“What is this orange stuff?” I’ve asked this question myself, and I’ve been asked by many other people. Today when I saw it while doing an aquatic studies class, I finally decided to investigate, and this time I spared the good folks over at the Cornell Cooperative Extension Office in Westport by looking it up on-line first. (I’ve developed a name for myself at the Coop. Ext. office, thanks to all the strange samples I send them for ID.)
When faced with a strange thing to identify, it helps to gather as much information as possible ahead of time. For instance, I’ve only ever seen this on raspberries growing along the roadside where I walk the dog. Today we found it along the outlet of Rich Lake. I suspected it was a fungus. So, I fired up the computer and did a search for orange fungus raspberry leaf. Here is what I discovered.
Orange rust is indeed a fungus. Originally labeled Gymnoconia peckiana (although I did find one source that calls it Gymnoconia interstitialis), this fungus has now been subdivided into two forms, based on morphological differences. These differences depend on the species of berry affected (black raspberry vs blackberry). So, now we have Gymnoconia nitens, which is common on blackberries, and Arthuriomyces pekianus, which occurs on black raspberries. While one source I found claimed that orange rust isn’t really a problem for the overall plant, most other sources state that it is a serious disease in the Northeast, affecting wild and even cultivated brambles. So far, red raspberries seem to be resistant.
The good news is that orange rust has no alternative hosts. In other words, its entire lifecycle is dependent on the blackberries and black raspberries. In the winter, the fungus hides out on the new roots underground, just waiting to reappear and spread the following year. You will know your plants have it when in late May and throughout June you find the undersides of your berry leaves coated with bright orange “stuff.” The cure: destroy the infected plants. Rip (dig) them up in the early spring (before the pustules erupt), get thier roots, and destroy them.
UM…pustules? Yes – if you look at the plants in early spring, before the leaves have completely unfolded, you may find glandular bodies on the leaves. These are the pustules (actually, they are called sori, which is the scientific name for a spore-containing structure; ferns reproduce with spores and you can often find their sori on the underside of their fronds). When they mature, they look like black specks and the surrounding tissue is yellowish. After maturing, they erupt, sending their spores out to populate the world; this is the orange “stuff” you see on the underside of your leaf.
The next question that comes to mind is “how” – just how are you supposed to destroy the infected plants. There you are with your pile of dug up infected berries – what are you supposed to do with them? Do you burn them? Bag them up and take them to the dump? I couldn’t find an answer on-line, so I broke down and called Cooperative Extension (I could hear the cringe on the other end of the line). The official word is don’t burn them (it could spread the spores on the smoke); either bag and take them to the dump, or take them to some far away part of your property where there are no berries, put them on the ground and cover with plastic. Anchor the plastic well. The plants will die and compost. With no wind to spread the spores, and no nearby berries to infect, the fungus should die out.