Bacterial spot of capsicum
What to expect from resistant varieties

Allen Hibberd and Heidi Martin, Department of Primary Industries and Fisheries, Queensland.

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Figure 1.  When about 1% of a leaf is diseased with  bacterial spot the leaf turns yellow and falls

Introduction

Bacterial spot (caused by Xanthomonas campestris pv. vesicatoria) is the most destructive leaf disease of capsicum. Disease progress can be very rapid in warm, damp weather. Few, if any marketable fruit can be expected from plants diseased like the one in Figure 1. A crop that badly affected  will be a major source of inoculum and the disease may rapidly spread into neighouring crops.

If most of your plants are like the picture below, think about ploughing the crop out and replanting in another block, consider inputs so far, future costs and likely returns. An alternative is to ratoon (cut back) your crop, however this is not usually practical in commercial production. Destroy all the foliage you remove.

Plant breeders have used bacterial spot resistance genes to develop resistant varieties. Advertising from seed companies rightly extols the virtues of resistance to bacterial spot, but what can be expected from these varieties when grown in the field?

Races of bacterial spot found in Queensland

Worldwide, there are 11 known races of bacterial spot that affect capsicum. Not all of these have been found in Queensland.

In 1992 and 1997 Queensland capsicum crops were surveyed to determine the races of bacterial spot present and four races were found. Following a standard method of naming, these races are: Race 1, Race 3, Race 4 and Race 5.

Race 1 was the most common and it was the only race found in commercial crops of varieties susceptible to all races of bacterial spot. Race 3 was found in some commercial plantings of varieties resistant to Race 1.

Race 2 is common in southern USA but was not found in Queensland.

What happens when you plant resistant varieties?

Seed companies may sell varieties of capsicum that are resistant to some of the races of bacterial spot. Table 1 shows the resistance and susceptibility you can expect from varieties with resistance to various races of bacterial spot.

Table 1. Resistance and susceptibility to various races of bacterial spot

No variety has resistance to all races of bacterial spot, but to be effective, the variety used must have resistance to all the races present.

Based on current knowledge, there is no advantage in growing a variety of capsicum resistant to Race 2 only, because Race 2 has not been found in Queensland.

The races of this disease are unstable, meaning that the disease is prone to mutate and rapidly produce new races. Research has proved that Race 1 can quickly mutate to Race 3, Race 4 or Race 5. In the field, these mutations can happen within one season. Therefore, if you grow a variety resistant to Race 1 only, it may become diseased with Race 3 of bacterial spot in warm, damp weather. This applies in Queensland and elsewhere.

Some seed companies also sell varieties resistant to Races 1, 2 and 3, but past experience is that these soon became diseased with either Race 4 or Race 5. The speed with which Race 4 or Race 5 develops may not be as fast as the speed with which Race 3 develops, but the end result will still be diseased capsicum plants.

What hope is there?

There is a form of resistance to bacterial spot known as quantitative resistance. This resistance acts against all races, regardless of what the races are called.

This resistance is far more difficult for plant breeders to work with, so there has been less advance in breeding. But progress has been made and growers should demand that priority be given to breeding capsicums with this resistance. (Capsicum is now among the eight most popular vegetables consumed in Australia.)

In the meantime, by knowing which races of bacterial spot predominate, growers will be in a better position to select from seed companies those resistant varieties most relevant to their areas.

An integrated management program is essential

Use as many of the following practices as possible in an integrated program to manage bacterial spot.

Seed management

• Buy seed from a reputable dealer, ask the seed company for seed that has been tested for bacterial spot. Infected seed can be an important source of the disease in capsicums.
  
  
• Seed can be treated with hot water or calcium hypochlorite to kill the pathogen. Hot-water treatment is more thorough than calcium hypochlorite because it can kill bacteria inside the seed as well as those on the surface. 
  
  
• Take care when treating seed with hot-water as high temperatures can reduce seed germination if proper precautions are not taken. Plant seed as soon as possible after treatment. If you treat the seed after purchase, the seed company's liability and guarantees are null and void.

Farm hygiene

• Destroy old capsicum plants immediately after final harvest.
  
  
• Do not allow volunteer capsicum or tomato plants to grow between seasons or amongst cover crops, because they can harbour the disease.
  
  
• Control solanaceous weeds such as nightshades in and around the crop.
  
  
• If disease becomes widespread decide quickly about ploughing out the crop.

Crop management

• Do not rotate capsicum crops with tomato, potato or eggplant and do not grow these crops together.
  
  

• Physically separate successive plantings. When space is limiting, successive plantings in the same field can be separated by planting several rows of a plant that will form a tall, thick barrier, such as sorghum, sudan grass or sweet corn.

• Provide appropriate amounts of fertiliser and water. Plants that are stressed or those that are growing too luxuriantly can be more susceptible to disease. Low nitrogen and potassium, and extra high magnesium, calcium or nitrogen levels have been associated with increased susceptibility of crops to bacterial spot.

• Use drip rather than overhead irrigation. Water splashed during overhead irrigation spreads the pathogen. When overhead irrigation is the only option, water during the day with enough time to ensure that the plants are dry by nightfall.

Disease management

• Monitor crops thoroughly for bacterial spot and other diseases each week.

• Avoid working fields when plants are wet. As well as movement by splashing water drops, the pathogen can be spread mechanically on workers hands and on farm machinery when plants are wet. Work in infested areas last if possible.
  
  
• Thoroughly spray crops with copper fungicides at recommended rates. In Queensland, populations of bacterial spot have developed tolerance to copper sprays which means that copper is now less effective than it once was in controlling bacterial spot infections. 
  
  
• Disinfect spray rigs and other machinery with a general disinfectant product, such as a quaternary ammonium compound, after the job is completed or before using the equipment in uninfected crops.

• Report any disease control problems to DPI&F so that possible causes can be investigated.

Further information

The following DPI&F Notes provide more information. 

• Growing Capsicum and Chilli: Before you start
  
  
• Growing Capsicum and Chilli: Common questions.

DPI&F information and services

• To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call, from 8 am to 6 pm Monday to Friday (excluding public holidays).  E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.

• Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.

Industry links

• Industry links is a page of links to various sites of interest to horticultural growers.

Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

Last updated 28 May 2007