likely oedema, a physiological disorder that can affect all plants. It occurs
when the plant takes up more water than it can rid itself of via the
process of transpiration. In afflicted plants, internal water
pressure (turgidity) can become so high that some leaf cells rupture
or cell contents leak into inter-cellular spaces in leaf tissue.
Oedema is most often driven by excessive water retention in the soil,
but cool temperatures, high humidity levels, low light conditions, or
partial defoliation can individually or collectively be additional
contributors, as can anything else that slows transpiration. As well,
nutritional deficiencies of Ca and Mg are known contributors to the
malady. Symptoms vary by plant, but can include wet or weepy
areas on the foliage, blisters that turn from white or tan to dark
brown or black, then gradually turn corky/ scaly/ ridged, and
wart-like or gall-like bumpy growth.

Al

Thank you for the information. We didn't get the advertised El Niño in Southern California this year, so I've continued to water weekly - but maybe I need to cut back. I believe drainage is pretty good but I will double check that. How would I know if my soil is deficient in Ca and Mg, or should I just add them as a precaution?

The only real way to tell (if you have deficiencies or toxicities) is via a soil test. Adding nutrients as a precaution w/o reason can create an excess (toxicity), which has the same potential to limit as a deficiency. Check with your county cooperative extension agent for soil test info, or if you have a Master Gardener's Hotline, call them for info. If you have a cooperative extension agent, you can likely take a piece of affected plant to him for a second opinion. A hands-on look makes a positive diagnosis much easier, but it does appear to be oedema.

Al

Can I ask what is a 'county cooperative extension agent' please? Being from over the pond I have never heard of that term before. Thanks ppl.

Gill UK

Hi! Im on this page because I have an elephant bush that has a similar issue. I need your help as my plant has red ‘warts’ and brown ‘blisters’. Im confused as it does get beaten up by a bitey cat but im still not sure.

My plant is showing the exact same spots. It was fine and then we had a lot of rain. Its planted in a pot so I moves it inside out of the rain and suddenly these spots appeared. I moved it back outside so I am hoping it will heal.

Animals are regenerating systems in that they heal by growing new cells in the same spatial position as those dead, damaged, or diseased. Plants are different. They are generating systems, and grow a tissue type altogether different from the original in the former spatial position of dead/ damaged/ diseased cells. The corky, callus tissue at the site (scabs) where cells ruptured due to excessively high internal water pressure, caused by a physiological disorder called oedema, are as close as you'll be getting to healing.

You should be able to reflect on the plant's recent history as you read the short piece I wrote about 'oedema', to determine how to best avoid it in the future. The short answer is - use an appropriate substrate and use a 'tell' to 'tell' you when it's time to water, as opposed to a finger stuck in the soil or an inexpensive "moisture meter"

Oedema

Oedema is a physiological disorder that can affect all plants. It occurs when the plant takes up more water than it can rid itself of via the process of transpiration. The word itself means 'swelling', which is usually the first symptom, and comes in the form of pale blisters or water-filled bumps on foliage. Under a variety of circumstances/cultural conditions, a plant's internal water pressure (turgidity) can become so high that some leaf cells rupture and leak their contents into inter-cellular spaces in leaf tissue, creating wet or weepy areas. Symptoms vary by plant, but as the malady progresses, areas of the leaf turn yellow, brown, brown with reddish overtones or even black, with older damage appearing as corky/ scaly/ ridged patches, or wart/gall-like bumpy growth. Symptoms are seen more frequently in plants that are fleshy, are usually more pronounced on the underside of leaves, and older/lower leaves are more likely to be affected than younger/upper leaves.

Oedema is most common in houseplants during the winter/early spring months, is driven primarily by excessive water retention in the soil, and can be intensified via several additional cultural influences. Cool temperatures, high humidity levels, low light conditions, or partial defoliation can individually or collectively act to intensify the problem, as can anything else that slows transpiration. Nutritional deficiencies of Ca and Mg are also known contributors to the malady.

Some things that can help you prevent oedema:

* Increase light levels and temperature

* Monitor water needs carefully – avoid over-watering. I'd heartily recommend a soil with drainage so sharp (fast) that when you to water to beyond the saturation point you needn't worry about prolonged periods of soil saturation wrecking root health/function. Your soil choice should be a key that unlocks the solutions to many potential problems.

* Avoid misting or getting water on foliage. It slows transpiration and increases turgidity.

* Water as soon as you get up in the AM. When stomata close in preparation for the dark cycle, turgidity builds. If you water early in the day, it gives the plant an opportunity to remove (for its own needs) some of the excess water in the soil.

* Put a fan in the room or otherwise increase air flow/circulation. Avoid over-crowding your plants.

Using a 'tell'

Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor.

In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water.

Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'.

One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue.

Al