Produce growers face challenges in climate change (The Vegetable Grower)
(Editor’s note: Gordon Johnson is a vegetable and fruit specialist with the University of Delaware.)
Climate change has the potential to affect fruits and vegetables as temperatures increase.
Climate data from the region has shown a steady increase in average temperatures over the last 100 years with average night temperatures in summer months increasing the most.
The summer of 2019 was one of the hottest on record in the Northeast with many days in the 90s and nights in the 70s.
Many vegetable crops had losses due to the heat. Providing adequate moisture through irrigation is critical in high heat periods.
However, water cannot completely compensate for extreme heat.
As growers face the challenges of climate change, there are a number of tools or strategies that can be used to mitigate the effect of higher temperatures.
Managing mulch is one such tool.
This includes changing plastic film to white, silver or metalized colors for summer production and the use of natural mulches such as rolled small grain cover crops to reduce soil temperatures.
In tomatoes, high soil temperatures have been shown to reduce potassium uptake and increase fruit quality defects (white tissue and yellow shoulder).
Use of white plastic has been shown to reduce theses defects.
Radiation blocks or reflective materials can reduce heat effects by reflecting away some solar radiation.
Commonly, particle films are used as radiation blocks including kaolin (white clay) based or calcium carbonate (lime) based materials.
These are sprayed on plants during high temperature periods.
Particle films are commonly used to reduce sunburn in watermelons in southern regions.
Wax-based reflective materials have also been used in fruits such as apples to maintain color.
Research at the University of Delaware and University of Maryland has shown that tomato quality and yield is improved with the use particle films
Shading is another strategy. Commonly, shade cloth or netting is used for this purpose.
This netting comes in black, green, white, and reflective aluminum colors and is commonly used at the 20- to 30-percent shade levels.
Shading is applied during the hottest periods or periods when the plant is most sensitive to heat (such as tomato fruit development).
Research at the University of Maryland showed that shading tomatoes during fruiting can improve fruit quality and reduce culls. Research at the University of Georgia on peppers showed similar results with improvement in the number of marketable fruits.
In 2018 and 2019 we also tested effect of shade cloth on tomato and pepper marketable yield. In 2018 shade treatment did not have a significant effect on pepper quality or marketable yield.
In contrast, in 2019 shade treatments, especially 30-percent black, shaded plots produced more marketable peppers than the unshaded plots.
Yield of marketable first harvest (early August) for 30-percent black was 18 times higher than unshaded.
Yield of marketable second harvest (September) was two-times unshaded. Lettuce trials were conducted with shade cloth.
Shade treatments had reduced bitterness in the 2019 trial but in both the 2018 and 2019 trials, variety selection was more important in determining quality under heat stress.
While stress mitigation tools may be more commonly used in fruits and vegetables as the climate warms, adaptive changes should be considered for more long-term stress management.
One adaptive change would be to switch to crops that are more heat tolerant for summer production. Sweet potatoes would be an example of a very heat tolerant crop. Another adaptive change would be to alter planting dates.
By planting earlier in the spring (for summer maturing crops) or later in the summer (for fall maturing crops), you can avoid the hottest growing periods and have better production potentially. We have studied the effects of planting dates on broccoli and Brussels sprouts at the University of Delaware. Yields of marketable broccoli changed dramatically with planting date.
Two other adaptive strategies would be to change to switch to more heat tolerant cultivars (for summer production) or to varieties that mature in cooler periods (to match with later plantings).
Broccoli trials in 2017 and 2018 showed that Eastern Crown had superior heat tolerance. Snap bean trials in 2017 and 2018 identified three varieties with good heat tolerance.
Tomato trials in 2019 identified two experimental lines with limited white tissue development under high heat.
The most effective adaptive strategy is to breed vegetable and fruit crops that are more stress tolerant.
For example, currently the lima bean breeding program at the University of Delaware is making significant progress in understanding heat stress losses in lima beans and breeding for heat tolerance.
1-800-634-5021 410-822-3965 Fax- 410-822-5068
P.O. Box 2026 Easton, MD 21601-8925