I have had lilys in my garden for 10 years suddenly this month the seem to be dieing what could be th cause?

Hello Coombs have they been dryer than usual? They hate wet conditions with a good well drained soil. but yellow streaking is usually a sign of draught. keeping them moist during the growing season keeps them from becoming stunted or turning streaky yellow. feed them in spring with bulb food as the shoots emerge. Good luck

its not the dreaded lily beetle is it?

Well surely they're just dying back after flowering, ready to rest in the winter, aren't they? The topgrowth does disappear in the autumn.