Casual Agent:
Erwinia cartovora pv. Cartovora
Distribution:
Worldwide
Symptoms:
Soft Rot is mainly a problem on mature bulbs. Affected scales first
appear water-soaked and pale-yellow to light brown or gray. As the
rot progresses, invaded fleshy scales become soft and sticky with
the interior of the bulb breaking down. A watery, foul smelling
thick liquid can be squeezed from the neck of diseased bulbs.
Conditions for Disease Development:
Bacterial soft rot is most common on onions in storage or transit;
however, this disease can develop on onions in the field before
harvest, after heavy rains and 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.
Entry into the bulbs is only though wounds such as those caused
by transplanting, mechanical injuries or sunscald. Also, onion maggots
can carry bacteria and introduce them while feeding. This disease
is favored by warm, humid conditions with optimum temperature range
of 20 to 30 degrees C. However, during storage or transit, it can
develop when temperatures are above 3 degrees C.
Control:
Avoid overhead irrigation where possible, and control insect pests
such as onion maggot. 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, and provide the
appropriate temperature and humidity with good ventilation to prevent
moisture condensation from forming on the bulbs.
