What’s that? It looks like someone sprayed the blackberry plants with bright orange spray paint.
Unfortunately, it isn’t paint. This is a fungal infection known as orange rust. It is two fungal species, Arthuriomyces peckianus and Gymnoconia nitens. Orange rust fungus infects the entire plant: the roots, crown and shoots. This fungus doesn’t usually kill the plant, but it weakens it to the point that it doesn’t produce much fruit. Once a plant is infected with orange rust, it is infected for life.
If you find a blackberry plant infected with orange rust, you must remove the plant completely, roots and all. Bag the plant in place, because dragging it away will only stir up the fungus and allow it to infect other plants. It is also common practice to remove all blackberry plants nearby since they are likely infected.
Have you seen orange rust?