If you read yesterday’s post, you learned that hemlocks all along the east coast are being killed by an invasive insect from Asia. What used to be dense stands of eastern hemlock are being swiftly replaced by an evergreen shrub called Rhododendron maximum. Ongoing research is being carried out at the Coweeta Hydrologic Laboratory in the southern Appalachia to better understand how this species transition relates to the surrounding biota.
By seeing this transition for myself, I know that there are clear ecosystem level transformations taking place. For one, these shrubs form dense thickets and block out much of the sun’s rays. From personal observation, I saw no other plants growing underneath well-established Rhododendron thickets. There’s simply not enough energy reaching the forest floor that promotes growth underneath this impenetrable shrub. With less light, temperature is also reduced beneath Rhododendron maximum. I’m just stating the obvious ecosystem alterations implemented by the encroaching shrub but you get the picture. The forest floor is changing.
The conquest of Rhododendron maximum is being met by yet another parasite; the organism featured in this edition of FUNGI FRIDAY! The dense thickets of Rhododendron maximum present perfect conditions parasitism. Parasites thrive when their host is plentiful and in close proximity, so with the hemlocks being widely replaced by Rhododendron, a parasitic fungus is also taking advantage. Exobasidium rhododendri is a gall forming plant parasite that easily accesses this vast resource pool of the newly encroaching Rhododendron.
Where you find dense thickets of Rhododendron you also find this fungal parasite. Exobasidium rhododendri isn’t necessarily detrimental to the plant but does knock it back a bit. Spores of this fungus specialize in penetrating leaf and stem tissue of several species of Rhododendron. Once inside, the spores germinate and send chemical signals into the plant that induce it to form a gall. The outer, white surface of the gall produces banana-shaped basidiospores that are ready to infect nearby thickets. Exobasidium rhododendri are native parasites that act as ecosystem stabilizers, allowing other non-dominant species to compete for space and resources on the forest floor.