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cepacta. pecunotyiic bacteria. and total bacteria from sorls , irrigationwater, onion foliage and onion bulbs in Georgia, U.S.A. Proc. l nt.Conf. Plant Pathog. Bact., 8th. (In press.)

Gitaitis, R. D., Sumner, D., Smittle, D., Gay, D., Maw. B., Hung,Y.. and Tollner, B. 1992. A semiselective agar medium foridentification and isolation of Pseudomonas viridiflava. causal agentof bacterial blight of onion. Pages 23-25 in: Proc. Natl. Onion Res.Conf.

Gitaitis, R. D., Sumner, D., Srniule, D., Maw, B., Gay, D., Tollner,B., and Hung, Y. 1992. Epidemiology of bacterial streak and rotof onion, caused by Pseudomonas viridiflava. (Abstr.) Phyto-pathology 82: 1076.

(Prepared by R. D. Gitaitis)

Soft RotSoft rot is common in many vegetables, especially during

storage or transit. Under certain conditions, it can causesignificant losses in storage onions. Occasionally, it maydevelop in the field after heavy rains before harvest. Thepathogen is widespread in soil and irrigation water.SymptomsThe affected fleshy scale tissues are water-soaked (Plate 63)

and pale yellow to light brown (Plate 64) and become softas the rot progresses. The whole interior of the bulb may breakdown, and a watery, foul-smelling viscous liquid may oozefrom the neck if the affected bulb is squeezed. Foliagesymptoms appear as a wilt and whitening of leaves and plants(Plates 65 and 66).Causal OrganismSoft rot is caused by Erwinia carotovora subsp. carotovora

(Jones) Bergey et al. The bacterial cells are rod shaped, gramnegative, and nonpigmented and have peritrichous flagella.The organism is strongly pectolytic, facultatively anaerobic,catalase positive, and oxidase negative. It hydrolyzes gelatinand does not produce indole, phosphatase, or lecithinase.Disease Cycle and EpidemiologyThe primary sources of inoculum are contaminated soil and

crop residues. Splashing rain, irrigation water, and insectsspread the pathogen. The bacterium gains entry into the bulbthrough the neck tissue of maturing plants; mechanical injuries;or damaged areas caused by the onion maggot, Delia antiqua(Meigen), storms, or other diseases. The bacterium can persistin the intestinal tract of onion maggot larvae and adult flies,which can spread the pathogen from onion to onion. Bulbswith mechanical injuries, bruises, or sunscald are particularlysusceptible to soft rot, especially if they are he Id under warm(optimum 20-30C) and humid conditions. Infection continuesif the temperature during storage or transit is above 3C.ControlOnion tops should be allowed to mature before harvest.

Bruising of bulbs during harvest and handling should beavoided. Onions should be sto red only after they have beenwell dried. Storage at 0e and less than 70% relative humiditywith good ventilation prevents condensation of moisture onthe surface of bulbs.

Selected References

Crete, R., Tartier, L., and Devaus, A. 1981. Diseases of onions inCanada. Inf. Serv. Agrie. Can. Publ. 1716E.

Sherf, A. F., and MaeNab, A. A. 1986. Vegetable Diseases and TheirControl. 2nd ed. John Wiley & Sons, New York.

(Prepared by S. K. Mohan)32

Slippery SkinSlippery skin of onions was first described from the United

States in 1899 and since has been reported in Australia, theUnited Kingdom, India, Indonesia, Thailand, Hungary, Spain,Bulgaria, and the former Soviet Union.SymptomsIn the early stages of the disease, the affected bulb may

not show any external symptoms except softening of the necktissue. If the bulb is cut longitudinally, one or two inner fleshyscales that are soft and have a cooked or water-soaked appear-ance are revealed. The rot progresses from the top of theinfected scales downward without spreading across to adjacentscales. The bacterium can infect other scales, and eventuallythe whole internal tissue may rot (Plate 67). In advanced stages,the affected tissue may dry out, and the bu lb may shrivel.The disease is referred to as slippery skin because the centercore of the affected bulb may slip out the top when the baseof the bulb is pressed.Causal OrganismThe bacterium that causes slippery skin is Pseudomonas

gladioli pv. alliicola (Burkholder) Young et al. It is a grarn-negative, nonfluorescent rod that does not form spores. Itcontains intracellular inclusions of poly-,B-hydroxybutyrate. Itis polarly flagellated, oxidase positive, and arginine dihydrolasenegative. It hydrolyzes gelatin but not starch and does notproduce levan from sucrose.Disease Cycle and EpidemiologyThe bacterium is primarily a wound pathogen and infects

leaves and maturing bulbs in the field or bulbs after harvest.Young, growing leaves are only slightly susceptible to thebacterium. Infection probably occurs just before or at harvesttime. The disease is usually more severe if tops are damagedby high winds or hail and subjected to wet or rainy conditionsprior to harvest. Mature bulbs are very susceptible and mayrot completely within 10 days at room temperature.ControlOnions should be harvested at proper maturity as soon as

tops lodge. Preventing injury, promptly drying bulbs aftertopping, and storing bulbs at 0-2 e will help reduce the disease.

Selected References

Burkholder, W. H. 1942. Three bacterial plant pathogens: Phytomonascaryophy/li sp. n., Phytomonas a/liico/a sp. n., and Phytomonasmanihotis (Arthaud-Berthet et Bondar) Viegas. Phytopathology32:141-149.

Roberts, P. 1973. A soft rot of imported onions caused by Pseudo-monas a/liico/a (Burkh.) Starr & Burkh. Plant Pathol. 22:98.

Sherf, A. F., and MacNab, A. A. 1986. Vegetable Diseases and TheirControl. 2nd ed. John Wiley & Sons, New York.Tesoriero, L. A., Fahy, P. c., and Gunn, L. V. 1982. First recordof a bacterial rot of onion in Australia caused by Pseudomonasg/adio/i pv. alliico/a and assoeiation with internal browning eausedby Pseudomonas aeruginosa. Australas. Plant Pathol. 11:56-57.

(Prepared by S. K. Mohan)

Sour SkinSour skin, first described in 1950, has been reported from

onion-growing areas all over the world. Losses often appearin sto red onions, but infection usuaIly begins in the field. Thedisease can be serious in individual fields, with yield lossesof 5-50%. Sour skin is primarily a disease of onions, but otherA//ium species are reported to be hosts.

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SymptomsPrirnary symptoms on onions include a slimy (but initially

firm). pale yellow to light brown decay (Plate 68) and break-down of one or a few inner bulb scales. Adjacent outer scalesand the center of the bu lb may remain firmo Externally, bulbsappear sound, but the neck region may soften after leaveshave collapsed. In advanced stages, healthy scales can slipoff during handling. Young leaves sometimes die back, startingat the tips.Causal OrganismThe cause of sour skin is the grarn-negative bacterium

Pseudomonas cepacia (Burkholder) Palleroni & Holmes, aversatile organism found as an inhabitant of soil and wateror as a pathogen of plants and animals. Bacterial cells arerods that measure 1.6-3.2 X 0.8-1.0 J . . L m ; they occur singlyor in pairs; and they are motile by means of tufts of polarflagella. Most strains produce nonfluorescent, yellowish orgreenish pigments, but the pigments may be of a variety ofcolors.

P. cepacia is capable of using a wide range of nutrients.A large number of organic compounds are used as sole carbonand energy sources for growth, including a large variety ofcarbohydrates, monocarboxylic and dicarboxylic acids, mono-alcohols and polyalcohols, aromatic compounds, amino acids,and amines. Substrates that are of diagnostic value (used bya majority of strains of P. cepacia but used only infrequentlyby other Pseudomonas species) include o-arabinose, o-Iucose,cellobiose, saccharate, mucate, sebacate, citraconate, andtryptamine. No organic growth factors are required. Cells accu-mulate poly-S-hydroxybutyrate as a carbon reserve material.

P. cepacia is obligately aerobic. The optimum growthtemperature is 30-35C. No growth occurs at 4C, and moststrains grow at 41 C. Denitrification is negative while nitrateis reduced to nitrite. It is oxidase positive and argininedihydrolase negative and can liquefy gelatin.Disease Cycle and EpidemiologyApparently, onions are relatively resistant to P. cepacia prior

to bulbing, or the environment does not become favorablefor bacterial multiplication until after bulbing. Infectiongene rally occurs through a wound when free water from rain,overhead irrigation, or flooding causes water congestion ofthe host tissue. The bacterium can gain entrance to the plantwhen onion tops are cut at harvest or through other woundsin the neck when the foliage falls over at maturity. Infectioncan also begin when water contaminated with bacterial cellsstrikes the younger upright leaves and flows down into theneck in the leaf blade axil. Young leaves are much moresusceptible than mature leaves, which are usually symptomless.Infection can remain latent in the growing onion, andsymptoms sometimes do not develop until the plant beginsto bulbo Bacteria spread more rapidly in water-soaked tissueand when temperatures exceed 30 C. Infection advances intothe bulb via the infected leaf and corresponding scale. Theinfection does not move into adjacent scales.Inoculum of P. cepacia has been associated with con-

taminated irrigation water. Splashing water from rain oroverhead irrigation may carry water- or soil-inhabitingbacterial cells onto the neck of the plant.ControlControl measures include proper maturing of the crop and

quick drying after topping and harvest. Since contaminatedirrigation water has been implicated in the spread of thepathogen, the use of recycled or irrigation runoff water shouldbe avoided. The method of irrigation has a substantial impacton the incidence of sour skin. Season-long overhead irrigationprovides a favorable environment for infection by P. cepa cia.whereas furrow irrigation results in almost complete absenceof the disease. In experimental plots, the final four or fivesprinkler irrigations were accompanied by increases in sour

skin of 150-300%. Where sour skin is a potential problem,changing from sprinkler to furrow irrigation, at least frombulbing to the end of the season, is advisable where feasible.

Selected ReferencesBazzi, C. 1979. Identification of Pseudomonas cepacia on onion bulbsin ltaly. Phytopathol. Z. 95:254-258.

Burkholder, W. H. 1950. Sour sk in, a bacterial rot of onion bulbs.Phytopathology 40: 115-117.

Kawamoto, S. O., and Lorbeer, J. W. 1972. Multiplication ofPseudomonas cepacia in onion leaves. Phytopathology 62: 1263-1265.

Kawamoto, S. O., and Lorbeer, J. W. 1974. Infection of onion leavesby Pseudomonas cepacia. Phytopathology 64: 1440-1445.

Teviotdale, B. L., Davis, R. M., Guerard, J. P., and Harper, O. H.1989. Effect of irrigation management on sour ski n of onion. PlantOis.73:819-822.

(Prepared by R. M. Davis)

Other Bacterial DiseasesAn instance of stalk and leaf necrosis in onion seed crops

caused by a strain of Erwinia herbico/a (Lohnis) Dye wasreported from South Af rica in 1981. The characteristic syrnp-tom was a rapid necrosis of seed stalks, leading to theirweakening and collapse under the weight of seed heads. Leaveswere similarly affected.Another bacterial disease of onion plants causing leaf

necrosis and rotting of the entire plant was reported fromew Zealand. The symptoms start on foliage as small, water-

soaked lesions that expand rapidly to produce a slimy, graybrown rot that may progress down to the leaf base and rotthe entire plant. The disease is favored by wet, humid ccndi-tions. Rotted plants have a strong, vinegarlike odor. Thebacterium may enter the neck of the bulb, and rotting mayprogress down one or more internal scales as water-soaked,yellow brown discolorations. In advanced cases, entire bulbsmay be completely decayed. The causal bacterium wasidentified as Pseudomonas margina/is (Brown) Stevens.A soft rot of onions grown during the cool season was

reported from Japan. Initial symptoms on leaves are small,water-soaked lesions that enlarge along the veins to leaf sheathsand eventually decay the leaf sheath and fleshy scales in thebulbo Two bacteria, Erwinia rhapontici (Millard) Burkholderand P. margina/is pv. margina/is (Brown) Stevens were shownto be the causal agents of this disease.

P. syringae pv. syringae van Hall has been reported to causeleaf necrosis in onion under certain growing conditions.

Selected ReferencesHattingh, M. J., and Walters, O. F. 1981. Stalk and leaf necrosisof onion caused by Erwinia herbicola. Plant Dis. 65:615-618.Ohuchi, A., Ohsawa, T., and Nishimura, J. 1983. Two pathogenicbacteria, Erwinia rhapontci (Millard 1924) Burkholder 1948 andPseudomonas margina lis pv. marginalis (Brown 1918) Stevens 1925,causing a soft rot of onion. Ann. Phytopathol. SOCoJpn. 49:619-626.

Wright, P. J., and Hale, C. N. 1992. A field and storage rot of onioncaused by Pseudomonas marginalis. N.Z. J. Crop Hortic. Sci.20:435-438.

(Prepared by S. K. Mohan)

Yeast Soft RotYeast soft rot of onions has been reported only from south-

eastern Washington and northeastern Oregon. The pathogenhas a wide geographic distribution.

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