Question
For the past couple years now, our crab apple tree has been subjected to these, from what I can tell, lumpy fungal things. It's not really similar to apple scab. The fruit seems unaffected, and it always starts late summer. Before fall, our tree looks almost ready for winter. I have been able to find one other picture of this on the internet, and nobody seems to know. I would deeply appreciate if you could help me help my poor tree.

Thank you.

Answer
Sounds like either Black Knot Gall or Cedar Quince rust. Both form growths on the  branches.
I would guess Cedar Quince rust.

Black knot galls (knots) are initially slight swellings on the stems. They are more visible the second year after infection, when they become hard, brittle, and coal black. These elongated, rough, black swellings may occur on twigs, branches, and sometimes the trunk. If growth of the fungus is on one side of the stem, the stem may be bent at the knot. If the knot girdles the stem, the stem beyond dies. Galls continue to spread in the branch and will be larger next year if they remain on the tree. Black knot galls may grow to a foot or longer. The disease does not typically kill a tree but causes deformed growth if left unchecked. About 24 species of Prunus are susceptible to this disease. Hosts include ornamental plums and cherries but not hawthorn. The fungus form spores on 1-year-or older knots, giving them a velvety green appearance. Removing knots controls the disease.


Cedar quince rust is damaging to hawthorn because it affects fruit, stems, and petioles. The galls that form on hawthorn stems give the stem a roughened, swollen appearance that is much more obvious when the orange masses of spores are visible. By mid- to late summer, the galls are black and roughened like black knot, but still not more than twice the normal stem diameter. They do not grow as large as black knot galls. The pathogen girdles twigs, causing tip blight on hawthorn. If you look closely at old cedar quince galls, you will see some small, cuplike fungal structures on the galls. They are white.

Primary diagnostic differences between these diseases are the host, gall size, and type of fruiting structures. Black knot is found on Prunus species (ornamental or edible plums and cherries), cedar quince rust on hawthorn, quince, and sometimes apple. Black knot galls are much larger, with green to brown sporulation on black. Fungal structures on cedar quince rust are white.

Here is a web link with pictures and how to treat the rust. http://www.mobot.org/gardeninghelp/plantfinder/ipm.asp?code=30