Mushroom Monday

Azalea Gall - Exobasidium spp.

Good evening, friends,

This week’s fungus is another one from Cape Cod, the Azalea Gall (Exobasidium spp). On our walk in Central Park yesterday we found a couple different types of galls - Hackberry nipple galls and Witchhazel cone galls - which were made by insects, but the azalea gall I found last week on a swamp azalea (Rhododendron viscosum) is caused by a fungus.

A couple years ago I took a class on galls and loved it. When people were asking about them yesterday I was able to regurgitate a little, but I realized I still didn’t have as good of an understanding for how frequently I encounter them. Fortunately, I still have my notes from the class so we can all get down on this gall refresher.

Fun Facts

A gall is a benign but structured growth of external plant tissue. Galls are formed either through hypertrophy - an increase in the size of cells - or hyperplasia - an increase in the number of cells. Exobasidium galls are created through hypertrophy and typically grow on the leaves of the plant. Galls on other plants can form on the leaves, flowers, stems, and buds - it depends on the type of gall.

A wide variety of organisms can produce galls: viruses, bacteria, fungi, nematodes, mites, aphids, moths, sawflies, beetles, fruit flies, gall midges, and gall wasps (Cynipidae). The gall-maker receives all the benefits (like shelter and nutrition) while the plant suffers from deviation in their direction of growth (and potentially more). While any benefit conveyed to the plant seems unintentional, there are people that believe the galls are defense mechanisms from the plant.

The gall, littered with insect frass (poop), grows right from the middle vein of the leaf.

98% of galls are found on angiosperms (flowering plants). Within North America, 50% of galls are found on plants in the family Fagaceae (oaks, beeches, and chestnuts).

There are at least two species of Exobasidium that grow on rhododendrons, E. vaccinii and E. rhododendri. In addition to the formation of galls, E. vaccinii also appears to cause leaves on the infected plants to turn red. All the leaves on our azalea were green so maybe this was E. rhododendri, but frankly the pictures of either on iNaturalist don’t look terribly similar as both species seem to display some amount of red on the leaf or the gall.

Swamp azalea flowers on the right compared to a gall on the lower left.

Ecology

The Exobasidium spores germinate with precipitation in the spring after overwintering in the buds of their host azaleas. As the fungus grows, parts of the leaves begin to turn fleshy and white. Over a matter of weeks, a pore surface forms on the galls from which spores are released. After the spores are released, the fleshy, white galls turn brown and drop to the ground.

Theoretically, the spores land on other azalea buds and the life cycle is repeated each spring. Once a plant is infected it is typically systemic and will have galls year after year. The fungus can also live in a non-pathogenic state as a yeast-like fungus in the soil.

The pore surface on one of the galls.

There was a study out of Canada (Reference 3) that indicated flame azalea (Rhododendron calendulaceum) reproduction is negatively impacted by Exobasidium. In the study, plants that had Exobasidium galls had either fewer flowers or smaller flowers, and subsequently produced less seeds. Another interesting study suggests that insects are the main vector of the fungus because plants the researchers protected from insects had much lower rates of fungal infection (Reference 4). The scientists also found Exobasidium in 3 different insects (two flies and a gall midge) out of the 181 insects they tested.

The gall had a taste and texture like an apple - it was surprising.

Some home gardening websites encourage people to remove the galls from their azalea to slow the spread of the fungus, but that’s only if they’re growing on landscaped plants in someone’s yard. Not sure how effective that would be, but I would be curious to see if it had an impact.

I ran into a great fun fact today on the origins of the term “dog days of summer”. While I always imagined the phrase was used to describe the type of heat that makes a dog lay down and pant heavily in the shade, it actually corresponds to the star Sirius - also known as the dog star - which is the brightest star in the night sky. Sirius begins to rise and set with the sun in late July and early August which are typically the hottest days of the year. It used to be thought that Sirius even helped contribute to that added heat.

Hydrate this week. Full moon on Sunday, 7/21. Peak illumination occurs at 6:17 AM though so you’re gonna have to set that alarm to see it,

Aubrey

mushroommonday.com

References:

1. https://ag.umass.edu/landscape/fact-sheets/exobasidium-gall-of-rhododendron-azalea

2. https://homegarden.cahnr.uconn.edu/factsheets/azalea-gall/

3. Lorne M Wolfe and Leslie J Rissler. 2000. Reproductive consequences of a gall-inducing fungal pathogen (Exobasidium vaccinii) on Rhododendron calendulaceum (Ericaceae). Canadian Journal of Botany. 77(10): 1454-1459. https://doi.org/10.1139/b99-108

4. https://apsjournals.apsnet.org/doi/10.1094/PHYTOFR-10-22-0107-R

5. https://www.inaturalist.org/taxa/130020-Exobasidium-vaccinii