Question: I have a plum tree severely affected by black knot disease, but it still produces a lot of plums. Can I still eat them?
Answer: Yes, you can eat them.
In general, when you see healthy fruits on a plant suffering from a disease, they still remain perfectly edible and safe to eat. This is not only the case for plums or cherries on black knot-ridden trees, but also tomatoes or squashes from plants suffering from powdery mildew and other leaf diseases or cherries or currants on plants with leaf spot, to give only a few examples. If the plant still produces fruits in perfect condition, don’t hesitate to harvest them and consume them.
Fruits superficially marked by disease are also edible. Apples with small amounts of scab or tomatoes slightly affected by late blight remain perfectly delicious and safe to eat: just remove the lesions with a knife.
Obviously, to what degree you tolerate these kinds of imperfection depends largely on you. There comes a point where so much of the fruit is covered with scabs or other lesions—or the lesions are so deep—that there’s not much left to eat!
Also, your tolerance will likely depend on the circumstances. Personally, I don’t hesitate in the slightest to consume imperfect fruits from my own garden, but I still seek impeccable ones when I’m at the grocery store. And when I harvest fruit to make juice or preserves, I’m much less picky than when I plan to display them in a basket on the table where everyone can see them.
That said, there are diseases, such as botrytis rot, that affect both foliage and fruits or fruits alone and really go too far. They alter the taste or texture of the fruit, giving dry, soggy or rotten fruits that are very unappetizing. Generally, they are of such inferior quality that the problem—and the solution—is obvious!
Not to say that even fruits severely affected by disease are harmful per se (there are few plant diseases that can affect humans, ergot [a disease of rye] being the main exception), but there is no reason to consume a fruit that is in such bad shape. When a fruit is thoroughly rotten, simply dump it in the compost bin.
So, sick plants can often produce delicious, healthy fruits … and fruits moderately affected by superficial diseases are also edible. Don’t hesitate to consume them!