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Excerpts from Jim Conrad's
Naturalist Newsletter
from the February 16, 2009 Newsletter, issued from the forest near Natchez, Mississippi; elevation ~400ft (120m), ~N31.47°, ~W91.29°:
SPOTS ON MAGNOLIA LEAVES
Each afternoon I seem to get a bit hungry around 4 PM so as Public Radio's All Things Considered comes on I start up a small campfire and fix some cornbread. It happens that at 4 PM the afternoon sun slants in low very prettily translucing through the Southern Magnolia's leaves next to my fire, as you can see below:
The glowing leaves always contribute to a sense of easygoing enchantment in the area, which I enjoy, but eventually I began wondering about all those spots speckling each leaf. When I finally went to take a closer look I found what you see at the top of the page.
The spots were much smaller than a BB but even with the naked eye they looked like little foliose lichens of the kind you find on rocks and trees where there's not much air pollution. Some search-engine work on the keywords "lichen magnolia leaves" brought up an illustrated paper from the University of Florida, in PDF format, which you can read online or download here.
The little dots were indeed lichens, of the genus STRIGULA; but there was more to it than that.
First of all, remember that lichens are actually compound organisms consisting of two completely different species -- a species of alga enabling the lichen to photosynthesize, and a species of fungus giving the lichen form, help retaining water, a reproductive mechanism, and other things.
It happens that Strigula's alga, whose scientific name is Cephaleuros virescens, can live on its own without being associated with a fungus, and is somewhat parasitic. On magnolia leaves the alga growing alone causes a leaf-spot disease called Algal Leaf Spot or "Green Scurf," which manifests itself as raised, greenish-brown to rusty spots.
Sometimes but not always the Cephaleuros algal spots combine with any of several GENERA of fungi, the most commonly reported being Massaria and Microthyriella. When this happens the two species together form the tiny, round, grayish lichen-spots seen on our magnolia leaves.
In our last picture notice that most of the spots are completely gray but some are partly green-orange. I'm interpreting the green-orange parts as Cephaleuros alga not yet combined with a fungus to form a lichen.
Does the Strigula lichen constitute a dangerous disease for the magnolias they parasitize?
The reports I've seen say that it's not a serious disease at all. It's clear that many leaves have been infested for years, and still look very healthy.
Both Strigula smaragdula and Strigula complanata occur in the southeastern US, where Southern Magnolias grow. On iNaturalist, user "mhodges1957" tells me that the difference between them is that Strigula complanata spores are composed of four cells, while from our pictures we can't see how many cells spores of S. smaragdula have.