High Temperatures and Tomato Fruit Production

Normal, healthy flower development and the production and shedding of viable pollen are stages in the larger process of fruit set. Research and grower experience indicate that these components of fruit set in tomato and other crops can be hindered at excessively high temperatures. Since fruit set is a requirement for fruit production, growers are right to ask if extra warm conditions influence tomato yield potential.
MUCH must be learned to answer this question reliably but the preliminary answer is “yes.” Temperature plays a role in fruit set but some of the details important to growers are less clear. For example, researchers must clarify how high the temperature must reach and how long it must remain above optimal before fruit set is hampered in most varieties. They must also determine how temperature and relative humidity (or other factors) work together in determining fruit set. These factors undoubtedly matter more or less depending on variety, so researchers must ask what makes certain varieties productive at high temperatures.
Currently, we may have more questions than specific answers about the impacts of high temperature on tomatoes in the field. However, the available evidence suggests that temperatures above approximately 90° F appear to hinder fruit set in some tomato varieties. Therefore, we set out to assess whether temperatures experienced so far in Wooster, OH in 2012 may be having undesirable effects in tomato fields.
We began by obtaining weather data for the Wooster Station of the OARDC Weather System (http://www.oardc.ohio-state.edu/newweather/stationinfo.asp?id=1). Data for the period June 1 to July 20 for the years 2008-2012 and 1988 were downloaded. For all years, daily reports citing the maximum temperature reached were obtained. In addition, for all but 1988, temperature readings taken every five minutes round the clock (288 readings per day) were collected. In total, more than sixty-nine thousand temperature readings were downloaded and analyzed.
Of the 69,430 temperature readings evaluated, 823 were higher than 90° F. For 2012, 656 of 13,826 the frequent readings exceeded 90° F and the high temperature for 13 of 50 (26%) days June 1-July 20 exceeded 90°F. By comparison, for the same 50-day period in 2008-2011, the number of days having a peak temperature above 90° F ranged from 1-4 (2%-8%) and the percentage of frequent readings above 90° F remained less than 0.5%.
It is obvious that the 2012 main season has been unusual but these numbers also help tell the story. Data are also available from other weather stations throughout Ohio (e.g., from the OARDC Weather System). For forecasts anywhere in the U.S. and some historical information, the NOAA/National Weather Service portal at http://forecast.weather.gov/MapClick.php?CityName=Wooster&state=OH&site=CLE&lat=40.8185&lon=-81.9329&FcstType=graphical is a favorite of the VPSL.
Many farmers remember the drought of 1988 — mostly not favorably. Temperatures remained high and rain did not fall. At OARDC, the weather station recorded a peak temperature exceeding 90° F on 22 of 50 (44%) days June 1-July 20. In short, some of the weather numbers for 1988 and 2012 are too similar.Little can be done to cool crops exposed to relentlessly high temperatures, especially when prolonged damaging temperatures are rare and difficult to forecast. Selecting heat tolerant varieties, maintaining proper soil moisture conditions and cooling crops as soon as possible after harvest are important steps, however.