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Planting and establishing an avocado tree can be hard work, which is why it’s so frustrating when their leaves begin to yellow. Yellowing leaves are never an encouraging sign, and unfortunately, pinpointing a cause can be difficult.
Causes of avocado tree leaves turning yellow include disease, nutrient imbalance, drought, and extreme heat. Fortunately, almost all of these problems have solutions.
While there are several diseases that afflict avocado trees, only two cause leaves to turn yellow:
- Avocado black streak (ABS)
- Armillaria root rot (oak root rot)
Avocado Black Streak (ABS)
Caused by an unknown virus or bacteria, ABS primarily targets the trunks and branches of avocado trees. Yellowing leaves, wilting, and leaf blotching are all characteristic of trees that have ABS.
You can tell for sure if your tree has ABS by looking for cankers on your tree’s trunk and branches. The size of the cankers will vary, but they produce a powdery, sugar-like substance. If you remove the bark underneath the cankers, you’ll notice lesions that are reddish-brown.
When ABS becomes severe, afflicted trees will die. If that happens, you’ll need to fumigate your soil before replanting.
There are currently no chemical treatments for ABS. However, ABS is more likely to occur in trees that are already under some stress. This means that you can treat and prevent ABS through consistent irrigation and fertilization (source).
Armillaria Root Rot (Oak Root Rot)
Armillaria root rot is a particularly maddening disease. Caused by a soil-borne fungus, the disease is already well underway in the root system before you notice any symptoms above-ground.
Yellow leaves, leaf drop, and wilting are all common symptoms of root rot. These symptoms indicate that at least some of the root system has died.
These symptoms could indicate drought stress as well. However, one sure sign of armillaria root rot is mushrooms growing around the base of the tree in the winter after it rains.
Unfortunately, if you’ve noticed these symptoms, there is virtually nothing you can do to save your avocado tree. Your tree may linger for a while, but eventually, the root rot fungus will kill the entire root system.
Furthermore, death of the tree does not mean death of the fungus. The fungus will remain in the soil and attack any other susceptible fruit trees, like peach or citrus.
If you suspect root rot, have your soil tested for the fungus. If it’s there, remove your tree and fumigate your soil before replanting.
The root rot fungus thrives in overly wet conditions, so you can prevent root rot in the future by allowing the soil to dry out between waterings.
Nutrient Excess or Deficiency
Avocado trees have precise nutrient needs and sensitivities that set them apart from other fruit trees. Yellowing leaves may be an indicator of the following:
- Excess salts
- Deficiencies of nitrogen, zinc, and/or iron
Saltiness is a problem that isn’t on the radar for many fruit growers. However, since many avocado growers live in humid, coastal areas, salt from water or excess fertilizer is a concern.
Avocado trees are very salt-sensitive. If your tree’s leaves are burning on the tips and edges, and if you notice mottled, yellow spots on the leaves, salt may be the problem.
Many growers exacerbate this problem unintentionally: they confuse the yellowing and burning with a need for fertilizer, which can add more chlorides to the soil.
Avoid over-fertilizing. Fertilization guidelines vary depending on the age of your tree, but a balanced 10-10-10 fertilizer once a month is sufficient.
Consistent, thorough irrigation is also helpful. By watering slowly and deeply, you can move excess salts deeper into the soil and farther away from your tree’s feeder roots.
Deficiency of Nitrogen, Zinc, and/or Iron
Nitrogen, zinc, and iron are all vital minerals for your avocado tree. Deficiencies of any of these three will cause your leaves to turn yellow, but since over-fertilization is a concern, you might need to look more closely at your leaves to determine which nutrients are missing.
- If leaves are yellow or pale green and have yellow veins, your tree needs nitrogen.
- If leaves are yellow in-between veins, and if the yellow spots start near the leaves’ edges, your tree needs zinc. Another sign of zinc deficiency is that areas of terminal growth will start to resemble feather dusters.
- If leaves are yellow between the veins and burned on the tips, your tree needs iron. This is especially common in soils that are highly alkaline (high pH value).
If your tree is low in nitrogen, start fertilizing on a consistent schedule. Balanced fertilizers contain nitrogen, so fertilizing regularly will solve this problem.
Zinc is available as a leaf spray and as a soil additive. Leaf sprays are most effective in mid-summer. Soil-based zinc applications vary in method but are often longer-lasting than sprays.
If your soil is alkaline, look for iron chelates specifically designed for high pH soils. These are most effective in late spring or early summer (source).
Drought or Underwatering
Drought stress and underwatering are simple problems with a simple solution.
If the leaves on your tree are yellow and flagging, but your tree is still producing quite a few flowers, your tree may just need more water.
Water your tree slowly and thoroughly, giving the soil enough time to completely absorb the water so none is left standing on the surface. Within a couple of days, your tree should perk up and start looking healthier.
From that point on, make irrigation a consistent part of your gardening routine. Depending on your local rainfall, a young avocado tree needs to be watered a few times a week; a mature tree needs watering one to four times per month. Consistent access to water is crucial to your tree’s health (source).
Remember it’s important to let the soil dry out between irrigations in order to prevent root rot. There’s an easy way to test if your soil is sufficiently dry before you water again:
- Grab a handful of soil from the base of your tree.
- Shape the soil into a ball.
- If the soil holds its shape, there is still plenty of moisture in the soil. If the ball of soil is crumbly–or if it can’t be shaped at all–go ahead and water.
Avocados are very exacting when it comes to temperature. They have a very limited tolerance to frost, which makes them great candidates for growers in warm zones, but they also don’t tolerate high temperatures very well.
An ideal temperature range for avocados is 70℉-88℉. If daytime temperatures are above 90℉ for several days in a row, leaves and fruit will turn yellow as a result of sunburn.
Many avocado growers live in regions where mid-summer temperatures can be far higher than 90℉, so this is no small problem. Every avocado grower in a hot climate needs to be prepared for heatwaves (source).
Sufficient water is the single more important factor in helping your tree cope with high temperatures! Start giving your tree extra water a few days before a predicted heatwave. During the heatwaves, plan to give it one and half times as much water as usual.
It’s also a good idea to lay mulch around the base of your tree to help shield the soil from intense sunlight and keep the roots cool. It will be easier for your tree to take in enough water if the roots are healthy. A three to eight-inch layer of compost, straw, or bark chips can not only keep soil temperatures down but can also prevent water loss from evaporation.
Yellowing leaves are worth your attention since they are a signal that your tree is stressed or unhealthy. With the exception of armillaria root rot and severe avocado black streak, the problems that cause yellow leaves are solvable. Your best course of action is to establish a consistent routine for watering and fertilizing your tree to keep it healthy for years to come.