West Sussex,
United Kingdom 
St Johns Wort plant with a problem (see pictures). Is it a type of Rust? - but doesn't exactly like pictures I've seen of 'rust' on websites.
The plant was taken as a root from a much larger plant (approx. 2 years ago) in the front garden, which is in robust health! Any ideas?
28 Jul, 2015
Answers
This has happened to me too. You can either trim it back or leave it to die down naturally in the autumn. I don't think it will kill your plant because Hypericum is a spreader.
28 Jul, 2015
Bamboo - Thanks for the answer - I think its more likely too much hot sun (not in last couple of weeks!) - the same plant in the front garden is in semi shade. I guess either I've put it the wrong place or may have watered it under strong sun. 'Will have to be more careful. I don't know the particular variety.
28 Jul, 2015
Arbuthnot - Thanks also for your answer - will probably just leave it to die down naturally. Encouraging to know its happened to someone else!
28 Jul, 2015
Looks like its suffered from drought at some point, and/or been frazzled in hot sun, rather than its being an infection. Which variety of Hypericum is it supposed to be?
28 Jul, 2015