"I've been working on conventional breeding for bacterial spot resistance since 1981, and after all this time I still don't have a variety with resistance to the bacteria. We've gotten very nice results with Bs2. It's a great technology. This is exactly the type of thing we need in agriculture now, where the conventional approach doesn't come up with a good solution."

—Prof. Jay Scott, Tomato Breeder, University of Florida

The challenge    

Bacterial diseases plague crops in warm, humid environments.  In the US, the majority of fresh market tomatoes are grown in the southeast, particularly in Florida.  For over 60 years, the major unsolved disease problem in tomatoes has been Bacterial Leaf Spot caused by Xanthomonas.  Tomato growers have relied on chemical treatments based on copper compounds, but the use of these has led to the selection of copper resistant pathogens. As a result, these compounds no longer have useful effect, yet they continue to be used despite negative environmental impacts.  Other more benign treatments, such as bacterial viruses and compounds that act as general inducers of disease resistance have been tested, but these are considerably more expensive than copper, require special handling and storage, and only mildly effective.

 

Another significant disease of tomato is bacterial wilt, caused by Ralstonia. Bacterial wilt is a major disease on tomatoes worldwide in tropical and sub-tropical climates with plant mortality  up to 100%. The disease is endemic to the southeastern US, with northern Florida as part of a hot belt for bacterial wilt. 

Any effective field resistance that could be brought in against bacterial wilt on tomato and other crops could be a landmark, simply because it is 120 years since the bacterial pathogen Ralstonia solanacearum was first described by E.F. Smith, and, till now, no cost-effective management options are available for growers worldwide. Our first study field testing of EFR showed excellent potential against bacterial wilt, and we are very excited.”  Mathews Paret, University of Florida

The strategy

Disease resistance genes for bacterial spot and bacterial wilt have been sought in tomato and its wild relatives, but to date none have been isolated or bred into commercial tomato lines for lasting resistance.  Bacterial spot resistance does exist in pepper, a close relative of tomato, and a gene known as Bs2 was isolated, introduced into tomato and found to confer resistance.  EFR, another disease resistance gene from the cabbage family, was introduced into tomato to enhance the durability by adding another mode of action and to test for protection against bacterial wilt.

The science

2Blades supported the introduction of Bs2 into tomatoes.  A series of field studies first looked at the effect of Bs2 in a California tomato variety and demonstrated a significant positive impact on plant health and fruit yield over six years of multi-site field trials in Florida.  Subsequently Bs2 was bred into high-performing tomato varieties from the University of Florida breeding program and these backgrounds also showed strong disease resistance and yield benefits.  Resistance is achieved in the complete absence of copper-fungicides, which would allow growers to save on the costs of materials, about $28 million on copper and $14 million on mancozeb annually, and labor to apply these compounds and, more importantly, providing a better management strategy for Florida’s soils and waterways.   Best of all, yields of marketable tomatoes were typically doubled in Bs2-containing plants.

EFR was found to contribute to durability of resistance against bacterial spot, and a first trial carried in 2015 in northern Florida, demonstrated that EFR conferred excellent control of bacterial wilt.

Program impacts

Schwartz A, Potnis N, Timilsina S, Wilson M, Patané J, Martins J, Minsavage G, Dahlbeck D, Akhunova A, Almeida N, Vallad G, Barak J, White F, Miller S, Ritchie D, Goss E, Bart R, Setubal J, Jones J, and Staskawicz B (2015). Phylogenomics of Xanthomonas field strains infecting pepper and tomato reveals diversity in effector repertoires and identifies determinants of host specificity. Frontiers in Microbiology 6:535. doi: 10.3389/fmicb.2015.00535.

Horvath DM, Hutton SF, Vallad GE, Jones JB, Stall RE, Dahlbeck D, Staskawicz BJ, Tricoli D, Van Deynze A, Pauly MH, and Scott JW (2015). The pepper Bs2 gene confers effective field resistance to bacterial leaf spot and yield enhancement in Florida tomatoes Acta Horticulturae 1069: 47-51. doi: 10.17660/ActaHortic.2015.1069.5.

Hutton SF, Scott JW, Jones JB, Stall RE, Vallad GE, Staskawicz BJ and Horvath DM (2015).  The Good, the Bad, and the Ugly: What the Future Could Hold for Bs2 Tomatoes. EDIS report HS1259, Gainesville, FL: University Florida Institute of Food and Agricultural Sciences, http://edis.ifas.ufl.edu/hs1259.

Timilsina S, Jibrin M, Potnis N, Minsavage G, Kebede M, Schwartz A, Bart R, Staskawicz BJ, Boyer C, Vallad G, Pruvost O, Jones J, and Goss E (2014). Multilocus sequence analysis of xanthomonads causing bacterial spot of
 tomato and pepper reveals strains generated by recombination among species 
and recent global spread of Xanthomonas gardneri. Applied Environmental Microbiology 81: 1520–1529. doi:10.1128/AEM.03000-14.

Horvath, DM (2014). Effective transgenic resistance to bacterial leaf spot in Florida tomatoes. Webcast at Plant Management Network.

Horvath D, Stall RE, Jones J, Pauly MH, Vallad G, Dahlbeck D, Staskawicz B, and Scott JW (2012). Transgenic resistance confers effective field level control of bacterial spot disease in tomato PLoS One 7, e42036.  doi: 10.1371/journal.pone.0042036.

Sharlach M, Dahlbeck D, Liu L, Chiu J, Gomez J, Kimura S, Koenig D, Maloof J, Sinha N, Minsavage G, Jones J, Stall R, and Staskawicz B (2012). Fine genetic mapping of RXopJ4, a bacterial spot disease resistance locus from Solanum pennellii LA716. Theoretical and Applied Genetics 126:601-609. doi 10.1007/s00122-012-2004-6.

Scott JW, Hutton SF, Vallad GE, Jones JB, Stall RE and Horvath DM (2012).  Is it time for a transgenic tomato variety? Florida Tomato Institute Proceedings 8-9.

Nobel D and Scott JW (2012). 40 years of breeding success. Tomato Magazine 16:8-11.

Zhao B, Dahlbeck D, Krasileva K, Fong R, and Staskawicz B (2011). Computational and biochemical analysis of the Xanthomonas effector AvrBs2 and its role in the modulation of Xanthomonas type three effector delivery. PLoS Pathogens 7: e1002408. doi: 10.1371/journal.ppat.1002408.

Potnis N, Krasileva K, Chow V, Almeida N, Patil P, Ryan R, Sharlach M, Behlau F, Dow J, Momol M, White F, Preston J, Vinatzer B, Koebnik R, Setubal J, Norman D, Staskawicz B, and Jones J (2011). Comparative genomics reveals diversity among xanthomonads infecting tomato and pepper. BMC Genomics 12:146.  doi: 10.1186/1471-2164-12-146.

Scott JW, Vallad GE, Stall, RE, Jones JB, Dahlbeck D and Staskawicz BJ (2011). Bacterial spot race T4 resistance and yield enhancement in tomatoes conferred by the pepper Bs2 gene in Florida field trials. Acta Horticulturae 914:437-440.

Scott JW, Hutton SF and Strobel JW (2010).  Some highlights from the University of Florida tomato breeding program. Florida Tomato Institute Proceedings 53:9-10.