CULTURAL TIP: THE EFFECTS OF WIND ON AVOCADO TREES
Continual wind or infrequent wind storms damage avocado trees. Wind can have both chronic effects where the trees are exposed to continual amounts of air movement and acute effects where the tree are thrashed around by storm force winds. In California, wind has been damaging avocado trees ever since the industry started so is nothing new to the experienced grower. Windfall fruit and damaged trees are a fact of life for California avocado growers. The negative effects of wind on avocado trees differ depending if the exposure to wind is continual or very infrequent and severe. The cultural management methods used to minimize the damaging effect of wind depends on whether the wind is reducing yields and damaging fruit quality or if the grower needs to respond to a severe windstorm.
Avocado trees grown in windy parts of groves are usually stunted trees and have poor production compared to the more sheltered trees elsewhere on the grove. The fruit on the wind exposed trees is inevitably blemished by the fruit skin being injured from twigs and branches hitting the fruit at any time from when the fruit are "BB" sized through to harvest. The fruit end up with black marks, which in the supply chain can be mistaken for anthracnose post harvest rots, to russet scarring easily confused with persea thrips damage. A lot of skin damage is obvious on green, unripe fruit due to the contrast between the brown/black marks and the green skin. For Hass fruit skin damage is usually masked or hidden when the fruit ripen to a dark color. Such a change in the color of Hass fruit when ripe helps to improve the appearance of the fruit. For greenskin varieties blemishes on the skin remain obvious once the fruit ripen. The blemishes then detract from the visual appeal of the fruit.
One type of damage wind causes is superficial skin damage where only the top layer of the skin is crushed. This damage does not extend for the full thickness of the skin and heals fairly quickly. The healed damage is often small spots and takes on a black, sometimes shiny appearance. If the damage is done while the fruit are small the spotting will appear to spread as the fruit grows increasing the apparent amount of damage. This type of wind damage is easily hidden when the fruit ripen and is generally not serious with respect to quality.
Another type of damage is where there has been greater force hitting the fruit and the skin damage is deeper. The result is a tear in the skin that then heals by forming a scar or callus. This scar is rigid so when the fruit grows it rips and breaks apart making it necessary for the skin to keep repairing the damage by forming more and more scar tissue. The result is a "russet" type blemish on the skin that has angular "netting" and is sometimes referred to as alligator skin. When the fruit grow they don't expand evenly over the fruit and the scars fracture along lines of force. This is what causes the netting effect. If the skin is damaged when the fruit are very small the size of the russet area can cover a large percentage of the fruit by the time the fruit are harvested. This type of damage is serious as too much russet blemish will downgrade the fruit.
Apart from fruit blemish wind also causes damage to leaves resulting in ragged edges, leaves also tend to be smaller and the trees more compact and stunted. The wind is probably causing additional water stress to the trees affecting the uptake of minerals. Rocking of the trees in the wind and smaller canopy is likely to reduce root health leading to a more stressed tree overall. The only way to reduce the stress on the tree is to provide a wind shelter that reduces the energy of the wind.
Continual exposure of avocado trees to wind has an insidious effect of lowering yields and decreasing fruit quality that can be hidden to avocado growers. More obvious is the damage of a severe strong wind event. Trees blow over, limbs break and fruit is blown off or dropped by the tree. The amount of damage depends on how strong the wind was, the amount of fruit on the trees and the structure of the canopy of the trees affected. Sometimes other weather events occur with the severe winds that can make the damage worse. For example, strong winds after rain can make large trees more prone to falling over in shallow soils that may have been waterlogged. Large avocado trees with dense compact canopies can be vulnerable to blowing over or a severe rocking motion as the canopy acts like a large sail catching the wind. Heavy crops add a lot of weight to branches that may be balanced by the rest of the tree acting as a cantilever. Once the roots start to give way in the soil the tree tips over. Other times the weight of the fruit cause trees with poor branch structure to break splitting the tree or causing a large limb to fall to the ground. Once the damage has been done the grower is left with loss of fruit and the cost of having to clean up the trees.
The actions to take after a severe windstorm depend on the type of damage caused by the wind. If the only effect was to blow most of the fruit off the trees, the amount of fruit on the ground always looks greater than when in bins, the grower is left with the decision of marketing or trashing the fruit. The fruit still attached to the tree should be watched for signs of wilting where tree limbs have broken or the tree has tipped over exposing the roots. If the fruit wilt then the fruit will have their ripening triggered and likely be of poor quality when harvested. The fruit that do not wilt have enough of a connection between the tree and broken branch or the remaining root system to keep the fruit going until the grower is ready to harvest. Once the fruit has been harvested the tree or branch can then be trimmed. A key issue growers should always keep in mind with downed trees or branches is to ensure the area is safe and any tree trimming that needs to be done for safety reasons should be done as soon as possible after the windstorm.
Unless the trees are very young it is usually a bad idea to stand back up fallen over trees, as this often causes damage to the remaining root system. Fallen trees or branches with wilted leaves are usually not going to recover and complete removal of these trees or branches is often necessary. Staking young trees in a area prone to strong winds is a good idea until the trees grow enough to have sufficient strength to support themselves in the wind. Trees that rock in the wind can have their roots damaged so the stake support of the trunk should be firm while those trees where the wind is weaker can be held more loosely to the trunk.
Trees that break off at the graft union or near the ground will need to be removed and replaced. Avoid planting a new tree very close to where the old tree was planted. Unless the stump is removed and the soil sterilized the roots from the old tree will still be present and when rotting can be a good source of infection by phytophthora root rot for the new tree. Trees that have tipped over and have large amounts of roots exposed can be left alone if the tree looks like it will recover, new shoots will emerge from the trunk and form the basic structure of a new tree. The large old branches can be removed over time ultimately reshaping the tree to a more manageable shape.
Reducing the effect of severe wind events can be achieved by ensuring the trees are well sheltered using windbreaks and making sure the tree shape is maintained so that the trees do not act as a sail. California avocado growers don't often see the need for windbreaks as they take up valuable space and use water where avocado trees could grow instead. It is also difficult to put a dollar value on windbreaks that is until a windstorm comes along and the windbreak stops half the fruit from being blown off the trees. Then the value of the windbreak is priceless as the grower still has fruit to harvest. Losing fruit and trees from a severe windstorm is part of the risk of growing avocados in California requiring careful thought on site selection, tree shape and structure and possible use of windbreaks when establishing a grove. Wind management should be an important part of the cultural management activities each year.
