High temperatures may result in smaller fruit

Apples sizing has significantly slowed or halted from the end of July until late August. Fruit size has been reported to be smaller and variable across the province. Size reduction could be due to low soil moisture and the response is quite variable, depending upon soil type, rootstock, variety and crop load management. Interestingly though, small fruit has also been found in irrigated orchards and this observed size reduction may be a result of hot temperatures as well.

Ontario has experienced many hot day time temperatures. Norfolk County, Niagara Region and areas surrounding Toronto and Ottawa have had 20-30 days of temperatures above 30°C. The rest of the province has experienced 10-20 days of temperatures above 30°C. The main concern for these high temperatures has been to protect apples from sunburn and bitter rot but heat stress can cause a reduction in fruit size as well.

Early research has shown that temperatures above 30°C and also high night time temperatures can be detrimental to growth. With temperatures above 30°C, net CO2 fixation is great reduced, resulting in a carbon deficit. From research conducted at Cornell, fruit size increases linearly as day/night temperatures increase from 12/7 to 33/28 at 6 mm fruit diameter. After three weeks past bloom or 11-18 mm fruit diameter, fruit growth is greatly reduced. Another study from Israel found that at temperatures above 29°C molecular development and growth process were disrupted and cell-expansion and cell-cycle genes were expressed differently.

There are products and practices that can help to reduce heat stress and improve fruit size as well as prevent against sunburn and bitter rot. Evaporative cooling through overhead irrigation misters can help cool trees down and is commonly used in apple growing regions that experience hotter temperatures, such as Washington State. Sun or heat stress protectant products such as Surround WP, Screen Duo and Caltrac can also help with heat stress.


  • Agriculture and Agri-Food Canada. Drought Watch Website.
  • Calderón-Zavala, G., A. N. Lakso, R. M. Piccioni. 2004. Temperature effects on fruit and shoot growth in the apple (Malus domestica) Early in the Season. In XXVI International Horticultural Congress: Key Processes in the Growth and Cropping of Deciduous Fruit and Nut Trees 636, 447-453.
  • Flaishman, M.A.,Y. Peles, Y. Dahan, S. Milo-Cochavi, A. Frieman, and A. Naor, 2015. Differential response of cell-cycle and cell-expansion regulators to heat stress in apple (Malus domestica) fruitlets. Plant Science, 233, 82-94.

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca