Another entry from BPotD Work-Learn student Cora den Hartigh today. She writes:
Late autumn is the time locally to observe this common fungus. Richard Droker (aka wanderflechten@Flickr) took these photographs of Rhytisma punctatum a few years ago (image 1 | image 2). This fungus and its relatives are common and widespread wherever maples are found, in Asia, Europe, and North & Central America. Thank you, Richard, for capturing this and for all your stunning microscopic and close-up photography!
An asocmycete, Rhytisma punctatum is known as the speckled tar spot fungus. It is characterized by black freckles on fallen maple leaves. Called “stromata”, the little black speckles are actually tough pods of tightly-woven mycelium meant to protect reproductive structures. The mycelium is made up of a web of hyphae, which look like tightly woven threads composed of chitin (the same substance found in insect exoskeletons and butterfly wing scales). Fruiting mushrooms are also woven from hyphae, but are more ‘soft’ than these little black pods because the hyphae threads are woven less tightly, like a crochet scarf rather than thick work gloves.
These stromata in the images are not mature yet, though! Dormant, they will overwinter on fallen leaves until the spring. Then, as the new maple leaves unfold and in the presence of ample moisture, Rhytisma punctatum will torpedo filamentous ascospores into the air hoping to colonize new growth. Though the force at which they torpedo these ascospores might only carry them about a millimetre, agents such as windy drafts, critters or water might aid them along. Tom Volk at the University of Wisconsin has posted some fabulous pictures of these ascospores.
With its stromata, Rhytisma punctatum resembles freckled pointillism (e.g., some of the work of Seurat). A very similar fungus, Rhytisma acerinum, has larger conglomerated blotches, or “stroma”, that exhibit more closely to impressionism (e.g., see Monet or van Gogh).
Usually this fungus is not considered to cause significant damage to the tree. It is often only observable when the black speckles develop in the fall. However, the fungus does cause nitrogen and phosphorus to be retained in the tissue of the leaves rather than withdrawn into vascular tissues in the autumn, which would certainly impact tree vigour.
What I find particularly intriguing about this photo is how some infected, fallen leaves will maintain chlorophyll-green around the fungus even as the rest of the leaf tissues senesce. Known as “green-islands”, these colourful spots indicate active photosynthesis. The fungus may be either producing or stimulating localized plant hormone production, e.g., cytokinins, in order to maintain and farm chloroplasts. It can then continue to feed itself and grow, prolonging its active life cycle beyond that of its host. This phenomenon is also known to occur with some leaf-mining insects and their bacterial endosymbionts (see: Kaiser, W et al. 2010. Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts. Proc. Biol. Soc. 277(1692):2311-9. doi: 10.1098/rspb.2010.0214 ). Here’s a portion of their article abstract:
“…We show that the phytophagous leaf-mining moth Phyllonorycter blancardella (Lepidoptera) relies on bacterial endosymbionts, most likely Wolbachia, to manipulate the physiology of its host plant resulting in the ‘green-island’ phenotype–photosynthetically active green patches in otherwise senescent leaves–and to increase its fitness. Curing leaf-miners of their symbiotic partner resulted in the absence of green-island formation on leaves, increased compensatory larval feeding and higher insect mortality. Our results suggest that bacteria impact green-island induction through manipulation of cytokinin levels…”