Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
Climate change has the potential to affect fruits and vegetables as temperatures increase, extreme weather events such as heavy rainfalls become more frequent, and there is a higher frequency of “false springs”.
Climate data from the Delmarva region has shown a steady increase in average temperatures over the last 100 years with average night temperatures in summer months increasing the most.
In the last 20 years, all but 3 of those years have had high temperatures above the 100 year average with 2 of the last 5 years having record highs (8 of the last 20 breaking records). We have also had 2 false springs with destructive late freezes in the last 5 years and some damage in 2017 from record high February temperatures.
Hotter temperatures can reduce yield in fruits and vegetables by lowering photosynthesis, increasing respiration, and causing reproductive failure (split sets, flower drop, reduced seed set, reduced fruit set). Quality of fruits and vegetables can also suffer due to higher numbers of size and shape culls (reduced pollination), increased internal defects (heat necrosis), and increased tissue damage (sunburn and sunscald).
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. Day-neutral strawberries had higher summer yields on white plastic in our trials in the past.
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.
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 20-30% 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 by Jerry Brust on tomatoes 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. Our research with shading of strawberries for summer production showed mixed effects with shading benefiting in some years but not in others.
Metabolic and developmental regulators may also have a place in stress mitigation. These are chemicals that are applied to plants and reduce stress through different mechanisms. Ethylene inhibitors such as 1-MCP and strobilurins reduce flower and fruit drop. Hormones such as cytokinins and jasmonates alter different biochemical pathways related to plant stress. Flower or fruit initiating hormones (auxins, gibberellins, cytokinins and combinations) can improve flower and fruit set. Unfortunately, we have few labels for use of these products in vegetable crops.
Water-based cooling can be employed to reduce heat loading in crops and crop environments. Evaporative cooling has been commonly used in greenhouses to cool air entering houses and reduce temperatures for greenhouse grown vegetables. Fogs and misters have also been employed for this purpose. In the field, low water volume sprinklers, either continuous or pulsed, have been successfully used during hot daytime periods for plant cooling. Irrigation timing can also be used to as a tool. For example, by starting drip irrigation soon after dawn, soil under black plastic mulch will remain cooler for longer periods during the day.
Some biological root inoculants have also been shown to reduce plant stress. Mycorrhizal fungi can act as root system enhancers, increasing the effective area for absorbing water from the soil. The University of Delaware has released a Bacillus subtilis bacteria for root inoculation that has been shown to improve plant stress tolerance.
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 are currently studying the effects of planting dates on broccoli and Brussels sprouts at the University of Delaware.
Two other adaptive strategies would be to change to more heat tolerant cultivars (for summer production) or to varieties that mature in cooler periods (to match with later plantings). Past research in a very hot year (2012) showed significant differences between seedless watermelon varieties in heat tolerance. The 7187 triploid watermelon variety (Nunhems) shows good heat tolerance and has been shown over many years to be widely adapted throughout the Eastern and Southern watermelon producing regions. We will be conducting variety trials with snap bean varieties (processing and fresh market) to test heat tolerance in 2017. We also have been evaluating southern highbush blueberries for adaptability to Delaware conditions as our climate warms.
Finally, 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.