Casual Agent:
Pseudomonas viridiflava
Distribution:
USA (Georgia, Florida)
Symptoms:
The first symptoms are oval, water soaked leaf lesions along with
tip-burn and streaks of varying length. The leaf streaks are green,
eventually darkening to black. Infection may become more severe and
spread down the leaf, causing collapse and drying of whole leaves.
Leaves may also become distorted and twisted. Bulb infection is characterized
by dark spots on outer scales and reddish-brown discoloration of inner
scales. Symptoms often develop in a ring-like pattern due to restriction
of the rot by the scales.
Conditions for Disease Development:
Disease occurs mainly in the winter and spring when temperatures are
cool. Epidemics are associated with prolonged periods of rain which
favor progression of the disease. Excess fertilizer stimulates disease
development, and it is thought that frost damage may predispose onion
plants to infection.
Control:
Applications of fixed copper compounds or streptomycin inhibit spread
of this disease, although bacterial strains resistant to copper may
occur. Excessive fertilizer applications may increase foiar symptoms
and should be avoided. Reduction of post harvest rot may be achieved
by harvesting onions at full maturity, by proper curing of bulbs with
forced hot air and by reducing wounding and bruising.
