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Mountain Laurel Leaf Spot - Mycosphaerella colorata

Mountain Laurel (Kalmia latifolia) in full bloom
Mountain Laurel (Kalmia latifolia) in full bloom

Good evening, friends,

For our first rendition of Toadstool Tuesday I’ve appropriately picked a fungus that couldn’t be farther from the traditional “toadstool” shape: the Mountain Laurel Leaf Spot (Mycosphaerella colorata). The past few weeks have been dry - so much so that fires in Canada are contributing to some rough air quality in the area. You can even smell the smoke in the air. On the bright side, it did rain today. If it weren’t for the lush, green scenery I might’ve mistook Manitou for Mordor today with the smoky skies, thunder, and general sense of unease. Hopefully the rain produces some fleshy fungi, but until then we will always have dots on leaves.

Mountain Laurel Leaf Spot

The native evergreen shrub, the mountain laurel (Kalmia latifolia), has been flowering all over the preserve this past week and it’s hard not to notice the beautiful blooms. The state flower of Connecticut. At the same time, it’s also hard to overlook that every leaf on the plant is riddled with lesions. The mountain laurel leaf spot (Mycosphaerella colorata, formerly known as Phyllosticta kalmicola, both less than desirable to spell and/or pronounce ) is a fungus that lives in and produces fruiting bodies on mountain laurel leaves. You’ll find it on basically every mountain laurel you encounter, there are claims that some cultivars are resistant to the fungus but I had trouble substantiating this information.

It should be noted that there are several fungi (Cercospora, Diaporthe kalmiae, Physalospora kalmiae, to name a few) that cause leaf spots/leaf blights on mountain laurel and you need a microscope to distinguish them, but M. colorata seems to be the most common. All we know for certain is that it’s not easy to be a mountain laurel.

Mountain Laurel Leaf Spot

One interesting observation is that the fungus doesn’t produce fruiting bodies on the fresh leaves that the plant grew this spring. It is thought that M. colorata is an endophyte that lives in the plant (unclear if in a beneficial, benign, or harmful manner), wrapping in and around the plant cells, but then produces the fruiting bodies in the leaves. It seems as if the reproductive structures, which create lesions in the leaves, are detrimental to the plant but according to the Connecticut Agricultural Experimental Station they “generally do not cause serious harm to the plant”. To me it appeared that leaves with lesions were shed pretty rapidly, but there were also plenty of leaves that were covered and still firmly attached.

After the leaf drops, the fungus mycelium continues to grow beyond the lesion and digests the leaf to the point where fruiting bodies (perithecia) eventually cover the majority of the surface area. It’s possible the fungus could exist within the plant from birth all the way through until the carbon is completely recycled back into the environment. Life is fascinating.

Mountain Laurel Leaf Spot
From my understanding, the small block circle on the top right is the lesion/fruiting body of M. Colorata. The other, larger lesions are produced by different fungi.

The fungus is an ascomycete, meaning it develops spores internally and ejects them into the world. There are apparently over 10,000 species in the genus and it seems that a lot of taxonomically ambiguous species get lumped under the Mycosphaerella umbrella.. The white circles in the middle of the lesions are the perithecia - the microscopic, flask-shaped fruiting bodies of an ascomycete. The fungus appears on mountain laurel leaves spring through summer, afflicting the leaves from the previous year. The mountain laurel grows up and down the Appalachian Mountain range ipso facto the fungus is found in the same region.

Mountain Laurel Leaf Spot

I’m hosting a walk in Stamford, CT for the Stamford Land Conservation Trust this Sunday at 10AM. All are welcome. More info here. The event time says 11AM on the website but it’s actually at 10. The rain should help invigorate some fungal growth.

Hopefully we can keep it rolling with even more rain, and less smoke, but


Here are a few bonus pictures from after the rain today:

A snail enjoying a snack (it kind of looked like rodent poop, but I don’t know)
A snail enjoying a snack (it kind of looked like rodent poop, but I don’t know)

I couldn’t close out Toadstool Tuesday without some actual toadstool-shaped mushrooms. These honey mushrooms (A. mellea) popped up right after the rain:

Honey mushrooms


1) Crous PW, Summerell BA, Carnegie AJ, Wingfield MJ, Hunter GC, Burgess TI, Andjic V, Barber PA, Groenewald JZ. Unravelling Mycosphaerella: do you believe in genera? Persoonia. 2009 Dec;23:99-118. doi: 10.3767/003158509X479487. Epub 2009 Oct 29. PMID: 20198164; PMCID: PMC2802725.


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