Tuesday, 9 April 2019

Onion sour skin

Onion sour skin

Pathogen: Pseudomonas (Burkholderia) cepacia

Synonyms: Burkholderia cepacia
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 in onions, but other Allium species are reported to be hosts.

Field symptoms often appear when one or two infected inner leaves turn light brown or yellow. A watery rot develops at the base of the leaves and proceeds to the neck, allowing the leaves to be easily pulled from the bulb. As the disease progresses, the outer bulb scales are infected. Infected scales develop a slimy pale yellow to light brown decay and may separate from adjacent scales, allowing the firm center scales to slide out when the bulb is squeezed. Infected bulbs often have an acrid, vinegar-like odor due to secondary invaders, especially yeasts, colonizing decaying bulbs. 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 healthy, but the neck region may soften after leaves have collapsed. In advanced stages, healthy scales can slip off during handling. Young leaves sometimes dieback, starting at the tips.

Yellowing of inner leaves and separation of bulb scales due to sour skin disease.

A rot of bulb scales usually occurs at or near maturity and sometimes in storage. The bacterium does not appear to be strongly invasive, attacking plants that are damaged or weakened. Kawamoto and Lorbeer (1974) found that Burkholderia cepacia infected only wounded onion tissue in artificial inoculation tests. Inoculation with wounding resulted in lesion formation, but spray inoculations, without wounding, were unsuccessful. Lesions expanded very slowly in the absence of free moisture, and the rate of lesion development increased with surface leaf moisture and/or increasing temperature to 32°C.

In earlier experiments, a decrease in inoculum concentration from 100 million to 1000 cells/mL delayed initial symptom expression from 1 to 5 days in young onion leaf blades injected with Burkholderia cepacia (Kawamoto and Lorbeer, 1972b). As cell numbers increased to over 1000 million cells/mL, the first symptoms of water soaking and wilting became evident. Bacterial cells were found to have migrated through the intercellular spaces and formed large masses (Kawamoto and Lorbeer, 1972a). Maintenance of moisture allowed the formation of small groups of bacteria in the intercellular spaces of both blade and sheath. Host cells, except xylem vessels and epidermal cells, were compressed by the bacterial masses, and the large parenchyma cells in the blade were the first to collapse. Later, the more compact parenchyma near the periphery of the blade collapsed and tissues became macerated. In the sheath, bacteria were abundant among the loosely organized parenchyma adjacent to the adaxial epidermis.

In the intercellular spaces of the closely packed parenchyma, beneath the abaxial epidermis, bacteria were usually restricted to dense, compact masses. Later it was found that bacterial numbers declined and none could be reisolated after leaves became dry and brittle. From these experiments, it was concluded that air-dried, diseased leaves were unlikely to be the source of inoculum in the field. These findings were in part confirmed by Cother and Dowling (1985), and they concluded that bacteria were likely to gain entry through the neck or leaf blades as the foliage falls over, and the epidermis breaks, at maturity. Infections were thought to arise from contaminated irrigation water, and the expression of symptoms was heightened in hot weather.

Individual leaves affected by sour skin wilt and dieback. Internally, leaves develop a soft, watery rot. The fleshy scales associated with infected leaves rot to form a tan-colored slimy ring in the bulb. Adjacent rings may remain healthy. The neck of infected bulbs is soft when pressed.

Pseudomonas cepacia is obligately aerobic. The optimum growth temperature is 30°C–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 Epidemiology
Apparently, 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 the corresponding scale. The infection does not move into adjacent scales.

Disease cycle of onion sour skin caused by Pseudomonas cepacia.

Pseudomonas cepacia is commonly spread by heavy rains, overhead irrigation, and flooding, which splash the bacteria onto young or wounded foliage. Infection typically occurs through wounds, including those made when onions are cut at harvest. Infection can also occur when water lands on upright leaves and flows into leaf blade axils carrying the bacterium with it. Sour skin is favored by rainstorms and warm weather and develops rapidly at temperatures above 30°C.

Disease Management
Cultural Practices
Do not damage foliage prior to harvest or bulbs during harvest because P. cepacia enters the plant primarily through wounds. Onion crops should be harvested at maturity and the bulbs dried quickly. Storing onions at cool temperatures, 0°C, with adequate ventilation to prevent condensation on the bulbs will reduce storage losses resulting from this disease.

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. Studies from California, United States, have shown that P. cepacia can be controlled successfully by substituting furrow irrigation for overhead sprinkler irrigation (Teviotdale et al., 1990). Earlier work had shown that disease incidence was greater in plots irrigated by overhead sprinklers than in plots irrigated by furrow the entire season or irrigated by sprinklers until bulbing and then by furrow (Teviotdale et al., 1989). There was no correlation between the amount of water applied and incidence of disease. Similarly, there was no significant difference in the number of rotted onions between two planting densities.

Contaminated water is thought to be a major source of disease as disease incidence was found to increase when surface drainage water was used for irrigation as opposed to water drawn from deep wells (Irwin and Vaughan, 1972).

Economic Impact
In general, P. cepacia is not considered to be a serious pathogen of onion or other crops. While yield losses of 5%–50% have been noted (Schwartz and Mohan, 1995), the disease is sporadic, particularly in maturing bulbs or in storage (Cother and Dowling, 1985; Jaccoud Filho et al., 1987).

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