Aloe pests and diseases

Going.Local
Published on May 14th 2020
15
A tree with green leaves
If you find yourself with a sick Aloe, the first step is to identify the problem. This is done by cataloguing symptoms (e.g. spots/wilting), linking it to a disease/pest and subsequently connecting it to a cause.
Here is a comprehensive list of symptoms to follow and help you take care of your plant.
A close up of a aloe
Leaf curl in aloes can result due to insufficient watering or when it is exposed to high light intensity.

OVERVIEW

This section is merely an introduction to give you an idea of what will be discussed. Where possible, the individual pest has been linked to plant profiles, or you can continue scrolling to view posts and photos associated with each.
ENVIRONMENTAL PROBLEMS:
  • Crisp / dry leaves - Sunburn (not applicable to old leaf die-off)
  • Curling leaves - Underwatering and/or too much sun
  • Discolouration from green to brown/red (whole plant) - Sun exposure (normal)
  • Leaves appear pale green at the center and plant leans to the side - Too little light (Aloe version of elongation)
  • Parts turn hard white/beige - Physical damage (hail, dogs, insects etc.)
  • Burst or torn leaves - Too much water in a short period
A close up of a crab
PESTS:
DISEASES:
  • Leaf spots / fungal infection
  • Aloe rust
    • Uromyces aloes
  • Stem rot / fungal infection
    • Pythium ultimo
  • Root rot / fungal infection
  • Soft rot / bacterial rot
    • Erwinia chrysanthemi

PHYSICAL DAMAGE

Cream / white or light brown scarring
Like humans, plants will cover a wound with a ‘scab’ that will appear off-white. This is due to several factors, but mostly physical damage to the plant. When the area is rimmed by a black or brown line it could be a fungal infection as a result of compromised tissue. In some cases where insects have been removed, the remaining tissue of the leaf can appear bronze.
  • Cause: Physical (hale, sunburn, spade/dog) or pest damage (snout beetle)
  • Treatment: Newly bought aloes should be gradually exposed to sunlight. Place it back in indirect light and give it slightly more light over a period of 2 weeks. Scars cannot be treated and will remain on the plant until the leaf dies. Do not remove the leaf as this poses a higher risk for fungal infection. Treat secondary fungal infections (see Diseases).
Burst / torn leaves
When a leaf bursts or a tear appears in the middle of a leaf it is the result of the plant taking up too much water. Unlike tropical species they cannot get rid of the excess water and the burst leaves either callus or get infected by fungal spores turning brown/black.
  • Cause: Too much water in a short period of time. (Either abnormally high rainfall or overwatering).
  • Treatment: Let the leaves dry or remove affected leaves. Plant in a grittier, rocky mix to stop it from happening again. Place powdered sulfur on exposed regions.
A close up of a aloe
Insect predation opens the leaf to fungal infections that can appear in a range of colours.

FUNGAL INFECTIONS

Fungal infections can look like spots, concentric circles or sections of discoloured leaves that appear black/brown/bronze/orange. In addition, some may cause dents/crusts, wilting of foliage, bumps or pustules on leaves.
Crown rot
If you observe your newer leaves discolouring or turning soft it is most likely a bacterial and/or fungal infection. If the leaves are black then it is most likely a fungal crown rot.
  • Causes: Overwatering, or watering from above.
  • Treatment: Cut away dead tissue and dust with an anti-fungal powder. Keep the remaining pieces as dry as possible until the area scars/calluses over. A healthy stem might produce new heads once placed in adequate soil. Adjust watering practices.
Rotting leaves with healthy roots
Some dual infections (bacteria and fungi) can result in rotting leaves with healthy roots.
  • Cause: High temperatures and stressed plants exposed to fungi/bacteria (Fusarium /Dickeya) infection.
  • Treatment: Unlikely, as the bacterial infection would have spread rapidly. You could try removing dead tissue, applying sulfur to exposed areas and letting the plant callus before watering again.
Standing evil (Stem rot)
Young plants will show a stem / base discolouration (slight brown) causing plants to fall over whilst leaves are green.
  • Cause: Pythium ultimum, which flourishes in wet clay soil at temperature < 20 C.
  • Treatment: The stem can be washed, dried and placed on new soil to root. Remember to use aerated soil with adequate drainage.
Root rot
When aloes appear dehydrated (even with wet soil) or the entire plant comes loose without roots, it is most likely root rot. The roots may appear slimy or dried depending on the age of the infection.
  • Cause: Overwatering, not enough aeration in the soil or a beetle larvae/mealybug infestation that exposed roots to fungus.
  • Treatment: Remove dead tissue, wash remaining plant and dust with powder fungicide. Let it dry for a period of 7-10 days before placing in new soil to root. Discard old soil.
Large leaf spots
A black, brown or red spot is often caused by one of several fungal species. This should not be confused with spider mite infestations which result in tiny spots littering the leaf. Fungal colonies infect the tissue in the immediate vicinity of the exposed tissue area causing it to die. Some may refer to dark black sunken spots as sooty mould (this is often due to an infestation of mealybugs or aphids exposing tissue).
  • Cause: Overwatering, lack of airflow and/or insufficient light causes fungal infections to develop. In some cases, insect predation or damage encourages fungal infection.
  • Treatment: Improve planting conditions by moving it to a sunnier area with better airflow. Apply fungicide to affected areas or remove affected leaves and let wounds callus (dry and scar over). Citrus seed extract / Copper oxychloride spray (fungicide) application will help affected leaves.
  • Beware: Aloe vera treated with a fungicide should not be used for household purposes.
Powdery mildew
Powdery mildew is a fungus that has a softer appearance due to the morphology of the fruiting bodies. The leaf may appear like it is covered in soft white fuzz.
  • Cause: Temperatures of 25 C and high humidity (stops at temp >34 C) in other words aloes planted in shaded areas or between lots of plants.
  • Treatment: Better placement in terms of sun and airflow. Minimize spreading by covering area with wet tissue paper, trapping spores, prior to leaf removal. Treat the infection with cinnamon or a sulfur-based fungicide.
Aloe rust
Rust is just a colloquial term for a fungus (Uromyces aloes) that appears in concentric circles of bronze/orange on leaves.
  • Cause: Fungus (Uromyces aloes)
  • Treatment: Affected leaves need to be removed carefully. The concentric circles are fungal hyphae that present spores outside the leaves. Any disturbance (via a spray or similar vibrations) will result in the spores becoming airborne and affecting more plants. To stop the spores from becoming airborne, cover the affected area in wet tissue paper, cut the leaf and discard affected leaves in closed bags. Treat cut sites with a powder fungicide.
  • Aloes known to be affected: Aloe baumii, A. glauca, A. maculata, A. saponaria, A. greatleadii, A. inyangensis, A. spicata, A. polyphylla, A. abyssinica, A. thraskii and A. littorals.

BACTERIAL INFECTIONS

Symptoms can include leaf/stem/trunk or tuber rotting, galls or areas on leaf that turn soft becoming light brown then dying (the latter can also be fungus infections).
Soft rot
  • Cause: Temperature 25-35 C, high humidity, waterlogged soil and excess nitrogen fertilizer, enters through damaged leaves.
  • Treatment: Prevention rather than cure. Avoid excess fertilizer, overwatering or watering from above and damage to foliage. Saving an infested plant tends to be unpractical, but you can try and remove infected tissues whilst removing and replanting in a sunnier location.
A close up of a aloe
Insect, or in this case caterpillar, predation on aloes can cause secondary fungal or bacterial infections.

PESTS

White scale
These look like small white stripes on the leaf that on closer inspection form individual white bumps. They are relatively easy to remove with a hose or wiping the leaf down with a clean cloth. There is also several commercially available pesticides and homemade remedies to remove/kill them. Two applications separated by 3 or more days should alleviate the problem.
For a homemade recipe and treatment see my prior post:
For more examples of white scale see:
Red scale
Red scale is rather easy to discriminate as it looks like the plant is covered in tiny fish scales.
  • Cause: Sub-optimal growth conditions
  • Treatment: You can treat it with a pesticide, however, the dead scales will still cling to the plant. You will have to physically wipe every leaf to remove the dead scales.
Snout beetle
Snout beetle predation results in white or off-white round circles of 5mm in diameter on the leaf surface. Larvae will bore into the plant leaving trails as it eats.
  • Treatment: Physically remove snout beetles or spray pesticide.
Red spider mite
The leaves take on a speckled appearance as the mites interfere with photosynthesis. Fine spiderwebs may also appear on occasion.
Treatment: Hose down the plant and apply the appropriate insecticide.
Whitefly
These tiny white flies or larvae attack leaves. The leaves tend to change colour then drop off. The symptoms are similar to spider mites, however whiteflies are easier to spot as they are larger.
  • Treatment: There are some biocontrol agents and chromatic traps available to curb whiteflies.
Caterpillars
Some caterpillars can wreak havoc on aloes by feasting on the newer leaves. They are easy to spot and remove. If left unattended the plant may suffer severely.
  • Treatment: Physically remove.
Aphids
Aphids tend to be a problem during the flowering season. If you have ever seen flower stalks develop and then get stripped before flowering it might be due to insect predation on the newly developed flower head.
  • Treatment: Remove with a hose, use a biocontrol agent like Aphidend (www.koppert.co.za) or apply an insecticide.
Abnormal growths: Gall mite / Aloe cancer
When a plant presents with abnormal growth, leaf or flower deformation the cause is most likely microscopic mites.
  • Treatment: Most will advise you to discard the plant. The only remedy is to sterilize an implement (with surgical spirits) and cut away the affected area. Do not be conservative and make sure to remove everything. Do not let the infected tissue touch any other aloes.
Examples of Aloe cancer:
BEWARE: Some proprietors may advise the use of Formalin/Formaldehyde (Formalin is buffered Formaldehyde) as treatment for Aloe cancer. Please take note that Formaldehyde is categorized as highly toxic to inhale, touch or consume and can affect your central nervous system. It is not advised for general use without advanced training.

FINAL NOTES

Aloes do fall prey to diseases in their natural habitat, however, well-placed plants should not need intervention. Always remember to read the material safety data sheet for any chemicals you plan on using and be safe.
Remember, we love to hear from you so let us know your thoughts in the comments below!
Aloe disease
Montagnella infection on the lower leaves of an Aloe. This is normally treated with a fungicide.
Bibliography:
(1) Van Jaarsveld, E. (2013). Waterwise Gardening in South Africa and Namibia. Penguin Random House, South Africa.
(2) Amura, C. (2020, 30 April). Aloe vera Complete Guide. My Garden Guide. https://mygardenguide.com/succulents/aloe-vera/#Cultivation_techniques
Free download for your phone or tablet
Download on the App StoreGet it on Google Play

Lots to see

Follow and read AlanGardenMaster’s articles as he develops his new one-acre plot. PimlicoDan shows city gardening in a whole new light, or follow DaisyDays on her adventures in the allotment and as a professional gardener. Just a few of the many personalities you’ll meet in our app. Free download for your phone or tablet.
Download on the App StoreGet it on Google Play