Late Blight Effectively Managed with Resistant Tomatoes on Long Island in 2012

Posted: March 30, 2013

A replicated experiment was conducted at the Long Island Horticultural Research and Extension Center in 2012 to evaluate new varieties and experimental hybrids that have resistance to late blight in terms of horticultural traits and susceptibility to diseases.
Late blight resistant Defiant (left) and susceptible variety Mountain Fresh Plus (right).

Late blight resistant Defiant (left) and susceptible variety Mountain Fresh Plus (right).

Resistant varieties are a valuable tool for managing diseases, particularly late blight, because it can be very difficult to control with fungicide applications started after onset and it cannot be 'tolerated'. Left unmanaged, late blight is much more likely than other diseases to completely destroy a crop and also to have devastating impact on other tomato plantings in a region due to the quantity of pathogen spores that can be produced and easily dispersed by wind.

Mt Fresh Plus was included as the industry standard for comparison. Most varieties evaluated produce standard red, round, slicer (beefsteak) type fruit. Plum, cherry, and campari (large cherry) types were also tested. All named varieties tested are commercially available. Experimentals from the Cornell Breeding Program also have resistance to early blight and Septoria leaf spot, common foliar diseases occurring in the northeastern USA.

Very good resistance of foliar symptoms of late blight was exhibited by all tomato varieties and experimental hybrids evaluated that have the Ph2 and/or Ph3 major genes for resistance: which were Plum Regal (homozygous Ph3), JTO-545 (heterozygous Ph3), Legend OP (Ph2), Matt's Wild Cherry (undetermined resistance, possibly Ph3), Jasper (undetermined resistance, likely Ph2 and/or Ph3), and Defiant PHR, Mountain Magic, Mountain Merit, and three experimentals from the Cornell University Dept of Plant Breeding (all heterozygous Ph2 + Ph3). Heterozygous means the hybrid has one copy of the resistance gene; homozygous means it has a copy from both parents which is expected to impart a higher level of resistance. Iron Lady is a new variety developed at Cornell that is homozygous Ph2 + Ph3. It is expected to have even better resistance than the others. High Mowing Organic Seeds is marketing it. The other resistant varieties are available from Johnny's Selected Seeds and Seedway.

Late blight became severe in New Yorker (Ph1). Severity of symptoms was similar to the varieties without major resistance genes, which were Mountain Fresh Plus, Juliet and Brandywine. The late blight pathogen genotype present in this experiment, US-23, was the dominant genotype present in 2012 in the USA and thus most likely will dominate in 2013. The Ph1 gene is also not effective for other genotypes of the pathogen that have occurred in recent years. Thus varieties with only this gene are not recommended.

Legend, the only entry with just the Ph2 gene, was numerically, but not significantly, more severely affected by late blight than the other resistant entries, except at the last assessment when extensive defoliation may have affected ratings.

Plum Regal and JTO-545, the two entries with just the Ph3 gene, were numerically more severely affected by late blight than the other resistant entries at all assessments. The difference was significant at the last assessment (12 October), which was 35 days after the last fungicide application for late blight. Fungicides applied may have suppressed late blight, especially on resistant varieties, but the late onset of treatment compromised efficacy.

In conclusion, best suppression of the US-23 genotype of the pathogen was achieved with tomato possessing both the Ph2 and Ph3 resistance genes. Only a few fruit with symptoms of late blight were observed on these entries.

Mountain Magic, Jasper, and Matt's Wild Cherry were the three resistant varieties receiving the highest overall rating in the 10 fruit evaluations (taste and color) conducted by public groups.

For methods, fruit descriptions, and tables with blight severity, please see Late Blight Effectively Managed with Resistant Tomatoes on Long Island in 2012 or request from Dr. Meg McGrath ( or from Tianna DuPont 610-746-1970.

Acknowledgments: This project was funded by the Agriculture and Food Research Initiative Competitive Grants Program Grant 2011-68004-30154 from the USDA National Institute of Food and Agriculture.

Contact Information

Margaret Tuttle McGrath
  • Department of Plant Pathology Cornell University Long Island Horticultural Research and Extension Center
Sandra Menasha Cornell
  • Cooperative Extension-Suffolk County