Protecting Citrus Trees From Heat Stress

Josh McGill Tip of the Week, Weather

By Anirban Guha

Heat stress could take a toll on citrus tree health and fruit yield. In Florida, the hot season lasts more than four months. Long, hot, oppressive summer days can heat up leaves and fruit and make the trees extra thirsty. Also, high soil evaporation, irregular rainfall and poor soil water-holding capacity can often lead to summer drought and make trees more vulnerable to heat.

Heat Stress
Citrus leaves can curl up as a reaction to heat and less water content. The red arrows indicate curled leaves in an HLB-affected Valencia tree on a hot sunny day in June.

What temperature is stressful for citrus?
If leaf temperature frequently hits 98 degrees, trees could start feeling mild heat stress. But citrus leaves heat above air temperature and can even reach110 degrees and higher on hot days.

Do citrus trees show signs and symptoms of heat stress?
Yes, inward curling of leaf edges is an early sign of heat stress. If timely care is not taken, leaf bleaching and rapid yellowing of canopy, growth retardation, reduction in flowering and fruit setting, increased fruit drop, and fruit sunburn can occur. However, heat reduces photosynthesis long before any visible signs occur.

Heat Stress
At the Citrus Research and Education Center, researchers have observed several benefits of (a) partial shading and (b) kaolin clay applications on canopy growth during summer and post-summer yield.

How can heat stress be prevented?
There are multiple strategies that growers can take. Frequent irrigation is recommended to meet extra water demand. Reducing competition from weeds can also prevent competition for water. Groves with row middles with good soil cover from cover crops or turf can stay much cooler than bare soil. HLB-affected trees may be highly sensitive to heat due to poor root systems and chronic nutrient deficiencies. Therefore, maintain a steady mineral nutrient supply throughout the summer. Avoid using any spray with oil in it as oil on leaf surfaces is damaging in summer heat. Any treatment that shades the tree will help prevent heat stress.

Do young plantings need extra care?
Yes, young trees are more sensitive to heat than mature trees due to a shallower root system and less storage capacity in the trunk. Individual protective covers and kaolin particle films shade young trees and help keep them cool.

What are some other tips?
If forecasts call for extreme heat events, consider building up water in the soil by applying mulch around the tree. Sun-protective kaolin clay can be sprayed on all sides of a tree.

Anirban Guha is a postdoctoral research associate at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

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