The lower leaves on my container dhalias are progressively going yellow and wilting and then going brown. It seems to happen as they're overshadowed by newer growth. Otherwise the plants seem healthy and have lots of new growth and buds. In other plants I'd say dying older leaves were either normal or nitrogen deficiency, but I'm loath to give the Dahlias any extra nitrogen (they have some controlled release 7-11-7 in the potting mix, which should be quite enough) and its the first year I've grown Dahlias so I'm not sure if this is normal or not?

I recently had to repot the plants since the original containers proved much too small. I know they haven't filled out the 12 gallon pots they're in. It has been hot, so I'm hoping its just a little bit of stress from repotting. They did wilt a little the first few days but they're staying upright all day now and its even hotter. I don't think its fertilizer burn since I wouldn't expect that to cause yellowing and it would affect the new growth. I don't think its viral since again the new growth isn't affected. Could it be some other kind of nutrient deficiency? They get watered twice weekly on a drip system which seems to be keeping the soil moist and there's no sign of waterlogging when I check the drainage holes. I don't think its spider mites, although since I've never had them maybe I'm missing the signs. Anything else I'm not thinking of?