Progress 07/31/00 to 07/31/05
Outputs
Data generated from this project has led to a better understanding of the epidemiology of seedborne plant diseases in Georgia. In particular, this work has a more detailed understanding of the epidemiology of bacterial fruit blotch, a serious disease of cucurbits caused by Acidovorax avenae subsp. citrulli (Aac). This research proved that there are at least two distinct sub-populations of Aac in the US. Group II strains are highly aggressive on watermelons and mildly aggressive on other cucurbits; while group I strains are moderately aggressive on all cucurbits. Group II strains have been associated with BFB outbreaks in watermelons across the US since 1989; however, group I strains have only been observed on melon and pumpkin since 1998. It is likely that this shift in population structure is due to the global production and movement of planting seeds. In greenhouse experiments, the stigmas of female watermelon blossoms could be colonized by Aac, resulting in seed
infection despite the lack of BFB fruit symptoms. This phenomenon possibly explains how infested seeds could be produced in seed production fields with no visible disease symptoms. Furthermore, by using green fluorescent protein mutants of Aac, the bacterium was observed to invade blossoms via transmitting tract tissues. Bacterial motility was not necessary for seed infection, but the exact mechanism of invasion remains unknown. By conducting caged field trials, it was shown that pollinating insects could serve as vectors to inoculate female blossoms and thereby lead to seed infection. Flower treatments with biocontrol agents were shown to limit blossom colonization and subsequent seed infection. A. avenae subsp. avenae, a pathogen of corn, was the most effective biocontrol agent tested, reducing BFB seedling transmission by 96.5% when used as a seed treatment. Additionally, when applied as a blossom protectant, this organism reduced seed infection to 13.8%, which was significantly
lower than the positive control (63% infection). This project also led to the development of effective polymerase chain reaction (PCR)-based seed health assays. Because of their high levels of specificity and sensitivity, and their applicability for the detection of multiple pathogens simultaneously in seeds, these techniques can be implemented in the short term to detect and exclude infested seedlots. Of these techniques, immunomagnetic separation and PCR displayed a sensitivity of 10 colony-forming units/ ml, and detected Aac in seedlots with 1 infested seed in 10,000. Additionally, using this approach, onion seeds were proven to be a source of inoculum for Pantoea ananatis, the causal agent of center rot. IMS and real time PCR is currently being evaluated and adapted for routine use by commercial seed testing agencies and seed companies. This project has resulted in the development of magnetic capture hybridization and real time PCR-based seed assays to simultaneously detect
Didymella bryoniae and Aac in watermelon seeds and Pepino mosaic virus and Clavibacter michiganensis subsp. michiganensis in tomato seeds. MCH-PCR has great potential to revolutionize seed health testing.
Impacts
This research has led to a better understanding of the epidemiology of bacterial fruit blotch of cucurbits, and other important seedborne disease in Georgia and the US. Seed health assays developed in this project are currently being implemented by seed testing companies and seed production companies to improve the efficiency detecting and excluding seedborne bacterial inoculum. Finally, new information about the epidemiology of BFB is being used to develop new strategies for limiting seed infection in commercial seed production fields.
Publications
- Fessehaie, A., and Walcott, R.R. 2005. Biological control to protect watermelon blossoms and seeds from infection by Acidovorax avenae subsp. citrulli. Phytopathology 95:413-419
- Walcott, R.R. 2005. Bacterial fruit blotch of cucurbits. The Plant Health Instructor. DOI:1094/PHI-I-2005-1025-02
- Walcott, R.R., Castro, A.C., Fessehaie, A. and Ling, K. 2006. Progress towards a commercial seed assay for Acidovorax avenae subsp. citrulli. Seed Science and Technology 34:101-116
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Progress 01/01/04 to 12/31/04
Outputs
To improve the management of seedborne bacterial diseases, biological and chemical seed treatments were evaluated. Acidovorax avenae ssp. avenae (AAA99-2), Pseudomonas fluorescens A506 and Brevibacterium spp. (WS-1)were were identified with antagonistic activity against Acidovorax avenae subsp. citrulli (Aac), the causal agent of bacterial fruit blotch of cucrbits (BFB) and evaluated as seed treatments. Watermelon seed with 63% natural infestation were treated with 108 CFU/ml of each biocontrol candidate, air-dried, and evaluated for BFB by the seedling growout assay. A506 and WS1 delayed the onset on BFB by 48 h and reduced disease by 11.9 and 38%, respectively. In contrast, AAA99-2 delayed BFB transmission by 12 days and reduced disease by 97%. AAA99-2 did not affect seedling germination. To investigate the effect of biocontrol agents on seedling emergence, a similar experiment was conducted using artificially infested seed under greenhouse conditions and 0.4% w/v
Kocide 101 was included as a negative control. As expected, BFB transmission under greenhouse conditions was lower than for germination boxes. Nevertheless, AAA99-2 was the best seed treatment, delaying BFB seedling transmission by two days. None of the treatments affected seedling emergence. These data suggest that biological control seed treatment might be feasible for BFB management. Unfortunately, since AAA99-2 was pathogenic to maize, it cannot be used, as a commercial seed treatment. However, it may be possible to identify more suitable biocontrol agents. Electrolyzed oxidizing (EO) water was evaluated as a seed treatment for seedborne bacteria. EO water demonstrated in vitro antimicrobial activity against Aac, but it failed to eradicate the pathogen from infested seeds. Acidic EO water generated at 14 and 20 A, and alkaline EO water was compared to 0.1% NaOCl and H2O for management of cabbage black rot (Xanthomonas campestris pv. campestris) and BFB. None of the treatments
completely prevented black rot transmission; however, 14 A EO water was the most effective treatment followed by NaOCl and 20 A EO water. Alkaline EO water was less effective than acidic 14 A EO water and NaOCl, and not significantly different to EO water generated at 20 A for black rot management. None of the treatments affected cabbage germination or seedling emergence. EO water was less effective on seedborne Aac but the best treatment was acidic 20 A EO water followed by alkaline EO water and acidic 14 A EO water. This might be because watermelon seeds are larger, and may protect the pathogen from exposure to the seed treatment. EO water did not affect watermelon seed germination or emergence. The data indicate that EO water can significantly reduce cabbage black rot and BFB seedling transmission. EO water did not completely disinfest the seeds and this may, in part, be due to the location of bacteria within seed or the possibility that the EO water is rapidly inactivated by seed
tissues. Adjusting exposure time or using different application techniques (e.g. vacuum infiltration) might improve the efficacy of EO water.
Impacts
Seeds are internationally traded and can serve as vectors to introduce nonindigenous pathogens into the US. By developing sensitive and efficient seed health assays and understanding the role that seedborne inoculum plays in disease epidemics, we will be better able to prevent economic and ecological damage. The findings of this work will help to determine the appropriate level of response to seedborne C. parasiticum inoculum. Additionally, this work will provide a method that can be used to test seeds for many types of pathogens. Finally, from this work it is clear that efforts are needed to curtail the introduction of exotic strains of Aac into the US. Failure to do this may increase the risk of BFB for other cucurbit crops.
Publications
- Walcott, R.R., Fessehaie, A., and Castro, A.C. 2004. Differences in pathogenicity between two genetically distinct groups of Acidovorax avenae subsp. citrulli strains on cucurbit hosts. J. Phytopathol. 152: 277-285
- Walcott, R.R., Gitaitis, R.D. and Langston, D.B. Jr. 2004. Detection of Botrytis aclada in onion seed using magnetic capture hybridization and the polymerase chain reaction. Seed Sci. and Technol. 32:425-438
- Gitaitis R. D., Walcott, R.R., Sanders, H.F., Zolobowska, L., and Diaz-Perez, J.C. 2004. Effects of mulch and irrigation system on sweet onion: II. The epidemiology of center rot. J. Am. Hort. Soc. 129:225-230
- Diaz-Perez, J.C., Giddings, D., Bertrand, D., Sanders, H. Randle, W.M., Walcott, R., and Gitaitis, R. 2004. Effect of mulch and irrigation system on sweet onion: I. Bolting, plant growth, and bulb yield and quality. J. Am. Hort. Soc. 129:218-224
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Progress 01/01/03 to 12/31/03
Outputs
Using DNA fingerprinting by pulse field gel electrophoresis and REP-PCR, two distinct groups were confirmed among 64 Acidovorax avenae subsp. citrulli strains collected from a range of cucurbits hosts from around the world. 82% of the group I strains were recovered from non-watermelon hosts and in general, they failed to utilize L-leucine. On the other hand, 94% of the group II strains were recovered from watermelon and 96% of them utilized L-leucine. 2-week-old watermelon cv. Crimson sweet, cantaloupe cv. Athena, pumpkin cv. Lumina and squash cv. Early yellow crookneck seedlings were susceptible to Aac strains representing each group. Group II strains were more aggressive on watermelon than other hosts. On the other hand group I strains were moderately aggressive on all cucurbit hosts tested. These findings suggest that recent increases in BFB on nonwatermelon cucurbits may be due to the introduction of nonindigenous strains. To facilitate rapid detection of Botrytis
aclada in onion seed, a magnetic capture hybridization and polymerase chain reaction (MCH-PCR) assay was developed. MCH-PCR reduces the time required to test onion seeds from 10-14 days to less than 24h. Additionally, MCH-PCR detected fungal DNA from aqueous suspensions containing 100 fg DNA/mL. MCH-PCR in combination with a Mini-Beadbeater DNA extraction protocol allowed detection of B. aclada in suspensions with 104 and 103 conidia/mL in 100% and 60% of the attempts, respectively. In suspensions with 10 conidia/mL the detection frequency was 10%. MCH-PCR also was more sensitive and efficient than conventional PCR, detecting the fungus in seedlots with 4.8 and 9.9 % natural B. aclada infestation (80 and 70 % of attempts). Differences between the detection frequency of MCH-PCR and the semi-selective agar assay were not significant. MCH-PCR detected B. aclada in seedlots with natural infestation levels ranging from 0.25 - 90%. To determine the significance of seedborne C. parasiticum
in Cylindrocladium back rot epidemics, naturally infested seed (0-7% infestation) were planted under greenhouse conditions and seedlings were observed for CBR. None of the seedlots led to CBR transmission. To determine if seedborne microsclerotia could be transferred to soil, uninfected peanut seed were planted into soil previously planted to infested seed. Again none of the seedlings developed CBR symptoms and attempts to recover the pathogen from the soil were unsuccessful. To more accurately explore the potential of infested seed to transmit C. parasiticum to soil, a PCR detection assay was developed using primers designed from the C. parasiticum beta-tubulin gene. This assay will be used in future epidemiological studies. One possible explanation for the recent increase in CBR in Georgia is the introduction of aggressive strains on seed. To address this issue, random amplified polymorphic DNA (RAPD) analysis was conducted on C. parasiticum isolates recovered from Georgia, Alabama,
Florida, and Virginia. The data indicated that there was little genetic diversity among these strains. The amplified fragment length polymorphism technique is currently being employed to confirm these observations.
Impacts
Seeds are internationally traded and can serve as vectors to introduce nonindigenous pathogens into the US. By developing sensitive and efficient seed health assays and understanding the role that seedborne inoculum plays in disease epidemics, we will be better able to prevent economic and ecological damage. The findings of this work will help to determine the appropriate level of response to seedborne C. parasiticum inoculum. Additionally, this work will provide a method that can be used to test seeds for many types of pathogens. Finally, from this work it is clear that efforts are needed to curtail the introduction of exotic strains of Aac into the US. Failure to do this may increase the risk of BFB for other cucurbit crops.
Publications
- Yates, I.E., Arnold, J.W., Hinton, D.M., Basinger, W., and Walcott, R.R. 2003. Fusarium verticilioides induction of maize seed rot and its control. Canadian J. Botany 81 (5): 422-428
- Buck, J. W., Walcott, R. R., Beuchat, L. R. 2003. Recent trends in microbiological safety of fruits and vegetables. Online. Plant Health Progress doi:10.1094/PHP-2003-01XX-01-RV.
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Progress 01/01/02 to 12/31/02
Outputs
To investigate the genetic diversity of A. avenae subsp. citrulli (Aac), strains were recovered from many cucurbits from the US, Thailand, Canada, Israel, China, Taiwan, Brazil and Australia. Using REP-PCR, pulse field gel electrophoresis and substrate utilization profiles, the strains were compared. PFGE was the most discriminating technique, revealing 23 distinct haplotypes. However, in considering all the data, two distinct sub-groups were observed. Sub-group I included strains recovered from cantaloupe, pumpkin, gourds and watermelon that generally could not utilize leucine. Subgroup II strains were recovered from watermelon and utilized leucine. Based on these observations host specificity of the two subgroups was investigated. Two-week old seedlings(n=5) of watermelon, squash, cantaloupe and pumpkin were spray-inoculated with suspensions (106 CFU/ml) of 5 strains from each subgroup and BFB incidence and severity were assessed. Watermelon and cantaloupe fruits
(n=5) were also inoculated 5 days after pollination with 5 strains from each subgroup and subsequently evaluated for BFB severity. Despite the clustering of strains based on DNA profiles, there were no significant differences in the ability of the strains to infect the cucurbit seedlings. In contrast, while the average disease severity rating of the subgroup I strains were 1.71 and 1.45 ( 0 to 5 scale with 5 being complete rot) on watermelon and cantaloupe respectively, the subgroup II strains were highly aggressive on watermelon (3.16) but were less aggressive on cantaloupe (0.99). The data support for the existence of host specificity among Aac strains; however, host specificity may be determined by a component of the fruit rind. Hence, efforts to develop BFB resistance should be based on fruit reactions. To improve detection of seedborne bacteria using PCR-based assays, efforts were made to optimize immunomagnetic separation and PCR. Optimum parameters for IMS-PCR require coating
immunomagnetic beads with 80ug purified anti- Aac antibody per 108 beads; using 108 immunomagnetic beads per sample and conducting immunocapture for 2 h. With these parameters, Aac was consistently recovered from suspensions with 10 CFU/ml seed extract. Additionally, these parameters allowed Aac detection from seedlots with 0.5 (25/5000 seed) and 0.1% (1/5000 seed) in 100 and 66.6% of the attempts, respectively. The ability to detect the pathogen in larger seedlots (n=10,000 seed) yielded inconsistent results. Hence, research is needed to improve pathogen extraction from large seedlots. Efforts were also made to compare magnetic capture hybridization and PCR (MCH-PCR) and an agar plate assay for the detection of Botrytis aclada in onion seed. Using naturally infested seedlots (ranging from 0 to 9.9%), the frequency of detection was higher with the agar plate assay (31/40) than with the MCH-PCR assay (21/40); however, this difference was not statistically significant. Despite this,
MCH-PCR was able to detect B. aclada in commercial seedlots that tested negative by the agar plate assay. This suggests that while the MCH-PCR may not be as robust as the agar plate assay, it may be more sensitive.
Impacts
Understanding the genetic diversity and possible host specificity among A. avenae subsp. citrulli populations may be important in developing disease resistance screning programs. This will be a critical aspect of the management of BFB in the future. Optimization of IMS-PCR and the efforts to scale-up this assay for use with large seed samples will assist with technology transfer and commercial implementation in the vegetable seed industry. Finally, the development of the MCH-PCR assay will not only provide a rapid and sensitive assay for the detection of B. aclada in onion seeds, but it provides support for the use of this approach to routinely detect phytopathogenic fungi in seed.
Publications
- Gitaitis, R., Walcott, R., Culpepper, S., Sanders, H., Zolobowska, L., and Langston, D. 2002. Recovery of Pantoea ananatis , causal agent of center rot of onion, from weeds and crops in Georgia, USA. Crop Protection 21: 983-989
- Walcott, R.R. 2003. Detection of seedborne pathogens. HortTechnology: 13:40-47
- Gitaitis, R.D., Walcott, R.R., Diaz-Perez, J.C., Wells, M.L., and Sanders, F.H. 2003. Transmission of Pantoea ananatis, causal agent of center rot of onion, by tobacco thrips, Frankliniella fusca. Plant Dis. (In Press)
- Walcott, R.R., Castro, A.C., and Gitaitis, R.D. 2003. The role of watermelon blossoms in seed infestation by A. avenae subsp. citrulli. Phytopathology (In press)
- Walcott, R.R., Gitaitis, R.D. and Langston, D.B. Jr. 2003. Detection of Botrytis aclada in onion seed using magnetic capture hybridization and the polymerase chain reaction. Seed Sci. and Technol. (Acepted 01/03)
- Gitaitis, R., Wilson, J., Walcott, R. Sanders, H., and Hanna, W. 2002. Occurrence of bacterial stripe of pearl millet in Georgia. Plant Dis. 86: 326
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Progress 01/01/01 to 12/31/01
Outputs
In a replicated experiment, seeds from watermelon fruits derived from female blossoms inoculated with sterile 0.1M phosphate buffered saline buffer, were found to be free of Acidovorax avenae subsp. citrulli (Aac). Samples of pulp from these fruits were not infected. Blossoms inoculated with 107 and 109 colony forming units (CFU) of an Aac mutant expressing the green fluorescent protein (AAC 8-3ST) yielded symptomless fruits. However, pulp from 24.3% and 50% of the fruits respectively, were infested with Aac. Additionally, 5.7% and 35% of the seedlots (seed from one fruit) were infested. Finally, 43.5 and 35% of the seedlots from blossom inoculated with 107 and 109 CFU respectively, led to BFB seedling transmission. BFB transmission ranged from 0 - 44% and 0 - 69% for blossoms treated with107 CFU and 109 CFU, respectively. The data strongly support the hypothesis that watermelon blossoms are pathways for seed infection. Pantoea ananatis could also infest watermelon
seeds via open female blossoms. Twenty two percent of fruits from blossoms inoculated with P. ananatis (107CFU) were contaminated and 46.7 and 67.8 % of the seedlots were positive by IMS-PCR and IMS-plating, respectively. To investigate the role of pollinating insects as vectors for Aac, honey-bees (n=35) collected from a field with an induced outbreak of BFB on June 30, and July 11 2001 were individually assayed. 45.7 and 14.3% of the bees sampled in June and July respectively, were found to be contaminated. The data suggest that bees and other pollinating insects can be Aac vectors. A survey of commercial peanut seedlots for Cylindrocladium parasiticum. Samples of 145 untreated, commercial peanut seedlots were assayed by visual inspection (n=400 seeds) and plating on semi-selective media (n=100 seeds). 24.1% of the seedlots were found to be infested with C. parasiticum by visual inspection and infestation levels ranged from 0.25% to 1%. Despite this, C. parasiticum was not recovered
from the samples. We conclude that while CBR can infest peanut seeds, the fungus becomes either non-viable or non-culturable in storage. There was no strong relationship between CBR infestation and peanut seed quality as determined by standard warm germination and conductivity assays. To improve the detection of seedborne pathogens, a magnetic capture hybridization and PCR assay was developed for Botrytis allii. This technique can be completed in 24h and represents a significant improvement in assay efficiency. MCH-PCR displayed a detection threshold of 10 fg of B. allii DNA. When conducted on DNA from dilution series of conidial suspensions, MCH-PCR facilitated detection of 10 (1/10 attempts) and 100(4/10 attempts) conidia/ml. MCH-PCR also detected B. allii naturally infested seedlots. An IMS-PCR assay for Pantoea ananatis, the causal agent of center rot of onion was developed and it displayed a detection threshold of 10 CFU/ml. This new technique allowed the confirmation of natural
infestation of onion seed by P. ananatis We hypothesize that infested seedlots introduced this pathogen into Georgia. Planting pathogen-free seeds may reduce the frequency of future center rot outbreaks.
Impacts
It is well established that infested seeds are the most important source of inoculum for bacterial fruit blotch disease. The impact of this disease can be significantly reduced by eliminating infested seedlots. By elucidating the blossom invasion pathway, we may find the most effective and economically feasible way to prevent the production of infested seedlots. The MCH-PCR assay will significantly improve the efficiency, sensitivity and specificity of testing seeds for plant pathogens.
Publications
- Walcott, R.R., Gitaitis, R.D., Castro, A.C., Sanders, F.H., and Diaz-Perez, J.C. 2002. Natural infestation of onion seed by Pantoea ananatis, the causal agent of center rot. Plant Dis. 86:106-111
- Gitaitis, R., Zolobowska, L., Culpepper, S., Sanders, H., Langston, D. and Walcott, R. 2001. PCR detection of the onion pathogen Pantoea ananatis on various weeds and crops in Georgia, USA. pgs 406- 408 in: Proceedings of the 10th International Conference on Plant Pathogenic Bacteria, Charlottetown, Prince Edward Island, Canada, July 23-27, 2000. Kluwer Academic Publishers, Dordrecht, The Netherlands. 454pp.
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Progress 01/01/00 to 12/31/00
Outputs
Acidovorax avenae subsp. citrulli (Aac) strains expressing the green fluorescent protein (AAC 8-3ST) were used to study the role of blossoms in watermelon seed infection. Female blossoms were pollinated and inoculated with 109 CFU/ml of (AAC 8-3ST), a wild type strain (AAC 94-21) or sterile buffer ( negative control). Fruit were allowed to develop to harvest maturity and seeds were extracted. None of the fruits displayed symptoms of bacterial fruit blotch disease (BFB), but 100%, 27% and 0% of the seedlots from blossoms inoculated with AAC 8-3ST, AAC 94-21 and PBS buffer respectively, tested positive for Aac. When samples of seed from each lot were planted under greenhouse conditions, no BFB symptoms were observed on seedlings. This was possibly due to inappropriate greenhouse conditions or levels of seed infestation were below disease transmission thresholds. Thirty asymptomatic watermelon fruits were sampled from a commercial field with a natural BFB outbreak in
Omega, GA. Seeds from 17% of the these fruits tested positive for Aac by IMS-PCR and seedlings from one lot displayed BFB symptoms. Results thus far indicate that Aac can colonize female watermelon blossoms and subsequently infect seeds without causing fruit symptoms. Efforts are underway to elucidate this seed infection pathway and to microscopically determine the location of bacteria within infested seed. Several candidate organisms including A. avenae subsp. avenae and Burkholderia cepacia were identified with antagonistic activity against Aac. The mechanism of antagonism appeared to be antibiosis. Initial efforts to demonstrate biological control activity on infested watermelon seed have been unsuccessful due to greenhouse conditions that were not conducive for disease development. A survey of commercial onion seedlots revealed that Aspergillus niger was the most prevalent fungus occurring in 87% of the lots tested. Other fungi observed included Alternaria spp., Fusarium spp.,
Penicillium spp., A. flavus, A. ochraceous, and Cladosporium allii. An IMS-PCR assay was developed for detecting Pantoea ananas in onion seed. The detection threshold of this assay was 105 CFU/ml of seed wash. By this method, P. ananas was detected in onion seeds recovered from fields with natural outbreaks of center rot disease. Seventeen percent of the seedlots recovered from two locations in Tifton, tested positive for P. ananas. Viable CFU were recovered from these seedlots and when seeds were planted in germination boxes, seedlings developed typical center rot symptoms. This is the first report of onion seed infection by P. ananas and seed transmission of center rot disease. Effective IMS-PCR techniques have been developed for Aac and P. ananas. These techniques display high levels of sensitivity and efficiency and have the potential to replace conventional seed testing assays for seedborne bacteria. We are currently developing equivalent techniques (magnetic capture
hybridization and polymerase chain reaction) for seedborne fungi. At present, we have developed a species-specific PCR primer set and an internal hybridization probe for Botrytis spp. and are evaluating parameters for testing onion seed.
Impacts
Much effort and money is invested in producing BFB-free watermelon seedlots since the consequences of this disease are costly for seed and commercial watermelon producers. Blossom colonization may explain why infested seedlots result from seed production fields with no obvious BFB symptoms. Addressing and managing blossom colonization may lead to efficient production of cleaner seedlots with respect to BFB. Biological control may be an effective way to protect blossoms and seeds from infection by Aac. Because of the highly controversial nature of BFB, seed testing is a critical quality assurance issue for seed producers and all seedlots must be tested. The current standard seed assay for BFB is the seedling grow-out test which requires greenhouse space and time, and is expensive to conduct. IMS-PCR is a robust and rapid technique with a detection threshold of ca. 103 CFU/ml. Additionally, it easily overcomes seed compounds that inhibit conventional PCR. There is great
potential for this technique to become the industry standard for testing seed.
Publications
- Walcott, R.R, Langston Jr. , D. B., Sanders Jr, F. H. and Gitaitis, R. D. 2000. Investigating intraspecific variation of Acidovorax avenae subsp. citrulli using DNA fingerprinting and whole cell fatty acid analysis. Phytopathology 90:191-196
- Walcott, R. R. and Gitaitis, R. D. 2000. Detection of Acidovorax avenae subsp. citrulli in watermelon seeds using immunomagnetic separation and the polymerase chain reaction. Plant Disease 84: 470-474
- Walcott, R. R., Langston Jr., D. B., Sanders Jr., F. H., Gitaitis, R. D. and Flanders J. T. 2000. Natural outbreak of a bacterial fruit rot of cantaloupe in Georgia by Acidovorax avenae subsp. citrulli. Plant Disease 84: 372.
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