Citrus canker and citrus scab, which is which? Minimize
September 2016. A moderator wrote: "After staring at these two images for a long time, I have decided I can't tell the difference between citrus canker and scab. They were taken in a country where both diseases are reported present.

Can anyone tell me what they are and why!"

A member from New Zealand wrote: I would say they are old canker lesions on a mature leaf. The dark margins of the lesions on the underside is not a feature of scab. Also, if you enlarge the lesions you will see that some tend to be zonate indicating they have expanded from the margins at an earlier stage. Scab does not do that. It forms a corky scab at the point of infection but while they can coalesce if close together they do not expand to any extent.

Some of the lesions appear to be quite eroded with age. Look at the young leaves on the same plant,  if there are yellow halos or greasy brown margins around the young scabs then it is a sure sign of canker. What is the host?

In direct contrast, a member from Argentina said: "To me they are both scab (apparently the same leaf up and down).

"I am a citrus canker expert and for other citrus diseases. see pictures in the site:

"You will find a comparison pictures of both diseases. On fruits there are no problems to distinguish. The problem it is in leaves. In Argentina, sour orange scab (Elsione fawcetti) is only on rootstocks: rangpour lime, Rough lemon, sour orange and so on. However, sweet orange scab (Elsinoe australis) is on fruti of lemon, orange and tangerines.

"All the pictures that are included in the book were tested to be sure what they are.

I agree that scab and citrus together in the same ransgpour lime leaf is difficult to distinguish but only for a short time.

Old leaves are easy. So, to me and with the info you provided they are scab. If we know the host it would be easier to explain".