Sour skin, first described in 1950, has been reported from onion-growing areas all over the world. Losses often appear in stored onions, but infection usually begins in the field. The disease can be serious in individual fields, with yield losses of 5–50%. Sour skin is primarily a disease of onions, but other Allium species are reported to be hosts.Symptoms Primary symptoms on onions include a slimy (but initially firm), pale yellow to light brown decay and breakdown of one or a few inner bulb scales. Adjacent outer scales and the center of the bulb may remain firm. Externally, bulbs appear sound, but the neck region may soften after leaves have collapsed. In advanced stages, healthy scales can slip off during handling. Young leaves so metimes die back, starting at the tips. (Click here to see an enlarged image).
Causal OrganismThe cause of sour skin is the gram-negative bacterium Pseudomonas cepacia (Burkholder) Palleroni & Holmes, a versatile organism found as an inhabitant of soil and water or as a pathogen of plants and animals. Bacterial cells are rods that measure 1.6–3.2 × 0.8–1.0 µm; they occur singly or in pairs; and they are motile by means of tufts of polar flagella. Most strains produce nonfluorescent, yellowish or greenish pigments, but the pigments may be of a variety of colors.
P. cepacia is capable of using a wide range of nutrients. A large number of organic compounds are used as sole carbon and energy sources for growth, including a large variety of carbohydrates, monocarboxylic and dicarboxylic acids, monoalcohols and polyalcohols, aromatic compounds, amino acids, and amines. Substrates that are of diagnostic value (used by a majority of strains of P. cepacia but used only infrequently by other Pseudomonas species) include d-arabinose, d-fucose, cellobiose, saccharate, mucate, sebacate, citraconate, and tryptamine. No organic growth factors are required. Cells accumulate poly-b-hydroxybutyrate as a carbon reserve material.
P. cepacia is obligately aerobic. The optimum growth temperature is 30–35°C. No growth occurs at 4°C, and most strains grow at 41°C. Denitrification is negative while nitrate is reduced to nitrite. It is oxidase positive and arginine dihydrolase 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 favorable for bacterial multiplication until after bulbing. Infection generally occurs through a wound when free water from rain, overhead irrigation, or flooding causes water congestion of the host tissue. The bacterium can gain entrance to the plant when onion tops are cut at harvest or through other wounds in the neck when the foliage falls over at maturity. Infection can also begin when water contaminated with bacterial cells strikes the younger upright leaves and flows down into the neck in the leaf blade axil. Young leaves are much more susceptible than mature leaves, which are usually symptomless. Infection can remain latent in the growing onion, and symptoms sometimes do not develop until the plant begins to bulb. Bacteria spread more rapidly in water-soaked tissue and when temperatures exceed 30°C. Infection advances into the bulb via the infected leaf and corresponding scale. The infection does not move into adjacent scales.
Inoculum of P. cepacia has been associated with contaminated irrigation water. Splashing water from rain or overhead irrigation may carry water- or soil-inhabiting bacterial cells onto the neck of the plant.
ControlControl measures include proper maturing of the crop and quick drying after topping and harvest. Since contaminated irrigation water has been implicated in the spread of the pathogen, the use of recycled or irrigation runoff water should be avoided. The method of irrigation has a substantial impact on the incidence of sour skin. Season-long overhead irrigation provides a favorable environment for infection by P. cepacia, whereas furrow irrigation results in almost complete absence of the disease. In experimental plots, the final four or five sprinkler 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 from bulbing to the end of the season, is advisable where feasible.
1. Bazzi, C. 1979. Identification of Pseudomonas cepacia on onion bulbs in Italy. Phytopathol. Z. 95:254-258.
2. Burkholder, W. H. 1950. Sour skin, a bacterial rot of onion bulbs. Phytopathology 40:115-117.
3. Kawamoto, S. O., and Lorbeer, J. W. 1972. Multiplication of Pseudomonas cepacia in onion leaves. Phytopathology 62:1263-1265.
4. Kawamoto, S. O., and Lorbeer, J. W. 1974. Infection of onion leaves by Pseudomonas cepacia. Phytopathology 64:1440-1445.
5. Teviotdale, B. L., Davis, R. M., Guerard, J. P., and Harper, D. H. 1989. Effect of irrigation management on sour skin of onion. Plant Dis. 73:819-822.
(Prepared by R. M. Davis)RETURN TO APSnet FEATURE STORY
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