By the following fall, the galls turn dark black and harden-off. Knots continue to expand each year resulting in galls that can be as large as a foot long and over an inch thick. Persistence And Transmission. Spores of the fungus develop on the surface of black knot galls in the spring. Spores are spread by wind or rain to other trees in and around the orchard or can reinfect new locations on the same tree.
Trees are most vulnerable during bloom.
Symptoms from spring infections first are seen as small swellings in the twigs in the fall. Additional spores are produced and discharged from the velvety green surface of young galls later in the spring and early summer. The summer spores cause additional gall formation on young twigs and branches. The fungus will survive in an infected tree for as long as that tree lives. Prune out knots with sterile pruning shears or a knife 4 to 6 inches below the swelling.