It can hit well over 100 degrees F here as early as April, and many of the apple trees are not fully leafed out yet. This leaves the bark exposed to the direct intense sun, which can kill it as seen above, almost invariably on the southern side. The damage is not readily apparent however, except to a nasty worm pictured below called the Pacific Flathead Borer. The adult beetle has a knack for finding this sunburnt bark and laying an egg on it. The grub hatches and eats through the botton of the egg and into the sun-damaged bark, which can no longer “sap” out the invader. It then proceeds to the edge of the healthy bark to feed on the cambium layer, tunneling along and leaving a trail of sawdust under the bark. If it makes it all the way around the young tree, it will girdle it and kill it from there up.
The preventative remedy for this is to apply a layer of sunscreen in the form of white latex paint diluted 50% with water. Paint the whole trunk, paying special attention to the southern exposure, the graft union near the ground, and the tops of any exposed branches that will get direct sun without leaf cover. Folks in northern climates also do this, but for different reasons; they do it so that the winter sun does not prematurely thaw a section of bark to start the sap flowing, which will freeze and burst in a cold spell.
We’ll cover more on finding the borers in a later post.