|
Damage Symptoms
Xanthomonas leaf
spot occurs on all cultivated crucifers and several wild crucifers.
The major symptom is leaf spotting, but lesions can also occur
on cotyledons, flower stalks, and fruit. Small leaf spots occur
scattered over the leaf surface as a result of invasion of bacterium
through pores.
Individual lesions begin as water-soaked
flecks that enlarge to form lesions about 3 mm (occasionally
5 mm) in diameter that are surrounded by a narrow chlorotic zone
that is most evident when leaves are viewed with back lighting.
Lesions tend to be more numerous near large leaf veins where
they may coalesce to form necrotic streaks along the sides of
the veins. Dry, dead tissue may fall out of the lesions giving
the leaves a shot-hole or tattered appearance. Leaf spots can
be so numerous at times that the entire leaf will collapse.
Conditions for Development
The bacterium persists
in debris from infected plants in the soil, but it does not survive
in the soil after the crop debris has deteriorated. The bacterium
can also survive on other crucifer crop and weed plants, and
it is seedborne. Extended periods of leaf wetness are essential
for disease development which can occur over a wide range of
temperatures. Heavy dews of long duration during cool weather
and also frequent rains during hot weather can result in extensive
disease development. The bacterium can be spread from plant to
plant by splashing rain or by people and equipment moving through
the field and making contact with the plants while they are wet.
Control
Rotate with non-cruciferous
crops or allow sufficient time for decomposition of debris from
the previous crucifer crop before replanting. Use pathogen-free
seed produced in an arid climate without overhead irrigation.
Avoid working in the fields when the foliage is wet. Resistant
cultivars are not available.
|
