Introduction

Field tomatoes are a long season crop with high water requirements. An average cultivar requires about 40 cm (15.7 in) of water over the growing season, with the need for moisture increasing until full fruit load is developed. The most critical times for moisture are during flowering, fruit set, and fruit sizing.

Moisture stress

When tomatoes experience moisture stress:

  • Fewer flowers develop per truss, meaning lower numbers of fruit are set, resulting in lower yields.
  • Total soluble solids increase, resulting in higher recovery in the processing plant and generally improved taste of fresh or processed tomatoes.
  • Fruit size decreases, which may result in unmarketable fresh market fruit and greater harvest loss in mechanical harvesting of processing tomatoes.
  • Blossom-end rot increases, leading to black specks and high mould counts in processed product. Affected fruits are unmarketable and loss can be significant in susceptible varieties. If severe, loads or fields of processing tomatoes can be rejected. Agricorp offers a salvage benefit for processing tomato fields severely affected by blossom-end rot.
  • Plant canopy temperature increases resulting in reduced transpiration and photosynthesis. At fruit temperatures above approximately 30°C, development of red colour is inhibited. This is an additional stress to the plant that leads to reduced growth.

Tomatoes are more tolerant of moisture stress than crops such as pepper and cucumber. They can adjust their physiological processes to conserve water while maintaining some growth. Early exposure to moisture stress makes the plant more tolerant of moisture stress later in the season. While this allows the tomato plant to survive where some crops would suffer irreversible damage, prolonged water stress does reduce yield as the plant uses energy to make these adaptations.

Although the plant can survive dry conditions, optimal yield and quality will not be achieved. Irrigation of tomatoes can result in higher and more consistent yields, better quality, less blossom-end rot and less cracking.

The effect of soil structure

Fields with good soil structure and higher organic matter hold more moisture and allow roots to penetrate better. Compacted layers limit the spread of roots. In tomato, 85% of the root system is in the upper 30 cm (12 in) of soil, but tomato roots can extend beyond 1 m (39 in) in depth. Cover crops can be grown and the residue used to mulch the soil, reduce water loss, and moderate soil temperatures. However, this requires advance planning and could have a detrimental effect in a wet spring. In irrigated fresh market tomato production, plastic mulches can be used to conserve water. Minimizing tillage can also reduce moisture loss.

Evapotranspiration

Evapotranspiration (ET) is the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.

In the early 1980s, Dr. C.S. Tan at Agriculture and Agri-Food Canada found that the marketable yields of processing tomato were directly related to seasonal ET. Below 88 mm of ET, tomato produced no marketable yield on a Fox sandy loam. Above 88 mm, yield increased linearly, to an optimum when 300–400 mm of water was available to the crop.

Optimum seasonal ET would be expected to be higher with current higher yielding varieties and production practices. Research on processing tomatoes in Ontario has shown yield increases of up to 81% on a range of soil types with the use of properly scheduled irrigation. However, under dry conditions irrigation is most critical on lighter textured soils with low water holding capacity. It is estimated that approximately 40 to 50% of processing tomato acreage is being irrigated in 2023.

Visit our Adverse Weather page for more information on dry conditions and low water.