CULTURAL TIP: MANAGING LEAF HEALTH FOR BETTER PRODUCTION
Having healthy-green avocado leaves is very important for getting the best yields from avocado trees. Not only should the leaves be green, but they also need to be of a good size, well distributed around the tree and in a layer thick enough to shade the fruit and branches from the damaging effects of the sun. Unfortunately, in California avocado leaves are often damaged in some way by pests like Persea mite or by toxic elements such as chlorides. This has the effect of reducing the overall area of the leaves that use light to make the sugars which the tree uses to support the crop it is carrying. While it is likely that avocado leaves can tolerate some damage before their ability to produce sugars is harmed, to get high yields of large fruit having undamaged leaves is generally a good principle to follow.
Fortunately, it is easy for an avocado grower to assess the health and quality of the leaves on their trees. An obvious sign of trouble is when leaves are yellow or yellow-green. The human eye is very good at picking up slight changes in the color green and with practice a grower can tell when leaves begin to turn from a dark green color to a more yellow-green color. The yellowing of leaves is due to the chlorophyll pigment breaking down, unmasking the more stable yellow pigments in the leaves. If detected early enough, the yellowing of the leaves can be reversed with carefully calculated applications of fertilizer. Fertilizer applications at the right time and amount can supply leaves with minerals that may be lacking and can also be used to reduce the yellowing that may occur during winter and in spring when the trees are in bloom. Good leaves at flowering could be considered a best practice goal as such leaves are mature and should function well in supporting flowering and newly set fruit with their sugar production.
More difficult to manage is the damage to leaves from avocado pests. This damage generally cannot be reversed as pests, like mites, destroy the cells in the leaves and, for some types of damage, can cause the leaves to fall. While it is not known for certain how much pest damage is needed before leaf function is reduced, it is reasonable to assume the greater the damage to the leaf, the lower the amount of sugars the leaf will produce. The best situation is to manage the pest populations in the grove so that leaf damage is minimized. Monitoring of pest populations should be a normal part of the cultural management activity on California avocado groves so that changes in pest levels are found early and timely control measures can be applied. In particular, a build-up of brown mite and Persea mite populations can quickly result in badly damaged leaves.
Small leaves and too few leaves on an avocado tree can tell an avocado grower a lot about the stress on the trees. Small leaves suggest poor root health and low nutrient availability to the tree. The result of small leaves is that the tree is less likely to have a good crop and to produce large fruit. If the tree has small leaves it should be checked for root disease and cankers. Should disease be found, the appropriate treatment should be applied. Trees with poor drainage and/or too wet soil can also develop small leaves as the root system is reduced and nutrient uptake is limited. Improved drainage and dryer soil should result in the tree producing an increased amount of leaves, as well as larger leaves. Too few leaves are likely to lead to a number of problems, with the most common related to sunburn of the fruit and branches. Sunburn on branches can lead to the establishment of branch cankers that may reduce the production of fruit from the tree. If the limbs of neighboring trees can easily be seen through where the canopy is thickest, corrective action may be needed to increase the number of leaves on the tree.
The health and number leaves on the tree are the most visible sign of tree health and condition. Making sure the trees are managed to have a good canopy should be an important cultural management goal for California avocado growers.
