I noticed this on a branch of my September ruby apple, I live in Saskatoon. I’m hoping you might ge able to identify it and if it needs treating. It’s a thin black layer, almost like mildew, that you can scratch away with your finger. Someone suggest fire blight and somebody else suggested black soot. Obviously the treatments are very different abd these people aren’t experts so I’m Hopi g you can help. Thanks
Thank you for contacting Toronto Master Gardeners regarding the black area on a branch of your apple tree.
The description you provide: “almost like mildew which you can scratch away“ suggests that it is a case of sooty mold. The excellent article by the University of California http://ipm.ucanr.edu/PMG/PESTNOTES/pn74108.html provides a description of the problem and a management strategy, as well as an appendix on pests, specifically ants which may be central to the problem. You may wish to confirm our diagnosis with an arborist
For a comparison with the possible issue of Fire Blight, the Fact Sheet from Ohio State University Fire Blight of Apples and Pears https://ohioline.osu.edu/factsheet/plpath-fru-22-0 describes damage of that bacterial disease.
We are very honoured that you enquired of the Toronto Master Gardeners. Closer to home, however, is the Master Gardener programme in Saskatchewan Email firstname.lastname@example.org
We wish for you a bumper crop of your September Ruby Apple this autumn.