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.

The 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 solanaceous weeds.

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 and leaching.

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.

CONTROL STRATEGIES

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.

1.The 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.

2. 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 commonly used.

3. 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 coppers.

4. 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 incidence.

5. Use calcium foliar sprays (chelates especially) with Nu- Film- 17 to enhance uptake.

6. Avoid using overhead irrigation.

7. Avoid contact by picking or spraying (esp. high pressure) when the plants are wet.

8. Rotate with non- host crops and control volunteer plants and solanaceous weeds.

9. 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.

10. A 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 dilemma.

11. Vaporgard may also be used to protect fruit from sunburn in cases where BLS has caused defoliation. Typical leaf damage without being treated