|Bacterial Leaf Spot Control Strategies
By Steve Ansermino, Dr. W.
Schreuder, Dr. P. Grobler
Bacterial leaf spot (BLS) has been a
major concern for growers of capsicum and tomato throughout Australia. It
is caused by the bacterium Xanthomonas vesicatoria (Xanthomonas campestris
pv. vesicatoria). Seven races of the pathogen have been described. Up to
10 are suspected, but only races 1 and 3 have been isolated and recorded
in Queensland. Race 1 is usually the most prevalent in recent outbreaks.
pathogen affects leaves and fruit of capsicum and tomato. Symptoms first
appear on lower surfaces of leaves as small (up to 3 mm), dark, irregular
water- soaked areas. These lesions become angular and the surface may
appear greasy with a translucent center and black margin. The lesions
enlarge to approximately 6mm in diameter, may dry and crack and develop a
yellow halo. Spots on upper surfaces are sunken, and lower surfaces are
raised. Lesions tend to have scab- like appearance. Leaves may tear and
become twisted due to uneven marginal leaf growth. Many leaves may drop
off, exposing fruit to sunburn. Lesions tend to be more numerous on the
young foliage. During prolonged wet periods leaves will take on a blighted
appearance rather than the typical leaf spots. The primary source of
infected plants is seed and seedlings, although the pathogen can persist
for about 1 year on diseased residues, or longer on volunteer plants or
The disease is spread by splashing
water and by implements or workers in the field when the plants are wet.
Disease development is promoted by moist, windy conditions. Infection
generally occurs through mechanical wounds by insects, wind driven sand
and rain, and by high pressure spraying. Warm (24- 30÷ C) temperatures and
sprinkler irrigation or heavy rains favour disease development. Overcast
weather also reduces calcium uptake. This in combination with warm, humid,
moist conditions is ideal for bacterial diseases.
CALCIUM & COPPER
Blossom end rots are common in varieties
which have poor calcium uptake. This would be aggravated by cold
conditions i. e. Ca is taken up passively with transpirational flow. Sandy
soils and ammonium or urea based fertilizers reduce soil calcium
availability. In California, trials on bell pepper disease tolerance have
shown highest resistance in varieties most efficient at calcium uptake.
Calcium deficiency results in weaker cells more susceptible to wind damage
Copper fungicides such as Liquicop serve
as a method to decrease and inhibit bacterial populations on foliage
surfaces. The infection period (time between infection and symptom
expression) normally varies between 7- 14 days for BLS. However, these
bacteria can live epiphytically (i. e. on the leaf surface) on their host
for long periods. As soon as the climatic conditions are favorable the
population increases, infection takes place with symptoms expressed 7 - 14
days later. Latent infections may also occur, meaning that the bacteria
infect without expressing any symptoms.
To control these bacterial diseases, farmers need to spray copper in a
preventive program. We have numerous examples on tomatoes, peppers and
beans where growers start to apply copper (including Liquicop) before and
after rain, resulting in significant differences in control. When the
copper was applied before rain, the epiphytic population was decreased and
infection potential is far less than when compared to spraying copper
after rain (infection period). Remember once infection has taken place,
copper sprays will decrease the epiphytic population and thereby inhibit
the spread of the disease only. However, if the disease pressure is very
high the percentage decrease in bacterial population caused by the copper
spray, may not even have an effect on visible disease control since the
remaining population is still too high.
The foundation of bacterial control is
sanitation. Once BLS has gained a foothold, it is very difficult to
eradicate. Exclusion and prevention are most important.
use of disease free seed and transplants is important for the early
control of BLS. Varieties such as Rubix, Lisa, Helix, FA769 and FA2001
have shown strong tolerance in the field.
With high volume applications under high disease pressure, 500ml/ 100L
Liquicop applied as a preventative spray will give a strong barrier to
infection. Increasing the spraying volume does not necessarily mean better
results. Spray volumes of 500L/ ha with directed spraying is recommended
as long as good coverage is achieved, although volumes up to 1000L/ ha are
Copper sprays can provide moderate levels of protection, but must be
sprayed as a preventative and not a curative i. e. from nursery into
paddock. Include mancozeb according to DPI off- label permit 70027 (QLD
only) or NRA permit 3430 (all except Vic). Spray at 7 day intervals until
transplanting and then at a maximum of 14 day intervals. Spray more
regularly if prolonged wet weather occurs. Attempt to anticipate wet
weather and spray before it. As a preventative spray Liquicop is far
better and more friendly than other coppers. Liquicop and mancozeb sprayed
together has shown superior levels of control versus other crystalline
Addition of 300ml/ ha Nu- Film- 17 to preventative sprays will also
enhance distribution and adhesion of chemicals on the leaf, especially
during wet weather. The resin base also artificially thickens the cuticle
and help seal superficial cracks in the leaf helping reduce disease
Use calcium foliar sprays (chelates especially) with Nu- Film- 17 to
Avoid using overhead irrigation.
Avoid contact by picking or spraying (esp. high pressure) when the plants
Rotate with non- host crops and control volunteer plants and solanaceous
Good sanitation practices including cleaning all equipment used in
diseased fields, ploughing in all debris immediately after harvest.
Sanitation of equipment can be done safely and effectively using Sporekill.
more advanced method of long term control involves interplanting tolerant
and resistant varieties. Because of the current use of monoculture and
uniform genotypes within pepper fields, the disease spreads rapidly once
the pathogen is present and environmental conditions remain favourable.
Variety mixtures are believed to present the pathogen with an evolutionary
Vaporgard may also be used to protect fruit from sunburn in cases where
BLS has caused defoliation. Typical leaf damage without being treated