Kemp, G. A. and I. L. Nonnecke.
A fall frost in 1953 of five degrees was sufficient to kill all but two plants. These two plants showed no evidence of any damage, even the flowers appeared unaffected.
A second group of this material was obtained this year, which included 150 recent accessions. These were placed in the cold frames for hardening off prior to a proposed MaY 3 transplanting. Cotton sashes and a double layer of canvas tarpaulins afforded some protection to these plants when on the afternoon of April 24 the temperature dropped below 32 deg. F. Seventy-two hours later the temperature had not risen above 32 deg. F. and a forecast low of 15 deg. F made it expedient to move all the plants to a warmer location. The recorded low during the 72 hour period was 18 deg. F while the high was 32 deg. F which was reached only momentarily, and the average temperature during this period was 24.4 deg F. Within 24 hours after moving the plants inside it was obvious which plants were going to survive the exposure. Approximately two per cent of the original 1700 plants survived. In most of the surviving lines the survival percentage was between 10 and 15 per cent, several lines, however, had as high as 75 to 84 per cent survival.
The problem of tomato frost tolerance appears to be very complex. Evidence has come forth indicating survival of certain species, varieties or plants but only under the specific set of environmental factors which happen to be present at a given time. For instance tomato plants which seem to survive one set of deleterious factors are not necessarily going to survive another frost at some other date. Worse yet is the fact that because the parent survives frost it is no sure thing that the progeny will show any signs of survival. A closer study of this whole problem is now under way.