Avocados have seen a monumental rise in popularity over the last decade. Their nutritious, creamy texture makes them a revered fruit to grow at home and your avocado tree will make you the envy of your neighbours!
However, they take on average 7 years to bear fruit so planting an avocado tree is a long-term investment. They can be prone to disease and require exacting conditions making them quite vulnerable.
Brown spots on your avocado leaves can be a common occurrence and indicate the first signs of an issue. Let’s look at some of the reasons brown spots appear.
Why Does My Avocado Plant Have Brown Spots
Avocados have a preferred set of conditions in which they will thrive. They originate from the tropical states of Mexico and the rainforests of South America, so they enjoy a humid and warm climate, specifically sandy loam soil and require adequate water and sunlight.
If any of these elements goes off kilter it will start to impact the basic plant process of photosynthesis which will deplete the leaves of chlorophyll and lead to chlorosis.
Their delicate nature can also make them vulnerable to specific pests and diseases. Each one of these issues will ultimately lead to discolouration and varying types of brown spots and patterns appearing on the leaves.
6 Causes Of Brown Spots on Avocado Leaves
Damaged, mottled and discoloured foliage is a clear indicator that something is amiss when it comes to growing most plants. As for avocadoes, brown spots on the plant’s leaves can mean anything from insufficient environmental conditions to serious pest infestations and disease.
Brown avocado leaves typically occur when the plant is over or underwatered, subjected to a lack of humidity or when lighting conditions are just not so.
Here’s a closer look a the most common causes of the leaves of your avocado plant turning brown.
1. Inadequate Watering
Avocado trees are tropical plants therefore they are used to receiving a lot of water. It is easy to underestimate the amount of water they need to thrive and stay healthy.
If you don’t provide adequate water, you could dehydrate the tree and cause the leaves to turn brown and fall off. This could also cause discolouration in other parts of the tree too.
However, it is a very fine balance, and you still need to avoid overwatering your avocado tree, or it could develop root rot.
Over-watering can also lead to nitrogen deficiency as the more water in the soil the weaker the dilution of nitrogen so, even though the tree is absorbing the same amount of water, it will be absorbing less nitrogen. A nitrogen deficiency causes the avocado leaves to turn yellowish and dark spots will suddenly appear.
Treatment for Inadequate Watering
If you do not live in one of our more tropical States you will need to water your avocado every 5-7 days until the soil is saturated. If you are in a more arid, hot but dry climate you will need to water your tree more.
If you believe the brown spots have been caused by nitrogen deficiency, then water moderately and regularly and add ammonium nitrate to the soil. I also like to use natural nitrogen supplements around my tree including my old coffee grounds, bone meal, grass clippings, fallen leaves and manure.
2. Humidity Levels
Although linked to watering, humidity levels need their own section when it comes to avocado trees. They are native to regions with very high humidity levels and thrive when humidity is between 70% and 80%.
If the humidity level drops too low the moisture that is available will be diverted to the fruit and other vital areas, causing the leaves to dry out and turn brown. This can lead to bigger problems for your tree than just losing its leaves.
Treatment for Low Humidity
If you think lack of humidity is the problem, you need to increase the humidity level around the tree. You can do this using a humidifier or a pebble tray, or if your tree is still quite small, you can mist it once a day.
3. Excessive Sunlight
Avocado trees like plenty of light and warmth but they can be scorched by too much direct sunlight. This will often occur when you plant a potted tree out in your garden. It can be shocked by the sudden influx of direct sunlight.
Scorching causes brown blemishes on the leaves and if not treated the leaves will wilt and drop off.
Treatment for Excess Sunlight
Consider moving your plant to an area that does not have such harsh sunlight. If your plant is potted or an indoor tree an Easterly-facing window or spot in your yard is good.
If your avocado tree is planted in your garden, you can cover it with shade cloth or even an umbrella to protect it from a precedented period of harsh direct sunlight.
4. Deficiencies
Avocados are famed as a rich source of potassium, so they can be prone to deficiencies as so the fruit needs such large quantities.
You will notice if your tree is lacking potassium by the appearance of scattered small light brown specks between the veins of the leaf with chlorosis appearing on the leaf margins. As the leaf ages, the specks will turn to large areas of dead tissue giving an almost scorched appearance to the tree.
Deficiencies of other trace elements will also manifest in a similar way with leaves taking on a scorched appearance.
If you notice any of these signs it is imperative that you treat the deficiency quickly before it starts to impact the avocados.
Treatment of Deficiencies
The best way to correct potassium or any other mineral deficiency is to use a fertiliser. In the case of potassium, one rich in potassium nitrate or potassium sulphate is wise.
5. Pests
Avocado trees can be especially susceptible to pests. They enjoy the juicy leaves and thrive in the humid conditions that avocado trees enjoy. They can all be easily removed and treated but early intervention is key to stopping them from getting out of control.
According to the University of California, the Persea mite is generally only found in California. They feed on the underside of leaves and leave behind a tell-tale silvery webbing that is denser than that of other mites.
They cause small brown circles, concentrated on the veins of the underside of the leaf that eventually become so dark they are visible on the topside of the leaf.
The six-spotted mite feeds on the underside of the leaf in a similar way to the Persea mite but it leaves much larger brown blotches in its wake.
The blotching is usually dark brown, but it can take a purplish colour too. If you find this shade of spots and they are also irregular in size and shape, they likely have been caused by the six-spotted mite.
The Avocado lace bug gets its name from its lacy appearance. In small numbers, they will not do much harm, but they do feed on the juices in the leaves.
In high enough numbers, lace bugs can create a mottled yellow-to-brown spotting effect on the leaves they’re feeding on from sucking the juice from the leaves and the brown droppings they leave across the underside of the leaves.
Treatment for Pests
Too much pesticide, even natural insecticidal soaps, can weaken an avocado tree, leaving it more susceptible to other types of infections, be it fungus infections, bacterial infections, or pest infestations.
For all of the pests listed above, it is recommended to spray with water alone, at high pressure, to blast mites off the leaves and get rid of their webbing.
You could clip off impacted leaves and your tree should soon return to health.
It is worth noting that the avocado lace bug lays eggs in the leaves so repeated treatments may be needed.
6. Diseases
Again, the delicate nature of the avocado and the humid conditions in which it thrives also make it susceptible to infections which can spread rapidly in such warm and wet climates. Two key ones that impact avocado trees are as follows:
Anthracnose
If you are experiencing a higher-than-usual leaf drop, check the leaves on the ground to see if there are brown spots and brown leaf tips. If you do, chances are the tree has the anthracnose virus.
The Rincon, Fuerte and Wartz varieties are more susceptible to the anthracnose virus. The Hass avocado is much less prone.
Algal Leaf Spot
The leaves on the low-hanging branches of your avocado tree and the ones most susceptible to algal leaf spots, particularly, the bottom 2 feet.
The spots are raised and dark brown or orange and look like dots of rust. If you scrape the surface, they will be dark grey underneath.
The leaf spot it leaves just looks unsightly, but it will not harm the actual fruit of the tree. You could consider pruning impacted leaves off.
Treatment for Disease
The most effective method of prevention against disease is to keep avocado trees pruned to 2 feet above ground level. Fungal infections like algal leaf spots can be treated with a copper fungicide, but that will only prevent new spots from developing.
Existing spots caused by most fungal infections are better pruned away and the leaves are carefully disposed of away from your compost heap.
Keeping looking out for any impacted leaves and pruning away then ensuring humidity and water levels are good and consistent should stop any devastating spread.
Verdict: Brown Spots on Avocado Leaves
A fruiting avocado tree makes the most fantastic centrepiece to your home garden, but they do require precise conditions in which to thrive. Brown leaves are generally the first indicator that something is off-kilter.
Identifying the cause and acting accordingly should ensure that the issue does not start to impact that most sought-after fruit – a delicious homegrown avocado!
Citation
The University of California – Pests in Gardens and Landscapes
Ben Hilton
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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.
