The Cause of Fuzzy Growths on Rose Bushes

You may have encountered a peculiar fuzzy substance growing on your rose bush in past seasons or maybe even this season. What is this thing that looks like it stepped out of a Dr. Seuss book? Is it nature, or did my neighbor spray something funky nearby? Is it poisonous or can I touch it (and maybe pet it?–just curious). Am I going to lose all of my roses?

Never fear! You’ve likely encountered a rose gall. This garden peculiarity is something rosarians worldwide have encountered for hundreds of years.

It’s normal to question what’s going on when your plant experiences something unusual. Let’s dive into mossy rose gall (also known as rose bedeguar or Robin’s pincushion) so you can feel confident in the future about handling this disease that really isn’t a disease.

The Long Answer

Let’s dig deeper into what causes galls to form, if they affect overall plant health, which roses are most susceptible, how to deal with the galls, and a common look-alike. Oh, and there’s some really weird historical facts about this phenomenon, too!

What Causes a Gall to Form on a Plant?

Galls form because of the interaction between two living organisms, typically a plant and an insect, disease, or mite. When the affecting organisms burrow into the plant or infect the tissues, it causes the plant to change its normal growth.

The cynipid wasp female uses its ovipositors to insert eggs into leaf buds of the roses in the spring. This initiates gall growth. Research shows that the larvae later produce chemicals to continue gall formation.

The tissues swell around the gall maker. With the cynipid wasp, the gall provides protection from weather, parasites, and predators. Basically, the gall ensures the eggs and larvae live a sheltered life.

Galls are often rich in proteins and carbohydrates, so the larvae are also being enriched by the casing, as well. Eventually, the larvae of the cynipid wasp pupate and emerge out of as an adult wasp, usually in the spring or summer. The Diplolepis rosae adult wasps are usually females and ready to lay eggs quickly–no male needed.

The galls are typically one to two inches in diameter, depending on the number of larval chambers within the gall. Each chamber contains a single larval wasp. More eggs equals a larger gall.

Do Mossy Rose Galls Affect Overall Plant Health?

Although they look alarming, these galls do not affect the overall health of your rose. The galls are simply unsightly. But, some would say, in the beginning stages, they are peculiar enough to keep around.

Occasionally, the growth above the gall may weaken and either fall off or need to be pruned. The gall is likely inhibiting the movement of nutrients to the end of the stem.

Which Roses Are Most Susceptible?

There is no clear evidence indicating which roses are more prone to mossy rose gall, but gardeners seem to notice a pattern between bushes that are affected more often.

You will most likely see mossy rose gall affecting common hybrid tea roses or multiflora roses in the United States. Early reports show that Rosa canina was also one rose found to commonly be infected in the U.S. The issue is more common on roses residing in Europe.

Also, there is some evidence to suggest that stressed roses are more likely to be “infected” compared to healthy roses.

How Do You Prevent or Treat Mossy Rose Galls on Roses?

As mentioned before, the galls are not harmful to the plant, but you’re probably eager to get them out of your garden–especially when they turn ugly grayish brown.

Unfortunately, there is no real cure or prevention. Some people suggest spraying for the wasp, but most sprays are not only ineffective (the wasp can fly away, and you don’t know when they’ve deposited eggs in the plant), but they do more harm than good. 

First, the larvae exist deep within the gall for protection, so sprays will not affect the larvae, and second, the wasps are only around to lay their eggs and move on.

The best way to get rid of galls is to prune them out. Some gardeners report that pruning out the galls will reduce the amount of mature female wasps that hatch in your garden and therefore reduce your chances of galls. This may or may not be true.

Regardless, if you don’t like the look of the galls, prune them out and destroy the affected stems by burning or burying them. You can use these Felco pruners to get the job done. 

Mossy Rose Gall Look-Alike

While you may not think that the spiny rose gall looks exactly like mossy rose gall, you may have run across this look-alike before. Spiny rose gall is also caused by a cynipid wasp, but this one is Diplolepis bicolor, not D. rosae.

Spiny rose gall is round with many pink spines on the surface. Reports show that spiny rose gall is usually exclusive to wild roses. Again, there is no proper treatment for spiny rose gall. It is only an aesthetic issue.

Interesting Historical Facts

Interestingly, humans used “bedeguar galls” to cure many ailments throughout history. The word bedeguar comes originally from Persia and means “wind-brought.” Undoubtedly, early gardeners didn’t understand that the galls came from wasps.

A few hundred years ago, people collected bedeguar galls from affected roses and usually dried and powdered them. A mixture of gall powder and honey supposedly cured baldness. Other historical accounts show people believed in using the mossy galls as treatments for diarrhea, dysentery, worms, and scurvy. The Italians believed for a short time that the powdered galls could cure the bite from a venomous snake!

Mossy rose gall is also called “Robin’s pincushion,” referring to a woodland sprite in English folklore named Robin Goodfellow. This sprite also shows up in Shakespeare’s play A Midsummer Night’s Dream as the character Puck. At early stages of development, the galls resemble a sewing pincushion, and the random nature with which these galls develop likely inspired this name.