Evaluation of Late Blight Resistant Tomato Varieties, 2012

To find out how tomato varieties carrying the Ph2 and Ph3 late blight resistance genes actually perform against the US-23 genotype in the field, separately from how their resistance is described in seed catalogs.

Production system. Certified organic field, natural infection, fungicides applied only after symptoms appeared.

Analysis. Leaves were examined for symptoms from 31 July to 12 October and severity assessed visually on each date. Figures below are the range across those assessments, so the low end is early-season and the high end late-season.

Natural infection by the US-23 genotype of Phytophthora infestans, the genotype dominant in the northeastern United States at the time. The authors conclude that the best suppression was achieved by varieties possessing both Ph2 and Ph3, rather than either gene alone.

Visual severity rating under natural infection. Plants are exposed to whatever inoculum arrives on its own rather than being deliberately inoculated, and the proportion of leaf tissue showing symptoms is assessed by eye on repeated dates. Natural infection is the realistic test but not a controlled one: disease pressure varies across a field and between seasons, so a low severity means the variety resisted what actually arrived that year, not that it would resist a heavier or different challenge.

This trial grew 13 entries. AMSeed publishes results for 10 of them. The rest are listed by name below with the reason their numbers are withheld, so the trial is represented as what it actually was rather than as the slice we happen to cover.

Cornell rated 13 entries; AMSeed publishes results only for entries it can resolve to a variety it holds. Severity is a range because it was assessed on repeated dates through the season. Natural infection means these figures describe what arrived on Long Island in 2012, not a controlled challenge, and fungicides were applied once symptoms appeared, so they are not unprotected results either.