I don't think it's sunburn because the pattern usually favors greater involvement of the thicker part of the leaf and less involvement toward leaf margins where the tinner tissue more readily gives up heat to the surrounding air. Heat isn't what initially CAUSES sunburn, but it excites the molecules involved and increases the formation of the oxidase that actually causes the chemical reaction we call sunburn. BTW - the oxidizing agent produced is none other than the product so often used to bleach hair and treat wounds - hydrogen peroxide.
There are a number of potential causes for the purple coloration of leaves and stems.The most common cause of purple leaves (very common at this time of year) is a deficiency of phosphorus (P) because P is required to make ATP and ATP is needed to turn sugars to starch and to load sugar into phloem for transport. No P = no ATP = no move sugars/starches = anthocyanin (purple pigment) builds up = plant turns purple. The most frequent cause of a P deficiency is cold soils, which is why so many plants planted out too early turn purple. Is your soil temp >55*?
A P(hosphorous) deficiency is not the only suspect cause of purple leaves. Nearly any environmental condition that puts the brakes on growth and the accompanying use of sugars, but does not limit sugar production (photosynthesis) can cause anthocyanin buildup and purple leaves.
If only the outer edges of the leaves are purple, it may be a K or Mg deficiency. If the center of the leaves are also purple, it could be too much Ca in the soil or the result of too much water in the soil or soil compaction blocking uptake of P and Mg.
Yes, fresh manure is high in nitrogen and soluble salts and has the potential to cause both fertilizer burn and/or ammonium toxicity, the later being the most likely of the two if it is manure-related.
Al
