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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 X. anthomonas: 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 what races of the pathogen are present on your farm?
That’s a difficult question to answer. Up to now, the only way to know is through differential testing. That
means planting a number of different bell peppers with varying BLS resistance packages and monitoring
which cultivars develop symptoms. For example, if you detect BLS development in ‘Aristotle X3R’ (which
has resistance to races 1,2, & 3); then you possible have races 4-10 present on your farm. If you were
to plant ‘Turnpike’ in that same field and you have BLS development in it, then you possibly have race
6 or 10 present, because ‘Turnpike’ has resistance to BLS races 0-5 and 7,8,9. It’s extremely important
to know what races of BLS are present so you can chose the proper cultivars to grow. Choosing the
proper cultivar will do two things: significantly reduce the chances of BLS development and significantly
reduce the number of copper applications on your bell pepper crop. As a note, there are a few non-bell
peppers available with BLS resistance packages (see the 2024/2025 Commercial Vegetable Production
Recommendations Guide).
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.
Moving forward in 2024. More sampling and surveying are planned for the 2024 production season in
New Jersey. Growers who are interested having tomato or pepper samples collected from their farm for
species determination and copper resistance testing are encouraged to contact their county agent so
arrangements can be made. For more information on this research and control options visit https://plantpest-advisory.rutgers.edu/wp-content/uploads/2022/04/Update-on-BLS-survey-2022.pdf.