153 Early infection of bacterial head rot of broccoli.
156
turn dark brown to black (154). The tissue becomes soft, collapses, and emits a bad odor. Secondary molds and bacteria cause further decay. Bacterial head rot symptoms can resemble those of Alternaria head rot. With bacterial head rot, fungal sporulation is absent unless secondary molds colonize the diseased florets.
Causal agents
Bacterial head rot is caused by a complex of pathogenic bacteria including the following: Erwinia carotovora subsp. carotovora, Pseudomonas fluorescens, P. mar-
ginalis, P. viridiflava, and perhaps others. The roles of
the different bacterial agents may differ. Some species, such as P. fluorescens, produce a surfactant-like chemical (viscosin) that allows bacteria to more readily penetrate the very waxy surface of broccoli florets. Other bacteria do not make such substances and cannot reach broccoli tissues on their own. If multiple species are present, the viscosin-producing species allow the non-producers to likewise penetrate the surface wax and infect the broccoli tissues.
Disease cycle
Severe disease is always associated with periods of cool temperatures and wet, foggy, or rainy weather when crops begin to form the early broccoli flower heads. Frequent overhead sprinkler irrigation can promote this disease. Infection is also tied with fluctuating day/night temperatures that allow water to condense on the heads. Excessive nitrogen applications increase disease severity. Secondary organisms are very often present on the diseased florets and this leads to rapid breakdown of tissues and difficulty in disease diagnosis.
Control
Some differences in cultivar susceptibility exist, based on architecture and exposure of the broccoli heads. Broccoli heads having domed, and not flat, head shapes are less likely to have severe head rot. Plant these less susceptible cultivars, if available. Avoid planting broccoli during the wet, rainy part of the year. Avoid using overhead sprinklers, or schedule irrigations to reduce the number of applications during head formation. Increasing the interval between irrigations from 2 to 8 days reduced head rotting by 50% in Oregon. Protectant copper sprays have been used with some success under UK conditions, but such have not proven useful in California.
References
Canaday, C. H. and Wyatt, J. E. 1992. Effects of nitrogen fertilization on bacterial soft rot in two broccoli cultivars, one resistant and one susceptible to the disease. Plant Disease 76:989–991.
Canaday, C. H., Wyatt, J. E., and Mullins, J. A. 1991. Resistance in broccoli to bacterial soft rot caused by Pseudomonas
marginalis and fluorescent Pseudomonas species. Plant Disease 75:715–720.
Darling, D., Harling, R., Simpson, R. A., McRoberts, N., and Hunter, E. A. 2000. Susceptibility of broccoli cultivars to bacterial head rot: in vitro screening and the role of head morphology in resistance. European Journal of Plant
Pathology 106:11–17.
Hernandez-Anguiano, A. M., Suslow, T. V., Leloup, L., and Kado, C. I. 2004. Biosurfactants produced by Pseudomonas
fluorescens and soft-rotting of harvested florets of broccoli
and cauliflower. Plant Pathology 53:596–601. Hildebrand, P. D. 1989. Surfactant-like characteristics and
identity of bacteria associated with broccoli head rot in Atlantic Canada. Canadian Journal of Plant Pathology 11:205–214.
Laycock, M. V., Hildebrand, P. D., Thibault, P., Walter, J. A., and Wright, J. L. C. 1991. Viscosin, a potent peptidolipid biosurfactant and phytopathogenic mediator produced by a pectolytic strain of Pseudomonas fluorescens. Journal of
Agricultural Food Chemistry 39:483–489.
Ludy, R. L., Powelson, M. L., and Hemphill, D. D. 1997. Effect of sprinkler irrigation on bacterial soft rot and yield of broccoli. Phytopathology 81:614–618.
Robertson, S., Brokenshire, T., Kellock, L. J., Sutton, M., Chard, J., and Harling, R. 1993. Bacterial spear (head) rot of calabrese in Scotland: causal organisms, cultivar susceptibility and disease control. Proceedings Crop Protection in Northern
Britain 1993:265–270.
Wimalajeewa, D. L. S., Hallam, N. D., Hayward, A. C., and Price, T. V. 1987. The etiology of head rot disease of broccoli.
Australian Journal of Agricultural Research 38:735–742.
Wimalajeewa, D. L. S., Hayward, A. C., and Price, T. V. 1985. Head rot of broccoli in Victoria, Australia, caused by
Pseudomonas marginalis. Plant Disease 69:177. DISEASES OFVEGETABLECROPS
B
ACTERIAL
D
ISEASES
154 Advanced infection of bacterial head rot of
broccoli.
157
Pseudomonas syringae pv. alisalensis
BACTERIAL BLIGHT
Introduction and significance
In California this disease is most often found on broccoli raab (Brassica rapa subsp. rapa) and more recently on broccoli and arugula. The disease is very damaging to broccoli raab. This pathogen has also been detected in the eastern USA. This is a recently described disease and pathogen, and one that is gaining in impor- tance on crops like broccoli raab.
Symptoms and diagnostic features
On broccoli raab (see chapter on Specialty Crops, page 389), small, angular, water-soaked specks occur on leaves. These flecks expand, turn brown, and become surrounded by bright yellow borders. As disease develops, the affected areas enlarge and coalesce, causing large sections of the leaf to turn yellow, then brown. With time the leaf can wilt and collapse, resulting in blight-like symptoms. On broccoli, the pathogen causes large, angular leaf spots that also begin as water-soaked lesions that later turn tan to brown (155).
Causal agent
Bacterial blight is caused by the bacterium Pseudo-
monas syringae pv. alisalensis.This pathogen appears to
be a new pathovar in the P. syringae group. The pathogen is an aerobic, Gram-negative bacterium that can be isolated on standard microbiological media. Colonies are cream to light yellow in color, smooth, and have a notably sticky consistency. When cultured on King's medium B, this organism produces a diffusible pigment that fluoresces blue under ultraviolet light. There is evidence that this pathogen is seedborne.
Disease cycle
Little is known about the specifics of disease develop- ment. The bacterium is splash dispersed by rain and overhead sprinkler irrigation. Leaf symptoms always begin on the lower, protected leaves where shading keeps the conditions humid and cool. As disease develops, the symptoms gradually move up the broccoli raab stem and eventually reach the top shoots. Seedborne inoculum would account for the occurrence of this disease in broccoli raab fields that never were planted to this crop before.
Control
Reduce or eliminate the use of overhead sprinkler irri- gation. Do not plant a subsequent broccoli raab crop in fields having undecomposed, infested crop residues. Copper applications provide only minimal protection against this pathogen.
References
Bull, C. T., Goldman, P., and Koike, S. T. 2004. Bacterial blight on arugula, a new disease caused by Pseudomonas syringae pv. alisalensis in California. Plant Disease 88:1384. Cintas, N. A., Koike, S. T., and Bull, C. T. 2002. A new pathovar,
Pseudomonas syringae pv. alisalensis pv. nov., proposed for
the causal agent of bacterial blight of broccoli and broccoli raab. Plant Disease 86:992–998.
Koike, S. T., Cintas, N. A., and Bull, C. T. 2000. Bacterial blight, a new disease of broccoli caused by Pseudomonas syringae in California. Plant Disease 84:370.
Koike, S. T., Henderson, D. M., Azad, H. R., Cooksey, D. A., and Little, E. L. 1998. Bacterial blight of broccoli raab: a new disease caused by a pathovar of Pseudomonas syringae. Plant
Disease 82:727–731. BRASSICACEAE B ACTERIAL D ISEASES
155 Tan lesions of bacterial blight of broccoli.
158
Pseudomonas syringae pv. maculicola
BACTERIAL LEAF SPOT
Introduction and significance
This disease is occasionally important during wet summers in Europe and other production areas, but is usually of minor significance. Cabbage is reported to be the most commonly affected crop, but bacterial leaf spot also occurs on cauliflower, collards, Brussels sprout, radish, and turnip. In the USA, this disease is often most important as a seedling disease that reduces quality and vigor of greenhouse produced transplants.
Symptoms and diagnostic features
Infections start as dark, water-soaked specks on leaves that can be seen from both leaf surfaces (156). Leaf spots typically remain small and measure 3–4 mm in diameter, but enlarging and merging of spots occurs if conditions are favorable. Older leaf spots turn tan and may sometimes have a purple border surrounding them. Severe infection can cause blotching, leaf distor- tion and premature leaf fall. Infected cauliflower heads may be discolored. On transplants (157), bacterial leaf spot may resemble early downy mildew infections that are not sporulating. The small, dark leaf spot symptoms are the reason for the older disease name of pepper leaf spot.
Causal agent
The cause of bacterial leaf spot is Pseudomonas
syringae pv. maculicola. The pathogen is an aerobic,
Gram-negative bacterium that can be isolated on standard microbiological media. Colonies are cream to light yellow in color and smooth. When cultured on King's medium B, this organism produces a diffusible pigment that fluoresces blue under ultraviolet light. Strains of this pathogen are host specific to crucifers. The pathogen is seedborne.
Disease cycle
The pathogen appears to survive between crops on infested debris. In the field the bacterium is splash dispersed by rain and overhead sprinkler irrigation. Disease development can be particularly severe on greenhouse produced transplants due to the use of overhead sprinklers, humid protected conditions, and the practice of clipping and mowing transplants to strengthen transplant stems.
Control
Reduce or eliminate the use of overhead sprinkler irri- gation. Do not plant a subsequent crucifer crop in fields having undecomposed, infested crop residues. Copper applications provide only minimal protection against this pathogen. In greenhouses, schedule irrigations to allow for maximum drying of foliage. Remove trans- plant trays that show symptoms of the disease. Copper applications in the controlled setting of greenhouses are probably more effective than in field use. Sanitize old transplant trays and bench tops so the pathogen is not spread to new plantings. Mowing of transplants greatly increases the risk of spreading the disease. Use seed that does not have significant levels of the pathogen. Appropriate seed treatments can also contribute to the management of seedborne inoculum. Treat infested seed with hot water.
DISEASES OFVEGETABLECROPS
B
ACTERIAL
D
ISEASES
156 Leaf spots of bacterial leaf spot of cauliflower.
156
157 Cauliflower transplants infected with bacterial leaf
spot.
159
References
Cintas, N. A., Bull, C. T., Koike, S. T., and Bouzar, H. 2001. A new bacterial leaf spot disease of broccolini, caused by
Pseudomonas syringae pv. maculicola, in California. Plant Disease 85:1207.
Koike, S. T., Smith, R. F., Van Buren, A. M., and Maddox, D. A. 1996. A new bacterial disease of arugula in California. Plant
Disease 80:464.
Lewis-Ivey, M. L., Wright, S., and Miller, S. A. 2002. Report of bacterial leaf spot on collards and turnip leaves in Ohio. Plant
Disease 86:186.
Peters, B. J., Asha, G. J., Cotherb, E. J., Hailstones, D. L., Nobleb, D. H., and Urwin, N. A. R. 2004. Pseudomonas
syringae pv. maculicola in Australia: pathogenic, phenotypic
and genetic diversity. Plant Pathology 53:73–79. Shackleton, D. A. 1996. A bacterial leaf spot of cauliflower in
New Zealand caused by Pseudomonas syringae pv.
maculicola. New Zealand Journal of Science 9:872–877.
Wiebe, W. L. and Campbell, R. N. 1993. Characterization of
Pseudomonas syringae pv. maculicola and comparison with Pseudomonas syringae pv. tomato. Plant Disease
77:414–419.
Zhao, Y. F., Damicone, J. P., and Bender, C. L. 2002. Detection, survival, and sources of inoculum for bacterial diseases of leafy crucifers in Oklahoma. Plant Disease 86:883–888. Zhao, Y. F., Damicone, J. P., Demezas, D. H., Rangaswamy, V.,
and Bender, C. L. 2000. Bacterial leaf spot of leafy crucifers in Oklahoma caused by Pseudomonas syringae pv. maculicola.
Plant Disease 84:1015–1020.
Xanthomonas campestris pv. campestris
BLACK ROT
Introduction and significance
Black rot is one of the most important diseases of crucifers. It is most damaging in tropical, subtropical, and other areas with warm, humid climates. For example, significant black rot occurs on cabbage grown in Africa. In cooler regions, losses are less severe, but the disease has become more prevalent in recent years, possibly because spread is favored by modern propaga- tion systems. The seedborne nature of the pathogen has resulted in its distribution throughout the world.
Symptoms and diagnostic features
Black rot symptoms can vary greatly depending on the particular brassica host, age of host when infected, par- ticular strain of the pathogen, and environmental con- ditions. Initial symptoms consist of small, water-soaked leaf spots. These then become brown specks with a chlorotic margin and can mimic other bacterial diseases (158). Other early symptoms are angular or V-shaped yellow lesions that often develop along the leaf edges. BRASSICACEAE
B
ACTERIAL
D
ISEASES
156 Early infection of black rot of cauliflower.
160
These lesions later dry up and turn tan or brown (159). Black veins are sometimes seen within these tan lesions. Severely infected leaves may wither and drop off the plant. If systemic infection occurs, the vascular tissues in petioles and main stems turn black (160). Some strains produce symptoms known as ‘blight,’ which are characterized by a sudden collapse of interveinal tissues and a lack of veinal necrosis. Systemic lesions may develop in the center of the leaf when temperatures are high (20-28º C), but most marginal lesions occur at 16º C. However, if temperatures are cool, black rot symptoms may not be expressed.
Causal agent
The cause of black rot is the aerobic, Gram-negative bacterium Xanthomonas campestris pv. campestris. The pathogen can be isolated on standard microbio- logical media and produces yellow, mucoid, slow growing colonies typical of most xanthomonads. Various starch-based media aid in the isolation and identification of this organism. Strains of this pathogen are host specific to crucifers and infect Brassica species, radish, crucifer weeds, and ornamental crucifers such as stock (Matthiola spp.) and wallflower (Cheiranthus
cheiri). This pathogen is seedborne, and different races
have been documented. Also, the highly virulent variants that cause the blight and plant collapse symptoms can be differentiated from leaf spot causing strains via monoclonal antibody and molecular tests. Strains infecting horseradish are X. campestris pv.
amoraciae; this organism is only a weak pathogen on
cabbage and cauliflower.
Disease cycle
Initial inoculum comes from infested seed, diseased weeds and volunteer crucifers, and infested but un- decomposed crop residues. Bacteria are dispersed by splashing rain or sprinkler irrigation from inoculum sources to susceptible plants. Cotyledons and leaves are infected through leaf openings called hydathodes, which are located where veins end at the leaf margin. Infection can also occur through wounds or roots. Symptoms only occur when sufficient inoculum builds up on the leaf and if weather conditions are conducive for infection and symptom expression. If conditions are humid and warm, and there is splashing water, disease can progress rapidly and cause extensive damage. In seed crops, infected plants may be symptomless, though the pathogen is present in the vascular system and is able to spread to pods and seed.
Control
Use seed that does not have significant levels of the pathogen. Treat seed with sodium hypochlorite or hot water. Produce seed in areas having a dry climate, and have such seed inspected and certified. Rotate crops with non-hosts for 2 to 3 years to avoid problems from infected crop residues. However, once crop residues are gone, the bacterium does not survive in the soil. Do not place seedbed or transplant areas adjacent to field pro- duction crops. Remove symptomatic transplants and DISEASES OFVEGETABLECROPS
B ACTERIAL D ISEASES 160 Vascular blackening of black rot of cauliflower. 160
159 Advanced infection of black rot of cauliflower.
161 surrounding plants and trays. Mowing of transplants
greatly increases the risk of spreading the pathogen. Use resistant cultivars if available. The application of chemicals to foliage is not very effective for black rot control.
References
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Plant Disease 84:1008–1014. BRASSICACEAE B ACTERIAL D ISEASES
162
Albugo candida