[SnooziSuzi]

Well, I have just been to the allotment and whilst I was there decided to pull some potatoes for tea.

I only have Rooster potatoes growing now as all of my Lady Crystal are gone.  I noticed that one plant was lighter in colour than my other potatoes and on digging the tubers were light, not pink so I guess this was one of last years crop that was missed in an overlapping bed I had there last year.

I noticed some spots on the leaves, but they only affected the lighter plant, none of my Roosters are affected at all (and on re-reading the Marshalls seed catalogue, Roosters have "very high resistance to all known potato viruses."  Champion!

I burned the haulms of the plant I dug up; none of the potatoes are affected so perhaps I caught it in time.

When I got home I googled blight and the types of spots I saw and it looks like Septoria Leaf Spot. 

Here is a link to a website in Idaho where they grow a lot of potatoes, but all of the texts point to it being a Tomato virus.

I know tomatoes and potatoes are closely related but

a) what are the chances of it really being Septoria Leaf Spot on potatoes
b) if it is fairly rare then were would it have come from to have affected only one potato plant and my toms (inside and outside) are fine?