Can't Say It Often Enough

Advice from Index Fresh

POST HEAT TREE MANAGEMENT

September 2020

HEAT STRESS

• Why are avocado trees so heat sensitive? Water is taken up by the roots and is essential for overall tree metabolism and carrying nutrients and metabolites to the growing portions of the tree canopy. It also serves an important function for overall temperature regulation for the tree since water vapor diffuses through the leaf stomata as part of the photosynthetic process. We know that avocado stomates close when temperatures exceed about 90F; this is modulated by relative humidity. When the stomates close, we break the process of water transport within the plant and water vapor is not released, thereby stopping the process of cooling.
• Think about it this way, the leaves act like a radiator to cool the whole tree. When stomates close, the radiator no longer functions, and the tree heats up.
• During high temperature events, especially with low relative humidity, the tree will have difficulty in taking up sufficient water to support the tree canopy, therefore it is important to hydrate the tree prior to predicted heat events to reduce heat stress.
• Overheating of the tree may lead to branch dieback, leaf scorch and fruit drop.

ACTIONS

• Irrigation and Fertilization: Check the soil moisture level! The amount of water and fertilizer should be adjusted according to the level of damage of the canopy. With leaf drop, less water is transpired and the typical irrigation program needs to be adjusted. If you have to reduce the water, it is always better to reduce the amount of water applied and maintain the irrigation frequency. Always check soil moisture to verfiy irrigation needs!!!
• Pruning: Similar to a freeze event the immediate damage may not be visible for several weeks so resist pruning the tree. Once ten days to two weeks have passed any damaged branches or leaves will be visible which can then be removed. Remove only damaged wood from the tree. If you have any doubts, do not prune. You should keep new growth (especially summer shoots) as much as posible for the next year´s flowering. Dried leaves on the tree can provide sunburn protection, so resist pruning.
• Sun Protection: Whitewash immediately (same day) against sunburn.
• Notes: Work should be performed during “normal” temperatures. Pruning tools must be cleaned and disinfected between each tree especially in areas and/or blocks with a known presence of diseases, such as sun blotch.

Disease control: Prevent “wood diseases” like Avocado Branch canker. Remove damaged branches before the winter rains arrive in November. Remember, Pruning tools must be cleaned and disinfected between each tree