Hello Boulder County Gardener,

First an important caveat. We are able to provide only limited diagnostic modes from photos; and because of COVID-19 restrictions, we cannot do much more. We cannot at present accept plant samples or examine things in more detail. That said, this past winter and spring have been challenging for established trees and shrubs along the Front Range. Our very cold October 2019 “first frost,” a largely dry winter, and another damaging freeze in mid-April have taken their toll on both evergreens and deciduous trees. Many evergreens are brown in whole or in sections; some deciduous trees have not leafed out or only have leaves on a few branches. And the July dry spells with the sun and heat, trees and shrubs are further under stress. Hard to say precisely the climate effect on trees and shrubs for the longer term. Remember that most of the trees and shrubs we plant in landscapes were not adapted to our Front Range climate. This is a fundamental tenet of gardening here on the high steppe.
Colorado Gardening: Challenge to Newcomers – 7.220
https://extension.colostate.edu/topic-areas/yard-garden/colorado-gardening-challenge-to-newcomers-7-...
Understanding Steppe Regions: A Key to Successful Front Range Gardening
https://denvergardeners.wordpress.com/2016/01/06/understanding-steppe-regions-a-key-to-successful-fr...


Chlorosis is the symptomatic term associated with reduction of photosynthesis in the leaves (lack of chlorophyll), and can result from a myriad of conditions.  But mainly (most often) it is a result of  problems in root uptake of minerals, particularly iron, manganese, or zinc; these are the mineral deficiencies that mostly contribute to interveinal yellowing which is the color that shows through when photosynthesis is operating poorly. Established trees and shrubs are pretty tough, but decline of photosynthesis as you’d expect will ultimately result in loss.

Here are a couple background articles on the chlorosis symptom to quickly read through:

https://planttalk.colostate.edu/topics/weeds-cultural-problems/2121-iron-chlorosis-trees/

https://planttalk.colostate.edu/topics/weeds-cultural-problems/2115-iron-chlorosis/


If the chlorosis symptom is a result of poor nutrient uptake due to intrinsic soil conditions such as alkaline soil,  usual remedies for landscape plants with chlorosis symptoms are of the following:,

(1) add iron sulfate to soil – effectiveness may be limited
(2) add iron chelates to soil – some are more effective than others depending on soil alkalinity
(3) addition of sulfur to lower soil pH – less feasible when alkaline soils are high in lime
(4) iron sprays to foliage – often impractical, especially on large trees, effect is only temporary
(5) and for trees, trunk injections – holes drilled in trunk may provide entry for disease organisms; best left to certified arborists

The preferred, least-costly treatment (with little risk, except your time and money) to temporarily relieve an iron chlorosis soil condition is to use a stabilized iron product as a soil amendment and/or a foliar spray.  Preferably look for a product with EDDHA iron, that is iron chelated in that formulation, water soluble, dark red microgranular form of iron EDDHA to be used for the correction of iron deficiency  by means of soil application. I believe there are several vendors.
As a random example:
https://www.vaniperen.com/story/how-to-choose-an-iron-chelate/
Most quality garden supply outlets will stock the EDDHA material.


More references that may assist a detailed understanding of the chemistry involved and the cultural care of mature trees and shrubs to reduce these conditions in the future.
https://cmg.extension.colostate.edu/volunteer-information/cmg-gardennotes-class-handouts/
#223, Iron Chlorosis
and
#657, Watering Mature Trees
#658, Mulching Trees
#659, Understanding Tree Roots
and
Iron Chlorosis
http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advi...


That said, solving a chlorosis condition of a mature tree or shrub is not trivial. A chlorosis problem may require additional investigation and may require experimentation.
Consider these points posed as questions:
Most often caused by insufficient root uptake of some of the minor plant nutrients, especially metallic minerals, most often iron?
The root uptake can be inhibited by root damage (from insects, or nearby construction or soil disturbance, or herbicides applied nearby, etc?
Mulched or not?
Too alkaline soil (high pH, common around here) can inhibit uptake of iron, zinc, manganese?
Excess soil compaction from over-watering or traffic?
Chlorosis induced by disease (bacterial or virus)?

Chlorosis symptoms may even be a consequence of the way the root system grew after the tree or shrub was planted years ago. Disturbing the root system of a mature tree by attempting to amend the soil mechanically is usually counterproductive.   Application of a soil surface drench or foliar spray containing iron and other nutrients may help. Supplemental water can help if the tree or shrub is otherwise insufficiently irrigated, BUT over-watering could enhance the chlorosis symptoms. You have not indicated how much irrigation water the plant receives. Most Eastern trees need supplemental water to grow well along the Front Range.  Preferably, you will need to do a lot of your own sleuthing.

If you decide to try a soil application of iron compound, make a soil drench with the EDDHA iron product and follow the manufacturers application procedure.

Eventually, if you encounter chlorosis symptoms in your landscape in a tree, as trees have relatively high value, you might call in a professional arborist certified by ISA. Here is a convenient list of tree contractors and you can ask to obtain a diagnosis from a certified person.
https://bouldercolorado.gov/forestry/arborists

On a personal note, I have lilacs with chlorotic symptoms also beginning this summer.  As I’ve been struggling with root weevils for years, I speculate that root damage from these critters plus the weather induction are the principal causes of chlorosis this summer as opposed to prior summers.
https://extension.colostate.edu/topic-areas/insects/root-weevils-5-551/
This summer I am attempting to treat the chlorotic lilacs with foliar sprays and chelated iron root drenches as an experiment to increase the photosynthesis before the end of this growing season, however I may eventually lose them.

Happy gardening,
-Diagnostics Team, Boulder County Master Gardener <personal data hidden>