Farming Fungus

In autumn the leaves of the bigleaf maple (Acer macrophyllum) turn yellow, and fall from the trees. This is our coastal maple tree with the great big leaves. If you look closely at some of those leaves, however, you will see that a few of them have big green spots, and in the big green spots are little black spots. Both the green and the black are due to a very unusual fungus, and its remarkable life cycle. It is green because the maple tar spot fungus (Rhytisma punctatum) is keeping pieces of the fallen leaf alive! The black spots are the fungus itself, or rather the reproductive parts of the fungus. The actual fungus is microscopic and lives in the green zones.

Maple tar spot fungus (Rhytisma punctatum) on Bigleaf maple (Acer macrophyllum) 

All photos by Kelly Sekhon

The tar spot fungus is only visible in the autumn, when it starts to reproduce. It is actually  there right through spring and summer, but shows no evidence of its presence. The leaves of summer show no sign of disease, and are completely healthy. This is a very successful parasite, in balance with its host. It is only when the leaves start to die that the fungus takes over. When those leaves die the chlorophyll disappears, and they turn yellow. Actually the leaves do not really turn yellow, the yellow was there all along, but was hidden by the green chlorophyll. When the chlorophyll breaks down the yellow becomes visible. It is a compound called xanthophyll. Xanthophyll also captures sunlight for the tree, but in a different frequency range than chlorophyll does. In addition it protects the chlorophyll from energy overload. Too much light is too much of a good thing, like too much ice cream. It can destroy chlorophyll. The yellow pigment dissipates some of this extra energy.

Newly fallen yellow maple leaves make a beautiful carpet around the bases of their parent trees, and the large green patches stand out vividly against the yellow background. Chlorophyll is contained within tiny spherical structures called chloroplasts, and the chloroplasts are densely packed within leaf cells, where they intercept incoming sunlight. Just before leaf fall a corky layer forms at the leaf base, cutting off the leaf from incoming water and nutrients. The chloroplasts then die. The xanthophylls are more stable and last a little longer. The tar spot fungus, however, produces a hormone which stimulates the chlorophyll, keeping the chloroplasts alive for a while, even after the rest of the leaf has died. These green zones continue to produce glucose, which feeds the developing fungus spores. The fungus is farming small areas on leaves that are no longer attached to the trees which originally produced them.

The next generation of spores, however, do not mature until the spring. The old leaves with their little black spore cases lie on the ground right through the winter. The new spores infect the immature leaves as they emerge. If a spore lands on the underside of a maple leaf it produces a microscopic fungus thread called a hypha. The hypha grows across the surface of the leaf until it finds a breathing pore - a stomate. Stomata are also microscopic and the undersides  of most leaves are covered by thousands of these little holes. Carbon dioxide and oxygen enter plants through the holes. The fungus grows through the stomate and establishes itself within the leaf, where it remains in an almost dormant state until the autumn. 

How do the spores get from rotten leaves on the ground to the tops of tall trees? They obviously do so successfully every spring, as tar spots are very common on fallen maple leaves. The spores are shot into the air from the spore cases, but only for a distance of about a millimetre. Possibly they are then captured by eddy currents, and drift upwards. Possibly they are splashed from the ground by raindrops and are carried in aerosol particles. Possibly they are carried by birds or flying insects. Possibly all of these modes of transport occur. There are so many possibly’s because we are unable to observe what actually occurs. These are microscopic processes beyond our level of vision. Most of the other processes that run the living world also lie beyond the level of our senses. To study them we need to use technologies that expand our senses - technologies such as microscopes, DNA sequencing, and biochemistry.

When the maple leaves turn golden next autumn, look for the green patches with their  little black dots. Next spring some of their offspring will be back in the treetops.