Tuesday, 9 April 2019

Bacterial disease of onion


Bacterial disease of onion

ONION

Bacterial soft rot in onion

Pathogen: Erwinia carotovora Subsp. carotovora, E. chrysanthemi, Pseudomonas gladioli, and Enterobacter cloacae

Synonyms: Pectobacterium carotovora subsp. carotovora, Dickeya chrysanthemi Bacterial decay, known as soft rot, is one of the most widespread and destructive storage diseases in onion. Soft rot generally starts in the field just before or during harvest. The disease causes serious losses to onions that have been wounded in the field or improperly handled and stored. Bacterial soft rot pathogens have very broad host ranges and can attack many vegetables, such as carrot, potato, cabbage, and lettuce. Infection generally requires a wound caused by heavy, wind-driven rain, hail, insects, or cut necks during harvest. Splashing water, aerosols, contaminated equipment and workers, and insects spread soft rot bacteria. Bacterial soft rot pathogens are commonly found in and easily disseminated by irrigation water. Erwinia spp. survive between onion crops in soil, crop debris, and pathogenically on other crops.

Bacterial soft rot is a major postharvest disease in onion bulbs and causes serious problems for growers and packers in New Mexico. Although several types of bacteria are associated with postharvest decay through secondary infection, only Pectobacterium carotovorum and Dickeya chrysanthemi produce pectolytic enzymes and can enter a bulb without preexisting injury to cause the specific disease known as “soft rot.” Bacterial soft rot is most prevalent during the rainy months of July and August or anytime when wet conditions prevail before or during harvest. 

Bacterial soft rot is a major postharvest disease in onion bulbs and causes serious problems for growers and packers in New Mexico. Although several types of bacteria are associated with postharvest decay through secondary infection, only Pectobacterium carotovorum and Dickeya Chrysanthemi produce pectolytic enzymes and can enter a bulb without preexisting injury to cause the specific disease known as “soft rot.” Bacterial soft rot is most prevalent during the rainy months of July and August or anytime when wet conditions prevail before or during harvest.

Symptoms

Symptoms of bacterial soft rot often appear as a soft, watery rot of individual scales that may advance and rot the entire bulb. A foul-smelling viscous fluid oozes from the neck when infected bulbs are squeezed. In the field, the youngest leaves or the entire foliage of affected plants appear bleached and wilted. Yield losses can be significant in the field and storage.

onion foliage
Symptoms on onion foliage, their collapse, and early season soft rotting in onion bulb due to soft rot bacteria.

Bacterial soft rot is mainly a problem with mature bulbs. Affected scales first appear water-soaked and pale yellow to light brown when infected by D. Chrysanthemi or bleached gray to white when infected with Pectobacterium Carotovorum Subsp. carotovorum. As the soft rot progresses, invaded fleshy scales become soft and sticky with the interior of the bulb breaking down. A watery, foulsmelling thick liquid can be squeezed from the neck of diseased bulbs.

Onion bulbs in the field may be affected before harvest, but infected bulbs usually go unnoticed until after harvest. Bacterial soft rot usually starts at the neck of the bulb and progresses downward along with one or more scales. Initially, the tissue is water-soaked and later it disintegrates into a soft, slimy mass. The decay does not spread readily from scale to scale. One or two scales may be completely rotten, while the remainder is fine. Eventually, the diseased bulbs can be detected by gently squeezing them, whereupon a watery fluid is exuded. An offensive sulfurous odor is usually associated with the liquid. Although these bacteria can directly enter plant tissue, existing wounds may intensify the infection rate. The pathogen also requires moisture to infect plant tissues. Therefore, susceptibility increases if bulbs have mechanical injuries, sunscalds, or bruises, especially when stored under warm, humid conditions.

Geographical Distribution

Worldwide

Disease Cycle

Bacterial soft rots are a primary problem in onions, but not in garlic. Free water is essential for the entry and spread of the bacteria. Wounds and senescent leaves are the means by which bacteria gain entrance into the bulb. The pathogens are soil-borne and may spread via irrigation water. The bacteria enter only through wounds created by insect feeding, or bruising, during harvest or packing. The disease spreads in storage if moisture is allowed to develop on bulb surfaces. Soft rot bacteria commonly exist in the soil and plant refuse. They enter onions through wounds and aging tissue under moist conditions. Excessive irrigation during hot weather seems to favor a high incidence of soft rot.

Onion maggots are particularly effective in spreading the disease. They feed on bulbs, creating wounds for bacteria to enter. The bacteria may persist in the intestinal tract of the onion maggot larvae and adult flies and thus can be carried from one place to another. Other sources of wounds that serve as an entry for bacteria include hail damage in leaves and developing bulbs, sunscald, and freezing. During harvest, the practice of cutting the onion tops creates openings for the bacteria. Also, bruising or other damage from mechanical activity leaves bulbs susceptible to infection, particularly if they are stored in warm, humid conditions.

Conditions for Disease Development

Bacterial soft rot is most common in onions in storage or transit; however, this disease can develop in onions in the field before harvest, after heavy rains, and also when leaves are drying. The main sources of inoculum are contaminated soil and crop residues. The bacteria are spread by splashing rain, irrigation water, and insects. This disease is favored by warm, humid conditions with an optimum temperature range of 20°C–30°C. However, soft rot can develop during storage or transit at temperatures above 3°C.

Disease Management

Cultural Practices
Avoid overhead irrigation once onions start to bulb (bulbing occurs about the time the bulb is twice the diameter of the neck) and control insect pests, such as onion maggots. Harvest only after onion tops are well matured. Provide for quick drying following topping, especially if temperatures are high.

Practice a 3-year or longer rotation to nonhosts such as small grains. Avoid reuse of irrigation tailwater and overhead irrigation. Prevent bruising and wounding of plants and bulbs during field operations, harvest, and handling. Control onion maggots and other insects that can vector soft rot bacteria. Sever all roots during lifting to promote rapid drying of foliage and necks before topping.

 bacterial soft rot
Disease cycle of bacterial soft rot in onion caused by Erwinia carotovora pv. carotovora.

Cure bulbs with abundant ambient air until necks are completely dry before storing bulbs at 0°C-3°C and 70% or less relative humidity. Disease spread and infection may be reduced by copper-based bactericides. Allow onion tops to mature before harvesting and avoid damaging bulbs during harvest. Store onion bulbs only after they have been properly dried under appropriate temperature and humidity with good ventilation to prevent moisture condensation in the bulbs.

The first step in controlling soft rot is to control other diseases and avoid injury to the plants. This includes controlling insects such as onion maggots that wound plants and transmit the bacteria. Allow the crop to mature completely before harvesting so that the neck areas are dried and they act as a barrier for bacterial movement from infected leaves to the bulbs. The tops should be dried out as much as possible after lifting and before topping. Mechanical toppers that cut the neck about ½ in. above the bulb should be designed to minimize bruising. Roller toppers should not be used because they rip and tear the neck leaves. At all stages of harvest and storage, onions should be handled carefully to avoid bruising.

Cure bulbs thoroughly so that the outer scales and neck tissues are completely dry. This is usually done with forced air in storage bins. Remove bulbs that show any sign of disease or injury before storage. There are no soft rot-resistant onion varieties available. Storms with hard rains, high winds, and hail cause wounds on leaves and bulbs that serve as entry points for bacteria. If these wounds, within hours after they occur, are protected with sprays containing copper-based materials, they will likely heal rapidly and infection will be reduced. Treatments after the appearance of symptoms will not control bacterial soft rot.

Chemical Control

In Colorado, copper bactericides provide some bacterial control when applied before disease onset. Sprays should be initiated 2 weeks before bulb initiation and continued on a 5–10-day spray interval depending on weather conditions. Apply in a sufficient volume of water to ensure thorough coverage. Include a low rate of a nonionic surfactant to further improve coverage.

Copper-tolerant strains of the pathogens are common in the United States. Tank-mixing copper bactericides with a low rate of an Ethylenebisdithiocarbamates (EBDCs) fungicide such as maneb is essential for effective disease suppression. Tank-mixing coppers with zinc or iron can also enhance their activity.

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