Sunburn
Have you ever purchased a perfect looking specimen Euphorbia or a cactus at a nursery only to take it home, plant it then several days or weeks later find that your perfect looking plant has developed what looks to be large rust-like marks? Well, because many Euphorbias are grown in controlled greenhouse conditions under soft lighting when they are moved outside into full sun the high light and heat ends up breaking down the chlorophyll on the most exposed side of the plant thus causing sunburn. Scientists have recently isolated a protein in plants - called UVR8 - which recognizes damaging UV-B light rays. When a plant is exposed to sunlight the plant’s gene expression changes allowing it to naturally produce its own sun block. However, taking a plant from a greenhouse environment and suddenly putting it outside in full sun doesn't give the plant much of a chance to acclimate to it's new environment before getting damaged by the harsh sunlight before the necessary cellular, protective changes are made. Fortunately for humans, we can regenerate our dermis quickly and without too much visible scarring. Euphorbia do not have the same luxury and instead, a brown scar is formed where the sunburn occurred. Unexpected sunburn can also occur in the Fall when there is dense fog or dew in the mornings then a strong sun later on in the day which magnifies the water droplets on the surface of the Euphorbia. This magnification can actually cause little brown spots the size of the droplets. Even a change in seasons can cause a burn to an Euphorbia new to the landscape. The circular declination of the sun changes from winter to summer and suddenly the existing plant may be receiving sun where it hadn't in the past. The best way to avoid sunburn on your greenhouse plant is to place it in an area where it is protected from the sun during the hottest part of the day. You may still get some sunburn but it will be limited. The better option may be to buy your Euphorbia from a nursery where it has already been growing in the sun and if possible when you go to plant it in your yard try to keep the directional orientation of the cactus the same as it had been at the nursery. Never take an Euphorbia that has been growing indoors and immediately place it in direct sunlight as it will burn severely.
Photo-toxic Poisoning
Damage on cacti due to photo-toxic poisoning looks very much like sunburn and in a sense is sunburn. Photo-toxic poisoning is caused when chemicals - usually insecticides or fungicides- are applied to kill a pest on the plant and the plant is exposed to direct sunlight afterward. You will notice that your cactus - that has been growing in full sun but had not shown any previous signs of damage - is suddenly showing leaf burn. Not all chemicals cause this reaction usually only those that contain some sort of oil. Since this problem is brought about by the grower's own action, it should be fairly easy to diagnose. If the damage appears shortly after chemically treating the plant, then photo-toxic poisoning is likely the cause. Once again, prevention is the key. If you must use chemical controls on your plants, be sure to keep the plant out of direct sunlight until you've had a chance to wash the residual chemical off the plant. If you are reading this after the fact, then you can't undo any damage that has been done. Get your plant out of the sun immediately if it has just recently been treated with a chemical but if you can't move it, then cover it with a light weight shade cloth until the the chemical is off of the plant.
Corking
Often times people who own cactus notice that their mature plants are starting to turn brown at the base. This hard brown outer layer is the dermis of the cactus and is a natural phenomenon and part of the aging process of many cacti and Euphorbia's called corking. The plant dermis is actually undergoing a strengthening and hardening almost like a tree trunk which is just a natural part of the aging process. If your cactus plant has corking around the base, then simply enjoy the beauty of a maturing plant. Injury
Sometimes when a gardener is moving, planting or weeding around a cactus the gardener may inadvertently scratch, bruise or otherwise injure the cactus without even knowing it. This injury may not be apparent immediately or may just seem like a slight discoloration at the time, but within a few days, a brown spot or obvious scar from a scratch will appear where the cactus or Euphorbia has healed. Like humans if the injury is deep enough the visible scar tissue will remain for the life of the cacti. If moving multiple cacti, take special care not to let them touch each other, protect the plants with pieces of carpet or plenty of newspaper. If what initially looks like scarring from an injury starts to spread then your plant may have been infected by a pest such as scale. We will address pest issues in a separate blog entry.
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