Ailing rodgersia

I dug up my rogersia this spring because it was being dwarfed by the rheum next to it. I know rodgersias are big, but it was no contest, really.

I put it in a very large pot and stood it in a pond, halfway up the pot. It responded immediately, putting out new shoots and looking generally satisfied.

I notice now that several of the biggest leaves are apparently drying out and dying off. I took it out of the pond in case it objected to water-logging but it still appears to be happening though the younger, smaller shoots look OK.

Any ideas as to the cause? Should I look for little visitors under the soil?

 

I'll put it in the shade right away.