Rust diseases of plants may of course be devastating, from black stem rust of wheat which contributed to famine after World War I to cedar apple rusts which must be controlled by orchardists and (sometimes) landscapers today. Yet, it must be admitted, they are fascinating. They can be autoecious (occurring on only one host plant) such as may-apple rust commonly seen in spring woodlands, but often they must complete their life cycles on wildly different hosts, such as wheat & barberry, juniper & apple.
About a month ago I came upon a rust disease I had not knowingly seen before, on an elderberry (Sambucus sp.) in a swampy area in Mohican State Park in north-central Ohio. I was attracted to it from maybe a hundred feet away when I spied bright-orange club shaped areas on the stems. I sought it out, stumbling through the swamps to mobs of spectators (including my wife) warning of everything from wet shoes to snakes of spectacular imaginations, I arrived to confirm it was rust – oh joy.
As far as I know this rust disease is not a serious issue for elderberry growers (a great ornamental and food plant), but contact me (firstname.lastname@example.org) if it is so. The fungus in this case is Puccinia bolleyana and the alternate hosts are sedges (Carex spp.). Elderberries of course have many uses, such as:
Elderberry wine and jams
Elderflower fragrances used in liqueurs such as the French St. Germain and the Swedish akvavit and along with the more powerful flavors of star anise and fennel in Sambuca
A Romanian drink named “suc de soc”, commercialized Fanta Shokata in Romania (at least so says Wikipedia).
Ornamentals with cultivar names from ‘Black Lace’ to ‘Madonna’. Hmm.
And of course, elder-wands, circa Harry Potter.
But this rust fungus is neat to look at and learn about. So, at least when it does not interfere with people’s livelihoods and pleasures, I Love Rust! And note the final picture, taken of a bridge in Massachusetts. We always talk in plant disease circles that rust pustules such as those seen on this elderberry are "signs" of the pathogen as opposed to symptoms such as leaf damage and yield loss. A Rust Sign indeed!