Have you heard about a problem with Burning Bush
(Euonymus alatus) getting a mildew this year? The
leaves have turned yellow green with small spots of
lighter yellow discoloration.
Powdery mildew is a common and usually not life-threatening
problem with Euonymus. Make sure the plant has good air
circulation, and be sure to clean up and destroy fallen
leaves which are infected. However, the symptoms of this
fungal problem would be whitish coating on the leaves, rather
than yellowed leaves. This makes me wonder if it is a
different problem such as scale (which is actually an
insect). Check and see if there are small bumps on the leaves
or stems. Scale can cause yellowed leaves. If your plant has
a small infestation, you can try scraping the scales off with
your fingernail, prune out the most infested parts of the
plant, and then apply dormant oil to the trunk and branches
before growth starts next spring, or apply superior oil
during the growing season. There are also other fungal and
bacterial problems that could be causing the spots.
See this fact sheet from Penn State for more on Euonymus scale.
Here is an archived link to additional information, which comes from
University of Illinois Extension. Excerpt:
Burning bush (also called Winged Euonymus): Euonymus alatus
Cold injury – Winter injury may be caused by very low
temperatures as well as drought stress. With excessively low
temperatures, the moisture in the cells freezes (due to
chemical compounds in plants, moisture freezes at various
degrees below freezing). Drought stress already has resulted
in limited moisture in the plant cells. Dry, freezing winds
during the winter reduces the moisture level even farther,
often resulting in dead plant tissue. Diseases can help
magnify or increase susceptibility to winter kill. Nectria
canker kills the sapwood tissue thus reducing or even cutting
off moisture to tissue further out on the plant. Winterkill
also makes plants more prone to infectious diseases and
insect problems.
Dieback/canker – See bridal wreath spirea. In addition
Botryosphaeria dothidea will infect and kill for similar
reasons.
Winged euonymus scale – Lepidosaphes yanagicola occasionally
occurs in the southern half of Illinois on burning bush. It
is an armored scale. And will attack several trees as well.
This scale can cause premature leaf drop, branch die back and
cause the plant to become more prone to winter injury. It is
found between the “wings” – the bark ridges. It does not move
to the plant’s leaves. The scale over winters as an adult and
lays its eggs in June. Eggs may be laid for up to a month.
Mating occurs before frost.
Euonymus scale – Unaspis euonymi – females are black and
males are white. The scale causes the foliage to develop
yellowish green spots. Heavy infestation results in early
foliage drop and often stems are killed. Eggs survive by over
wintering in the female body. The eggs hatch about early June
in Northern Illinois. Crawlers emerge and move onto new
growth or can be blown by wind to other plants.
Since I cannot diagnose the problem remotely, it makes sense
to take plant samples to a Master Gardener Clinic.