Good speculation on the rhododendron leaf damage! Jim in Wisconsin zoomed right in on the causes: the first photo was taken on a year where we had an unseasonable freeze right as leaves were expanding, and the second was taken on a year where we had unseasonably hot weather as leaves were expanding.
In both cases, the ultimate cause of damage is lack of water in rapidly expanding tissues. Once dormancy is broken, leaf and flower buds are highly sensitive to environmental extremes – they are expanding and are most sensitive to anything that interferes with water content.
During a freeze, leaf tissue water freezes, causing what’s called freeze-induced dehydration. It’s not the ice that causes the damage, but the lack of liquid water in the cells. Water freezes in the air spaces between cells, and osmosis draws water out of the cells into these intercellular spaces. Eventually the cells more or less implode once they’ve lost enough water.
During a hot episode, the roots can’t keep the rapidly expanding leaves fully turgid, and again necrotic areas appear as a result of water loss through transpiration and cellular “implosion.”
So both of these problems are caused by a lack of leaf tissue water – and it’s impossible to tell from looking at them whether it’s from cold or heat or salt or some other stress that reduces water availability.
Bottom line: keep track of seasonal abnormalities. It will help you to correctly diagnosis problems that show up some time later.