citrulli group II strain Aac5, which shares homology with XopU in Xanthomonas oryzae. The Agrobacterium-mediated gene transient expression system was used to study the effect of AopU on host immunity. The results showed that AopU localized on the cell membrane and nucleus of Nicotiana benthamiana, inhibited reactive oxygen species burst induced by flg22 and the expression of marker genes associated with pathogen-associated molecular pattern-triggered immunity, but activated salicylic acid and jasmonic acid signal pathways. Further investigations revealed that AopU interacts with E3 ubiquitin ligase ClE3R in watermelon, both in vitro and in vivo. Interestingly, the deletion of aopU did not affect the virulence of A. citrulli, suggesting that AopU may have functional redundancy with other effectors in terms of its role in virulence. Collectively, these findings provide new insights into the mechanism of plant immune responses regulated by A. citrulli T3Es.

Fermentation: An Unreliable Seed Treatment for Bacterial Fruit Blotch of Watermelon.

Acidovorax citrulli is a seedborne pathogen that causes bacterial fruit blotch (BFB), a global threat to watermelon production. Treating watermelon seeds to eliminate A. citrulli is a critical component of BFB management, and several strategies have been evaluated to mitigate the impact of the disease. In China, watermelon seed producers routinely incubate seeds in watermelon juice (fermentation) to reduce the risk of seed infection by A. citrulli and seedling transmission of BFB. However, there has been limited effort to evaluate the efficacy of fermentation in mitigating A. citrulli seed infection. The current study showed that fermented watermelon fruit juice could inhibit A. citrulli population growth and demonstrated that the low pH conditions, not the temperature dynamic, generated during fermentation might play a major role in A. citrulli growth inhibition and could induce the viable but nonculturable (VBNC) state in A. citrulli. We developed an effective method that was based on propidium monoazide PCR to detect viable A. citrulli cells under low pH conditions or in fermented watermelon fruit juice. We also provided evidence that VBNC A. citrulli cells induced by fermented watermelon fruit juice could not be resuscitated and did not retain their virulence on watermelon seedlings. However, VBNC A. citrulli cells could be resuscitated in Luria-Bertani medium. Based on these observations, we conclude that fermentation in watermelon fruit juice may not be an effective seed treatment for BFB because it may increase the seed infection by A. citrulli.

Oxalic acid produced by Aspergillus niger Y-1 is effective for suppression of bacterial fruit blotch of watermelon seedlings

Bacterial fruit blotch (BFB) of watermelon caused by Acidovorax citrulli (Ac) is a seedborne disease. Seed treatment with bacterial disinfectants is considered as an important measure for suppression of Ac infection. This study was performed to detect the antibacterial activity of the cultural filtrate (CF) of the saprophytic fungus Aspergillus niger Y-1 against Ac. The six-day-old CF, citric acid (CA, 4mmol/L) and oxalic acid (OA, 60mmol/L) were determined for in vitro antibacterial activity against Ac. CF and OA were determined as seed disinfectants for suppression of seedborne infection by Ac. Results showed that production of CA and OA by A. niger in potato dextrose broth was consistently detected by HPLC. The CF, CA and OA inhibited growth of Ac and their inhibitory effect disappeared when the pH values of the three solutions was adjusted to 7.0. OA was more effective than CA in suppression of Ac. The potting experiment showed that both CF and OA applied on watermelon seeds effectively suppressed BFB incidence on seedlings. The efficacy was comparable to the seed treatment with HCl (1%, v/v). This study suggests that the CF of A. niger and OA can be used as seed disinfectants for elimination of seedborne Ac.

Antibacterial properties and major bioactive components of Mentha piperita essential oils against bacterial fruit blotch of watermelon

Plant-derived essential oil is an alternative to antibiotics, eliminating the concern of developing antibiotic-resistant bacterial strains. In this study, using the half-divided Petri plate assays, 32 volatile essential oils were screened for their antibacterial activity against Acidovorax citrulli (Acc). Sweet basil and peppermint oils were the most effective against Acc, with subsequent trials showing that peppermint oil to be the most active. Using gas chromatography–mass spectrometry, the major compositions of peppermint oil were analysed. Among the various compositions of peppermint oil, menthol, neomenthol, isopulegone and 1,8-cineole were significantly active against Acc and each component at 0.2% concentration inhibited all bacterial growth. This study demonstrated in vitro and in vivo antibacterial activities of peppermint oil and its active components against Acc. These results suggest the use of peppermint oil as a potential antibacterial agent to treat seed with Acc.

Effect of chitosan solution on the inhibition of Acidovorax citrulli causing bacterial fruit blotch of watermelon

The production of watermelon in China has been seriously hampered by fruit blotch disease and limited control measures are now applied. Chitosan has been employed to control a variety of plant diseases and is considered to be the most promising biochemical to control this disease. The in vitro antibacterial effect of chitosan and its ability in protection of watermelon seedlings from bacterial fruit blotch were evaluated. Results showed that three types of chitosan, in particular, chitosan A at 0.40 mg mL⁻¹ significantly inhibited the growth of Acidovorax citrulli. The antibacterial activity of chitosan A was affected by chitosan concentration and incubation time. The direct antibacterial activity of chitosan may be attributed to membrane lysis evidenced by transmission electron microscopic observation. The disease index of watermelon seedlings planted in soil and the death rate of seedlings planted in perlite were significantly reduced by chitosan A at 0.40 mg mL⁻¹ compared to the pathogen control. Fresh and dry weight of watermelon seedlings planted in soil was increased by chitosan seed treatment, but not by chitosan leaf spraying. The results indicated that chitosan solution may have a potential in controlling bacterial fruit blotch of watermelon.

Acidovorax citrulli Seed Inoculum Load Affects Seedling Transmission and Spread of Bacterial Fruit Blotch of Watermelon Under Greenhouse Conditions.

Infested seed are typically the primary source of inoculum for bacterial fruit blotch (BFB) of cucurbits. An inoculum threshold of 1 infested seed per 10,000 seeds is widely used in seed health testing for Acidovorax citrulli. However, the influence of seed inoculum load on BFB seedling transmission has not been elucidated. In this study, watermelon seedlots (128 seeds/lot) containing one seed inoculated with A. citrulli at levels ranging from 1 × 101 to 1 × 107 CFU were used to investigate the effect of seed inoculum load on seedling transmission and spatiotemporal spread of BFB under greenhouse conditions. The relationship between A. citrulli seed inoculum load and frequency of BFB seedling transmission followed a sigmoidal pattern (R2 = 0.986, P = 0.0047). In all, 100 and 96.6% of seedlots containing one seed with 1 × 107 and 1 × 105 CFU of A. citrulli, respectively, transmitted the pathogen to seedlings; in contrast, the proportion of seedlots that yielded BFB-infected seedlings was lower for lots with one seed infested with 1 × 103 (46.6%) and 1 × 101 (16.7%) CFU of A. citrulli. The relationship between A. citrulli seed inoculum load and frequency of pathogen detection in seedlots using immunomagnetic separation combined with a real-time polymerase chain reaction assay also followed a sigmoidal pattern (R2 = 0.997, P = 0.0034). Whereas 100% of samples from seedlots (10,000 seeds/lot) with one seed containing ≥1 × 105 CFU tested positive for A. citrulli, 75% of samples from lots with one seed containing 1 × 103 CFU tested positive for the pathogen, and only 16.7% of samples with one seed containing 10 CFU tested positive. Because disease transmission was observed for lots with just one seed containing 10 A. citrulli CFU, zero tolerance for seedborne A. citrulli is recommended for effective BFB management. The seedling transmission experiments also revealed that temporal spread of BFB in 128-cell seedling trays increased linearly with A. citrulli inoculum load (r2 = 0.976, P = 0.0037). Additionally, the frequency of spatial spread of BFB from an inoculated seedling in the center of a planting tray to adjacent healthy seedlings over one-, two-, or three-cell distances was greater for lots with one seed infested with at least 1 × 105 CFU than for lots with one seed infested at lower inoculum loads (1 × 101 and 1 × 103 CFU/seed).

Acidovorax avenae subsp. citrulli newly reported to cause bacterial fruit blotch of watermelon in Greece

<i>Acidovorax avenae</i> subsp. <i>citrulli</i> newly reported to cause bacterial fruit blotch of watermelon in Greece

First Report of Bacterial Fruit Blotch of Watermelon Caused by Acidovorax avenae subsp. citrulli in Hungary.

Typical bacterial fruit blotch (BFB) symptoms were observed on cvs. Crisby, Suzy, Top Gun, and Lady watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai.) fruits in southeastern Hungary (Medgyesegyháza) in July 2007 when the mean maximum daytime temperature was greater than 32°C. Approximately 20 to 30 ha of watermelons were observed to be affected by the disease. Source of the infection was not determined, however, grafted watermelon transplants that were grown in this area had been imported from Turkey where the pathogen is present (2). Disease symptoms started with irregularly shaped, water-soaked lesions on the surface of the fruits. The lesions enlarged and the epidermis became brown and cracked. BFB symptoms were not readily visible on mature foliage. Colonies of the BFB pathogen were creamy white on nutrient agar (Difco, Detroit, MI). Strains were gram negative, oxidase positive, and produced acid from glucose aerobically. A cell suspension (50 μl of ~1 × 107 CFU/ml) from a 24-h nutrient plate culture was infiltrated with a hypodermic syringe into the intercellular spaces of fully developed intact tobacco (Nicotiana tabacum L. cv. White Burley) leaves to determine the hypersensitive reaction (HR) (1). A typical HR developed 20 h after leaf infiltration. Bacterial pathogenicity was tested on surface-sterilized, mature fruits of different plant species by injecting cell suspensions into the fruit tissues as previously described (each fruit was injected in five places; the negative control (sterile water) as well). Fruits were incubated for 7 days at 25°C and then observed for symptom development. Necrosis was observed at each point of inoculation with the pathogen for watermelon and green pepper (Capsicum annuum L.). Necrosis was also observed for cucumber (Cucumis sativus L.), zucchini (Cucurbita pepo L. convar. giromontiina Greb.), squash (C. pepo L.), and patisson (C. pepo L. convar. patissoniana Greb.). Necrosis was not observed when the pathogen was inoculated onto fruit of melon (Cucumis melo L.), tomato (Lycopersicon esculentum Mill.), and eggplant (Solanum melongena L.). Additionally, symptoms were not observed at the points inoculated with sterile water (negative control) for any of the fruits tested. To identify the pathogen, PCR was used with Acidovorax avenae subsp. citrulli-specific primers WFB1/2 (4). The 16s rDNA region amplified with a general bacterial primer pair (63f forward and 1389r reverse) (3) was cloned into a pBSK+ vector (Stratagene, La Jolla, CA) and sequenced by M13 forward and reverse primers (GenBank Accession No. AM850114). On the basis of the symptoms, biochemical tests (API 20NE; Biomérieux, Marcy l'Etoile, France), fatty acid methyl ester analysis (74.5 to 83.6% similarity), and 16SrDNA sequence homology (100% sequence similarity with AAC00-1), the pathogen was identified as A. avenae subsp. citrulli. To our knowledge, this is the first report of BFB of watermelon in Hungary.

Occurrence of Bacterial Fruit Blotch of Watermelon Caused by Acidovorax avenae subsp. citrulli in the Eastern Mediterranean Region of Turkey.

Turkey has the second highest production level of watermelon (Citrullus lanatus L.) in the world, with 3.8 million tons produced in 2005. In the spring of 2005, a severe outbreak of a disease resembling bacterial fruit blotch was observed on watermelon cv. Crispy growing in a production area of 35,000 ha in Adana Province in the eastern Mediterranean Region of Turkey. In 13 commercial watermelon fields surveyed in this study, incidence of symptomatic fruit ranged from 30 to 45%. Characteristic symptoms were dark, gray-green stains or blotches on the surface of fruits. Cracks developed in the rind and released an amber-colored substance. Bacteria were consistently isolated from affected fruits and they formed nonfluorescent colonies on King's medium B. Identification of isolated bacterial strains was based on the methods described by Schaad et al. (2). All strains were oxidase positive, gram negative, arginine dyhydrolase negative, and produced a hypersensitive response on tobacco leaves (Nicotiana tabacum cv. Samsun). Strains were identified as Acidovorax avenae subsp. citrulli (1,2) on the basis of the results of biochemical tests (1,2) and sole carbon substrate utilization (BIOLOG GN; Biolog Inc., Hayward, CA) with 75 to 93% similarity indices. A pathogenicity test was performed for nine identified strains by injecting a bacterial suspension (108 CFU/ml of saline) under the rind of three harvested watermelon (cv. Diyarbakir) fruits and into the stems of three seedlings. Saline was used as the negative control. The fruits and seedlings inoculated with each strain were incubated in polyethylene bags for 48 h at 25°C. Dark green water-soaked lesions developed on inoculated fruits and water-soaking and stem necrosis were observed on the seedlings 3 to 5 days after inoculation. The bacterium was readily reisolated from inoculated fruits and seedlings and identified as A. avenae subsp. citruli on the basis of BIOLOG GN. In 1995, bacterial fruit blotch of watermelon has been observed and eradicated in the Marmara Region of western Turkey (1). Therefore, to our knowledge, this is the first outbreak of bacterial fruit blotch on watermelon in the eastern Mediterranean Region of Turkey.

Wet Seed Treatment with Peroxyacetic Acid for the Control of Bacterial Fruit Blotch and Other Seedborne Diseases of Watermelon.

Prevention of seed transmission of Acidovorax avenae subsp. citrulli into the transplant house or field is the most effective control of bacterial fruit blotch of watermelon currently available. Peroxyacetic acid was evaluated as a disinfectant that might efficaciously eradicate A. avenae subsp. citrulli from contaminated seed and also be efficacious against other seed-transmitted diseases of watermelon. Peroxyacetic acid at low concentrations eliminated A. avenae subsp. citrulli, Fusarium oxysporum, and Didymella bryoniae from microbial suspensions. Treatments of seed contaminated with A. avenae subsp. citrulli and D. bryoniae with peroxyacetic acid at 1,600 μg/ml and higher for 30 min were effective in preventing seed transmission of bacterial fruit blotch and gummy stem blight. Hydrochloric acid treatments at 10,000 μg/ml, while effective in eliminating seed transmission to watermelon seedlings, can adversely affect seed germination, especially with triploid seed. Efficacious dosages of peroxyacetic acid can be applied safely to freshly harvested triploid watermelon seed without concerns for reduction in seed quality. A most effective wet seed treatment protocol involved a 30-min treatment with peroxyacetic acid at 1,600 μg/ml followed by seed drying at low humidity in a 40°C drying oven for 48 h.

凝集反応法によるスイカ果実汚斑細菌病の迅速診断法

The immunological agglutitation method, using either latex particles or Staphylococcus aureus conjugated with anti-Aac-IgG, was tested for diagnosing bacterial fruit blotch of watermelon caused by Acidovorax avenae subsp. citrulli (Aac). In either case, when the suspensions of various plant pathogenic bacteria were tested, only Aac yielded a strong positive reaction. When sap prepared from sections including lesions was tested, both methods produced positive reactions with all sections from artificially inoculated watermelon plants. Moreover, positive reactions were observed with lesions older than 28 days. These methods are very simple, and only about 5min/sample is needed to reach a diagnosis.

Seed Transmission of Acidovorax avenae subsp. citrulli in Cucurbits

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.

First Report of Bacterial Fruit Blotch of Watermelon Caused by Acidovorax avenae subsp. citrulli in Illinois.

In August 2001, symptomatic mature fruit of watermelon (Citrullus lanatus) cvs. Crimson Sweet and Royal Sweet were collected from six fields (48 ha) in Iroquois, Kankakee, and Mason counties in Illinois. The fruit surface had dark green, water-soaked lesions, and the rind's epidermis was cracked. Sections of infected rind were examined with a light microscope, and bacterial streaming was observed. Rind tissue was disinfested by rubbing the fruit surface with a paper towel moistened with 70% ethanol. Pieces of rind from the margins of lesions were placed in a test tube containing sterile-distilled water. The tube was shaken, and the suspension was streaked on King's medium B agar in petri plates. The plates were incubated at 26°C for 3 days. A nonfluorescent, gramnegative, anaerobic, oxidase-positive, rod-shaped bacterium was isolated that produced creamy white colonies on nutrient agar. The bacterium was identified as Acidovorax avenae subsp. citrulli based on an enzyme-linked immunosorbent assay test specific for A. avenae subsp. citrulli (kit PSA 14,800; Agdia Inc., Elkhart, IN). The disease was diagnosed as bacterial fruit blotch (1,2). The occurrence of bacterial fruit blotch of watermelon in Illinois is important because (i) watermelon is widely grown in Illinois, (ii) the pathogen also infects melons and pumpkins, and (iii) Illinois grows approximately 1,000 ha of melons and 8,000 ha of pumpkins.

Evaluation of Citrullus sp. Germ Plasm for Resistance to Acidovorax avenae subsp. citrulli.

In the greenhouse, 1,344 Citrullus spp. and Praecitrulllus fistulosus accessions were screened for resistance to Acidovorax avenae subsp. citrulli. Seedlings were inoculated at the first true leaf stage by misting with a water suspension of a virulent strain of A. avenae subsp. citrulli originally isolated from commercial watermelon in Florida in 1989. Seedlings were considered resistant if less than 20% of the cotyledons were necrotic and there were no lesions on the true leaves 10 days after inoculation. Twelve accessions had individual seedlings that were resistant to A. avenae subsp. citrulli. Selfs of seven of these accessions were susceptible in greenhouse and field tests. Selfs were obtained from five accessions from Zimbabwe and Zambia that possessed a level of greenhouse and field resistance to A. avenae subsp. citrulli that could provide control of bacterial fruit blotch of watermelon. Based on field evaluations, plant introduction (PI) 482279 and PI 494817 were judged to contain plants with the best sources of resistance; however, PI 500303, PI 500331, and PI 482246 also had plants with high levels of resistance.

First Report of Infection of Honeydew with Acidovorax avenae subsp. citrulli.

Honeydew fruits (Cucumis melo var. inodorus Jacq. 'Morning Ice,' 'Honeybrew,' and 'Green Flesh') with circular, 3- to 10-mm-diameter lesions that did not extend into the flesh of the fruit were collected from a field in Frio County, TX, in October 1996. Lesions were either water-soaked or had a scabby center with a water-soaked edge. Non-fluorescent, gram-negative bacteria were consistently isolated from lesions. Eleven representative bacterial strains had 0.835 to 0.950 similarity to Acidovorax avenae subsp. citrulli, using Biolog GN Microplates and the MicroLog data base release 3.50 (Biolog, Hayward, CA). Strains were tested for pathogenicity on watermelon (C. lanatus (Thunb.) Matsum. & Nakai 'Royal Sweet') and honeydew seedlings (cv. Morning Ice) by daubing suspensions (approximately 108 CFU/ml) of bacteria onto cotyledons of 1-week-old transplants. Water soaking, followed by necrosis, occurred after 3 to 5 days. A. avenae subsp. citrulli, the causal agent of bacterial fruit blotch of watermelon, was reisolated from lesions. Detached fruit of honeydew (various cultivars) and watermelon (cv. Emperor) were surface disinfested with 70% ethanol, injected sub-epidermally with bacterial suspensions in blemish-free areas, and incubated at 24°C. Water-soaked lesions developed on fruit 10 to 14 days later and the bacteria were reisolated from inoculated fruit. Fruit inoculated with sterile water did not develop symptoms. Female flowers of honeydew plants (cv. Honeybrew) were inoculated at the time of pollination with a suspension of bacteria daubed on the fruit surface. Pollinated fruits were sealed in a moist chamber for 48 h. Circular, water-soaked spots developed 10 to 14 days later. A. avenae subsp. citrulli was reisolated from these lesions. The affected 44-ha field utilized center pivot irrigation and the incidence of diseased fruit exceeded 50%. The source of the pathogen is unknown. Bacterial fruit blotch was identified in watermelon grown under overhead irrigation 0.8 km from this honeydew field 9 weeks earlier. In past years, bacterial fruit blotch has occurred in watermelon fields in Frio County (1). Honeydew from other fields in the vicinity, including one that was also sprinkler irrigated, did not show these symptoms. This is the first report of A. avenae subsp. citrulli causing a disease of honeydew fruit. Reference: M. C. Black et al. Plant Dis. 78:831, 1994.

First Report of Bacterial Fruit Blotch of Watermelon in Oregon.

Most of the watermelons, Citrullus lanatus (Thunb.) Matsum. & Nakai, consumed in the Pacific Northwest during the summer months are grown in the southern Columbia Basin under dry (<5 cm rainfall), low relative humidity (46 to 57%), and high temperature (29 to 41°C) conditions, using transplants, plastic mulch, and drip irrigation. During May 1996, irregularly shaped, water-soaked lesions were observed on cotyledons and first true leaves of watermelon cv. Sangria transplants growing in a greenhouse. Similar lesions were observed later on older leaves in a commercial field of cv. Millionaire. Microscopic examination of symptomatic tissue revealed bacterial streaming, and isolation on nutrient agar consistently yielded numerous creamy to off-white bacterial colonies. Bacteria from purified, single colonies were Gram negative and rod shaped. Physiological characterization by the Biolog GN Bacterial Identification System (version 3.5) showed a similarity of 0.971 to the Biolog description for Acidovorax avenae subsp. citrulli. Pathogenicity of two strains was confirmed in three separate tests by hypodermic needle infiltration of cotyledons or by stab inoculation into hypocotyls of 12 to 24 21-day-old cv. Crimson Sweet seedlings with aqueous suspensions of bacteria containing approximately 6.0 × 108 CFU/ml. Inoculum was prepared from 48-h-old nutrient agar cultures. Test plants were incubated in the greenhouse at 21°C, under a 16-h photoperiod. Hypocotyl and cotyledon inoculations produced water-soaked lesions within 24 to 48 h on both the hypocotyl and cotyledons or just the cotyledon, respectively, on plants inoculated by either method. No symptoms developed on control plants infiltrated or stabbed with sterile water only. Isolations from three symptomatic seedlings yielded colonies similar in morphology to those used for inoculation. Tests of two purified cultures by Biolog indicated the bacteria were A. avenae subsp. citrulli. The symptomatic test plants were transplanted to fields, and the maturing melons developed large, dark green, water-soaked lesions with irregular margins. Similar fruit symptoms were seen in commercial fields. Labels on seed used in commercial production and in our tests warned of risks related to fruit blotch. This is the first report of bacterial fruit blotch of watermelon in Oregon. This disease may have a significant impact on watermelon production in the Columbia Basin.

Wet Seed Treatments for the Control of Bacterial Fruit Blotch of Watermelon

The bacterium causing fruit blotch of watermelon was transmitted in seeds from fruit that had the typical water-soaking symptom; from fruit that had small, restricted, necrotic lesions; and from symptomless fruit adjacent to typically symptomatic fruit. Storage of seed at 12°C for 12 months did not reduce the level of seed transmission. Fermentation of seeds in watermelon juice and debris prior to washing and drying reduced the level of seed transmission from symptomatic fruit from 61% to less than 1%. Treatment of washed seeds from symptomatic fruit with 1% CaOCl 2 for 15 min was relatively ineffective in reducing seed transmission, but 1% HCI for 15 min was as effective as fermentation. Fermentation of seeds for 24 to 48 h followed by 1% HCI or 1% CaOCl 2 treatment for 15 min prior to washing and drying were the most effective treatments for eliminating bacterial contamination of watermelon seeds. These treatments did not adversely affect seed germination.

Bacterial Fruit Blotch of Watermelon: The Hypothetical Exam Question Becomes Reality

Bacterial Fruit Blotch of Watermelon: The Hypothetical Exam Question Becomes Reality

First Report of Bacterial Fruit Blotch of Watermelon in Texas.

First Report of Bacterial Fruit Blotch of Watermelon in Texas.