I'm new to Canna's this year and I have a ton of them potted on my deck and a ton in the yard planted in the ground. Several on the deck have started to get crunchy around the edges of the leaves. Is this some kind of disease, are they getting too hot (it's been in the high 90's and/or over 100 here for weeks) or is it something else? And how do I fix whatever it is? I absolutely love my Canna's and want to do whatever is needed to keep them gorgeous.