Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001688
11/12/99

Bacterial Blights


Symptoms: The sudden death of flowers, buds, leaves, young
fruits (Vis. 1) and succulent shoots characterizes a blight
disease. The blighted leaves, fruits and flowers are
black and shriveled and look as though they were scorched
with fire. They remain clinging to the shoots. Shoots
often have a bent, drooped tip resembling a shepherd's
crook.

Cause: Bacteria generally cause blights during wet weather
in early spring when buds are breaking. Cool spring
temperatures favor some blights and warm temperatures
favor others. Fire blight of crab apples, for example, is
favored by warm springs, and bacterial blight of lilac is
favored by cool springs. In wet spring weather, cankers
(Vis. 2) formed during previous infections ooze droplets of
bacteria. The bacteria are splashed by rain and blown
about as mists, or insects may visit the attractive ooze
and carry it about. Infection usually begins either
through nectaries of flowers or through the microscopic
breathing holes (stomates) in leaves. In lilac bacterial
blight, typical blight symptoms may occur on new growth.
When infection occurs later in spring when leaves are
more mature, the bacteria cause a leaf spotting disease.

Control: Avoid using high nitrogen fertilizers on
susceptible cultivars. Only fungicides containing copper
may be effective against bacterial blights and bacterial
disease, but copper may burn foliage on some cultivars.


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