Copper resistance in bacterial leaf spot of tomato and pepper crops has been detected at a high level in New Jersey the past few summers. While not surprising, copper resistance has been known to develop for decades now. Copper applications for the control of bacterial diseases in many crops has been a mainstay for decades now and is often applied in weekly protectant fungicide programs. With help from Dr. Nrupali Patel and Dr. Don Kobayashi, bacteriologists in the Department of Plant Biology located on the New Brunswick campus, a (NJ-VGA funded) survey was initiated to determine which species of bacterial leaf spot are most prevalent in New Jersey tomato and pepper crops. Bacterial leaf spot can be caused by four species of Xanthomonas: X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri. Currently, there are four races of BLS found in tomato (T1-T4; one for each of the 4 species stated above) and eleven races found in pepper (0-10). Differential tests in southern New Jersey using various bell pepper lines over the past 15 years has suggested that the number of races of BLS in pepper has increased over time; with all races present in the State to date. Lab testing results from samples collected from the small number of NJ vegetable farms the last three summers has shown the presence of X. euvesicatoria in pepper, as well as X. euvesicatoria and X. perforans in both tomato and pepper in the state, with ~60% of all samples testing positive for copper resistance.
How do you know what species of bacteria are present on your farm?
The only way to determine which species of bacteria are present in tomato or pepper crops on your farm are to have them identified through laboratory methods.
How do you know if copper resistance is present on your farm?
Growers who have used copper applications for controlling bacterial leaf spot in crops such tomato or pepper for many years should always monitor for efficacy. If you notice or have noticed a loss in copper efficacy over time, then there is a good chance copper resistance is present. Once copper resistance is detected, further applications will be unwarranted and ineffective. The only method to truly determine if copper resistance is present is through laboratory testing, however growers who pay close attention to efficacy should have a good idea if copper is still effective.
What can you do to mitigate bacterial leaf spot development on your farm?
In crops such as bell pepper, it comes down to growing cultivars with resistance to BLS and knowing what races are present on your farm. Many of the recommend commercial cultivars have varying resistance packages to the different races of the pathogen. Some cultivars, such as Paladin which has Phytophthora resistance has no resistance to BLS. Other “older” cultivars such as Aristotle X3R has resistance to races 1-3; newer cultivars such as Turnpike has resistance to races 0-5,7-9; while cultivars such as Playmaker and 9325 have resistance to 0-10 (also known as X10R cultivars). Unfortunately, BLS resistance in commercial tomato varieties are lacking, but efforts from around the world are making progress.
Got BLS?
If Bacterial leaf spot is already present on your farm this growing season, growers should 1) monitor the efficacy of their copper applications. If the lack of control is apparent, there is a good chance copper resistance is present; 2) Stop tying. Tying (or excessive tying) creates wounds on the plant which create entry points for bacteria and can quickly escalate the spread; 3) Work in plantings or blocks with BLS last each day if possible. Start harvesting in the cleanest plantings first, then work towards the worst.
Prepare now for next season
The easiest way to mitigate losses from BLS year after year is to switch to bell pepper varieties with X10R resistance packages where you can. Unfortunately, the lack of BLS resistance in most non-bell peppers makes controlling the disease more difficult. Some growers across the state have adopted the use of hot water seed treatment to help mitigate potential problems with BLS.
Does the hot water seed treatment work against viruses?
Hot water seed treatment has been reported to be effective against Cucumber mosaic virus, Pepper mild mosaic virus, Tobacco mosaic virus, Tomato mosaic virus in pepper and tomato. Unfortunately, the Tomato spotted wilt virus and Tobacco streak virus are not known to be seed-borne, and vectored by infected thrips. Tobacco streak virus which was just found in NJ.
Hot Water Seed Treatment Method – Something the think about now for next year!
Hot water seed treatment is a non-chemical alternative to conventional chlorine treatment which only kills pathogens on the surface of the seed. Heat-treatment done correctly kills pathogens inside the seed as well. If done incorrectly, it may not eradicate pathogens and may reduce germination and vigor. For cole crops, it is especially important to follow treatment protocols as seeds can split.
Seed heat treatment follows a strict time and temperature protocol and is best done with thermostatically controlled water baths. Two baths are required: one for pre-heating, and a second for the effective (pathogen killing) temperature. For cole crops, the initial pre-heating is at 100°F (38°C) for 10 minutes. The effective temperature is 122°F (50°C). Soaking at the effective temperature should be done for 20 minutes for broccoli, cauliflower, collards, kale, and Chinese cabbage, and 25 minutes for Brussels sprouts and cabbage. Immediately after removal from the bath, seeds should be rinsed with cool water to stop the heating process. After that, seeds should be dried on a screen or paper. Pelleted seeds are not recommended for heat treatment. Only treat seed that will be used in the current season.
As an alternative to hot water seed treatment, use 1 part Alcide (sodium chlorite), 1 part lactic acid, and 18 parts water as a seed soak. Treat seed 1-2 minutes and rinse for 5 minutes in running water at room temperature.
For more information on seed treatment please see Section 4.3 on page 149 in the 2024/2025 Mid-Atlantic Commercial Vegetable Production Recommendations Guide.
