Listed below is from the Dr. Lenny Wells on the UGA Pecan Blog.
Pecan trees leaf out from the bottom up. Thus the foliage in the lower canopy this time of year is usually much better developed, thicker, and can be a little darker in color.
The warm March temperatures led to an early budbreak and the foliage in the lower canopy got off to a quick start. Over the last week or two we’ve had periodic cold fronts move in following rains, which has slowed growth down a bit, especially in the upper canopy. This leads to a greater disparity between the foliage in the upper canopy vs the lower canopy and makes it look as though the tree is weak in the top. Additionally, cultivars with a later budbreak are also lagging behind earlier budbreak cultivars and appear comparatively weaker at this point.
This is nothing to worry about and the problem will correct itself as the temperatures and the soil warm up. Cool soil temperatures will also lead to Zinc (Zn) and Nickel (Ni) deficiencies as the trees are not able to take these nutrients up from the soil as readily when soil temperatures are cool. Symptoms of such temporary deficiencies include bare limbs, small, yellow leaves, rosette, and mouse ear. This is why foliar Zn and Ni sprays are always recommended early season regardless of soil levels of these nutrients.