This is what iron chlorosis looks like, in a blueberry shrub. Since there is generally adequate iron in the soil, the solution is to lower soil pH, typically by surface application of agricultural sulfur, around the drip line of the shrub. I can't say for certain if this is your issue, since you are growing apples, and I don't have experience with this species. Is there an optimum soil pH for apples?

It does look like Fe deficiency. Lowering pH probably isn't the answer. It depends on whether there is free calcium bicarbonate in the soil. Iron chelate or another form of Fe would be the normal treatment. This time of year I'd just let it go. The chelate is most effective if applied before symptoms show in spring.

Fruitnut, how would you apply the chelate FE, foliar or incorporated in the soil? Surely applying something now would help the trees prepare for winter. They are only first year in the ground here, and I would hate for them all to go into the winter weak.

The more I look at the orchard the more I can see a pattern where the yellowest looking plants are on the land that starts to slope away. It is the same type of soil where you see row crops showing FE deficiency also.

Sounds as if the issue is on shallower soil that has caliche near the surface. The iron chelate has worked best for me soil applied in early spring as growth begins. That's when I've had an issue due to cold wet soil. The symptoms can also show up here on calcareous soils when we get heavy rains this time of year. That's caused by wet soil making iron less available.

No treatment will do much to make the yellow leaves greener. It will make new growth greener. Foliar applied hasn't done much for me but can't hurt. If the trees have stopped growing then applying iron won't do much IME. The plants should be OK unless it's so bad that the upper leaves die. I've seen that happen and it could lead to branch tip dieback this winter.

Are you sure roots have adequate access to good old oxygen.

With blueberry shrubs, my experience has been that an application of soil sulfur, to the surface of the soil, will gradually improve the color of the leaves, over an 8 week period. However, I can speed up the process by mixing 5% white vinegar, from the grocery store, with cold tap water, which immediately lowers soil pH, in a few seconds. Using our local water, it takes 12 fluid ounces of vinegar(5%) added to 4 gallons of water, to get the pH down to around 5. This is perfect for blueberries, but maybe not quite right for apples?

HM They may have had a little too much rain this spring, but the whole orchard sits on a 4-5% slope and the soil is a silt loam. Water doesn't pool or sit anywhere.

Of course if the subsoil is closer to the surface in that one spot then it may be holding too much water in the root zone. The subsoil is more of a clay, we call it "badger dirt" because its the lighter colored stuff that badgers bring up, I guess it is the same thing as caliche that Texans talk about. I didn't notice much difference in how deep the topsoil was when I planted.

Maybe I will shut off those lines and make them miss a watering or 2 to see how they react.

If your irrigation water is sufficiently alkaline, that would raise the soil pH, and inhibit the uptake of iron.

HM I dug some test holes a couple feet outside the root zone on a dozen trees. The soil near trees with symptoms were noticeably wetter than the soil around trees with no symptoms. I guess they are probably done getting water for the year now. Thanks for all the tips.

Hmm, calcareous soil, iron chlorosis, irrigation water, sounds familiar. You may also have high bicarbonates in your irrigation water, apply sparingly or it will make your tied up micros. even worse on that calcareous soil. BTW, the addition of elemental S to soils with high free lime will in essence destroy the soil structure and lead to poor water infiltration if enough is added. The S is converted in the soil to sulfuric acid which dissolves the lime, a major component of calcareous soil. There goes the pore space, structure and water infiltration rate.

Get a sample of your soil, subsoil and water to find out what you really have, it'll be a whole lot easier to deal with your problem with that knowledge at hand. For the soil test, a standard test + % calcium carbonate, for the water test, pH, electrical conductivity (E.C.), total bicarbonates and more.

For the water + calcareous soil issue do a web search and you'll find plenty of info. to educate and/or confuse you, I hope the former. In the mean time, check out the link below, It'll give you a good start understanding what may be your problem.

Here is a link that might be useful: IFAS