Would someone tell me what is wrong with this plum tree?

It is some type of growth all over this plum tree which is around 15 years old and normaly produces well certain years.

It’s hard to tell from the picture. The blue-green stuff is harmless lichen. You can use dormant copper spray to control that. The other growths look like a major black knot infestation, which can be fatal if not removed. It spreads fast and is hard to eradicate once it gets beyond a certain stage.

Looks like some black knot as well.

It’s real easy to tell from the picture to anyone with much experience growing plums in the northeast and AJ certainly nailed it- it looks like your black knot is well beyond the “early stage”.

You have to either amputate small branches a couple inches below the galls or get real surgical and cut to clean wood removing all infected bark down to below the cambium. We spend hours and hours doing it every year and now is when the newest galls become apparent. I’m seeing a lot of it in the SE NY area this season but am holding off on cutting it out until I get up all the baffles and do all the fruit thinning in the orchards I manage. After cutting out the galls you can spray the wounds with the fungicide chlorothalonil. There’s probably a product at your local big box that contains it.

What types of trees does black knot hit?

Mostly just plums but sometimes causes a bit of damage to cots and cherries. It goes after native cherries with a vengeance sometimes which are often a source of inoculum.