Based on the additional info, I'm going to guess you have a combination of fertilizer burn and sun burn. I think there's a couple of things going on here. Container culture is very different than planting in the ground, and even If planted in the ground I wouldn't plant a Japanese Maple in a mixture of 50% compost. My guess would be your tree was showing signs of fertilizer burn from the compost, and the high temps aggravated your already stressed tree. I believe I remember reading that when nutrients are too high in the soil, a plant isn't able to absorb water from the soil. With the high temps the tree needed even more water but couldn't get it due to the nutrient imbalance. At this point, if I'm right, I think your only option is to get the tree out of the compost mixture. Did you bare root the tree when you repotted or did you pot up ( ie take the tree out of the pot, loosen the out side roots and pot into the large container placing the compost mix around the existing mostly intact root ball? Reason for asking is, if you only potted up you can likely plant the tree in the ground for this year or pull the plant out of the compost mixture and pot up using a soil with similar soil characteristics as the soil in the root ball. Be careful and try not to disturb the roots any more than necessary. You're getting late in the year to be potting up but leaving the tree in the compost is likely to do more harm. Also keep the roots moist as best as possible during the procedure. If the roots dry out, they'll die and cause even worse problems. The tree should be moved to a shady area protected from wind, after or during the pot up/planting, until it begins to improve. Also a note on full sun, some cultivar of Japanese Maples are listed as capable of being planted in full sun. This is a misnomer because full sun in Japan, western Washington, or even in the north east is different than full sun in California, Texas or Georgia. Although some of the Sango Kaku's are among the more tolerant of sun exposure when planted in the ground, I've read many examples of these trees showing sun exposure stress symptoms when in hotter climates in containers. Hope this helps, and also hopefully one of the others will come along and correct me if I've given information that's too far off the mark. PS The above recommended procedure would only be a temporary fix. Next spring, just as the buds begin to swell, the tree would need a full repot into an appropriate container medium if you wanted to put it back in a pot or keep it in a pot which ever the case may be. Blake

Have never encountered Osmocote in a tablet form - that's a new one on me! Generally it comes in a jar with just the loose granules and I apply a small amount (1-2 Tbsp depending on container size) each spring as new growth begins. It lasts pretty much all season although I do supplement with a liquid (Dyna Gro) from time to time during the summer months as it provides a lot of the trace elements/micronutrients that Osmocote omits. I would think one tablet is sufficient for most containers. And I am not sure I really consider the development of dead wood on JM's as 'die back'. Certainly die back can occur from winter cold damage as well as disease issues but the typical dying off of small twigs and stems is a completely natural phenomenon with JM's and nothing really to be concerned about. For whatever reason, many of these smaller stems or twigs just fail to survive and you seem to see it more with the weeping dissectum types so I am guessing it has much to do with congestion and shading. There are other trees that develop a similar trait - Styrax japonica and various birches comes to mind - so this is not unique to JM's.

"It's beautiful. If you YouTube pruning JMs, you can get a lot of information." I'm sorry but of all the places where one can find good pruning info, I would rate YouTube at the bottom of the list. ANYONE can post to YouTube and most of those that do have no earthly idea what they are doing!! And with JM's in particular, pruning is much more of an art than a science and a very light hand is necessary. Typically with a tree this young, very minimal pruning is required. There is a current post on the Trees forum regarding a tiny mail order JM that was rather whacked up by the vendor before shipping but has now been reduce to a virtual short, bare stick by excessive and unnecessary pruning. What would have been the preferred approach is to have left the tree alone for its first season (or two!!) to see how it responded to the initial bad pruning cuts and then - and not before - make some remedial cuts to direct growth and encourage a better branching structure. With JM's, especially the smaller growing or dwarf varieties, narrow crotch angles and/or low branching are NOT the issues they would be with larger, faster growing trees. Because of this, one seldom ever sees JM's with structural issues. Generally the only structural problems one encounters are the inability of the low weeping dissectums to stand up to snowloads and that is only because the branch network supports the snow rather than lets it pass through as it would with a bigger, less "platformed" tree. And careful thinning can address that.

As you say,there's not a great deal you can do Stephen,JMs being a comparitively soft bark plant(and I believe named so somewhere)will often get die back on the youngest growth.Some more than others,the thin twigs are easily damaged by the elements and infections.When large portions of die back occur it's usually a sign of something else,more often an infection.All I can suggest is use a copper spray at fall time to help keep out infections and shelter from the worst of the elements,though not always possible.Also don't fertilise late in the year encouraging new growth that won't harden off before dormancy. I now spray copper every year as we rarely have freezing temps anymore in the south to slow down the spread of infections during dormancy....my plants suffer greatly.