Aloe Pests and Diseases

By Bob Williams (February 2020)

Almost every member of HSCSS has an aloe in his or her collection, and many not in the succulent hobby have an Aloe vera plant on the kitchen windowsill in case of burns. A number of pests and diseases can attack – and damage – these plants.

Haworthias and gasterias are very closely related to aloes. Some of the same pests and diseases that affect aloes also attack these plants. In my research, I came across a website – https://agaveville.org – that is very informative for not only agaves, but also aloes, haworthias and gasterias.

Let’s talk pests. In my research, our plants can be affected by six insects. Some insects endemic to Africa, such as the aloe beetle, are not a cause for concern here. Scale, aphids and mealy bugs were discussed in Mike Hellmann’s article on pests in the January 2020 Digest. There is nothing more I can add.

Spider Mites

The article also addressed spider mites. The mite that finds aloes delectable is the red spider mite. If you have an infestation of these, you will see small, pale markings on the surface of the aloe leaves. Aloes do not like these mites at all. Even flowers may be distorted, as the mites attack the emerging flower stalks.

Visible mites like these are easily contained by a sprinkling of insecticide powder in the centers of plants. The mites succumb to most insecticidal spray chemicals, as well.

Snout Beetles

Next on our list of pests is the snout beetle. This is aloe enemy No. 1, so quick action is required when you find these insects on your plants. The beetle, which can be up to 3/4 inch long, aims at the center of an aloe plant. It wedges itself between the leaves to insert its snout and drink the leaf sap. This leaves a telltale dark spot that dries into a pea-sized dry spot with a puncture mark in the middle.

Once the beetles have mated, eggs are deposited at the base of a leaf. The newly hatched larvae bore straight into the stem, where they spend the remainder of their larvae cycle. The rot and destruction caused by the larvae are what eventually kills the aloe.

Physically removing and killing the beetles or a sprinkle of insecticidal powder should take care of the mature beetles, if they have not been there too long. The number of bore holes and their distance from the center of the plant will tell, at a glance, how long the beetles have been active. A large number of bore holes away from the center of the plant almost certainly means there are beetle larvae destroying the plant from inside.

If this is the case, you have few options. If the plant is small, a good trash can is in order. If the plant is large, start cutting the stalk of the plant above the infestation. If more bore holes exist, cut some more. When you do not see any more holes, dip the stalk in rooting hormone, let it dry for a short time and see if it will root. A good systemic insecticide can be used as a preventative precaution.

Gall Mites

Another mite is particularly attracted to aloes. This is the gall mite. If you have an infestation of these little buggers, your aloes have a condition called gall cancer. Very often, the first sign of gall mites is a new but crooked flower cluster that emerges from a plant. Upon closer inspection, you may see the first signs of frilly growth on the flower stalk, which develops into unsightly galls on the small flower stalks that form the cluster as it matures.

The same galls may also start as an irregular growth on the bases of older leaves, often where an earlier flower stalk has dried. Tree aloes are often the source of infestation. Huge galls form on old flower stalks and, if left untreated, can spread their inhabitants for many years.

The gall mites travel through the air. Cut away the affected tissue with a sharp blade and treat the cut with a strong solution of aphicide or other systemic insecticide. The whole plant can be sprayed with the manufacturer’s recommended solution of the same insecticide a day or so later. Aloes that touch an infected plant are particularly vulnerable.

Fungus Gnats

One last pest that may affect your aloes is fungus gnats. Fungus gnats are generally the result of overly wet soil. Allowing the soil to dry out will kill the gnats annd prevent them from spreading. Fungus gnats are easy to prevent altogether by not overwatering. If overwatering is an issue, you may be inviting disease.

Basal Stem Rot

Now that the critters are out of the way, we turn our attention to diseases. The first and most familiar disease is basal stem rot. The result of cold and wet conditions, this condition leads to rotting stems. Aloe tissues affected by basal stem rot turn black or reddish brown.

It is possible to take a stem cutting above the rotten portion to save pieces of the plant. (As you can see in the picture), this plant is a goner. The rot travels up the stem, so early detection is a must.

Aloe Rust

The next disease on the list is aloe rust. Caused by the fungi Phakopsora pachyfhiza and P. meibomiae, this fungal infection causes black or brown circular spots on the aloe leaves. The fungus invades the outer leaf structure and oxidizes the organic compounds in the leaf structure called phenols. The result is a spot that becomes blackened and hard. This rust also can occur on haworthia and gasteria leaves.

The good news is that this condition does not kill the plant, and new growth will not show signs of the rust. Most aloes lose their leaves as they grow. When they fall off, throw them in the trash.

Many plant species can serve as hosts for these fungi. Some have built up immunity, while the aloes, haworthias and gasterias have not. The spores of the fungi are carried by the wind and can travel a long distance. The conditions that favor infection are extended periods of leaf wetness and a temperature range of 60 to 82 degrees F. Temperatures above 86 degrees prevent development of the disease.

Spores are produced 10 days after infection and released about three weeks after that. As long as the environment is moist with moderate temperatures, spores will be generated.

There are some ways to prevent aloe rust from occurring. First is to not let water stay on the leaves. Good air circulation can help keep leaves dry. Get rid of any leaves that you have cut off or have fallen off as soon as possible.

Dust plants with sulfur powder every one to two weeks. This will not kill the rust, but will prevent germination of new spores. Spray a solution of one teaspoon of baking soda in one quart of water to help fight the rust. Foliar fungicides may also help. In one article, the author recommended using copper oxychloride.

Sooty Mold

Our last condition is sooty mold. Sooty mold is a fungal infection that is secondary to an infestation of aphids or mealy bugs. Aphids and mealy bugs are pests that suck moisture out of plants and leave a clear, sticky substance called honeydew behind on the leaves. The honeydew creates a moist atmosphere that eventually develops into sooty mold. Sooty mold does not kill the plants; the insects will do that.

To address sooty mold, wash off the honeydew. This substance can be hard and lessen the amount of light reaching the leaves, reducing photosynthesis.

Sources:

Sunbird Aloes – http://www.sunbirdaloes.co.za
The Cape Aloe – http://pza.sanbi.org/sites/default/files/info_library/cape_aloe.pdf
Garden Guides – Aloe Plant Disease – https://www.gardenguides.com/93474-aloe-plant-disease.html
Diseases of Aloe vera – https://www.slideshare.net/DineshDalvaniya/presentation1-aloevera-disease