Azalea Leaves Pale

Asked August 12, 2020, 7:54 AM EDT

Good morning. Can you diagnose what’s causing my azalea leaves to look pale. While they were planted only in April, the leaves used to be a deeper green color. I have recently, within the past couple of leaves, applied PlanTone around the canopy but beyond watering (not overwatering) them That’s all I’ve done. Thank you, in advance

Prince George's County Maryland

7 Responses

This is classic - and extensive - damage from a sap-sucking insect pest called Azalea Lace Bug. The feed underneath the leaves and also lay eggs in specks of black goo that are glued to the leaf surface, also on the underside. Plants that are more stressed, particularly by being exposed to too much hot afternoon sun, are more vulnerable to infestation.

The most innocuous treatments include blasting the bugs off with strong jets of water and coating the upper and lower surfaces with horticultural oil. (If you try oil, do not apply while temperatures are above 85 degrees. Repeat treatments will be needed, as per label instructions.) If unsuccessful, a stronger approach of spraying with a systemic insecticide (acephate), which is absorbed by the leaves, may work. This latter technique is of questionable use at present because the leaf tissues are already damaged and an above-average number of leaves may be shed by the plant next spring as the new growth appears. This undamaged new growth would have the higher priority for treatment of any type, since little can be done to protect the existing foliage at this point - the damage can't really get much worse and the insects may have already left. Monitoring for their re-appearance starting around mid-April next year will tell you when to treat; if using systemic, apply just after bloom.

https://extension.umd.edu/hgic/topics/lace-bugs-trees-shrubs
https://extension.umd.edu/hgic/insect-pests-rhododendrons-and-azaleas-identification-and-management

Miri

Thank you, Miri, for your reply. Is it not possible the plants are suffering from a lack of acidity in the soil. I have 8 of these bushes in 2 different locations in my yard. If this azalea lace bug is the prognosis grim?

High pH for acid-loving plants like azaleas can cause leaf discoloration, but the symptoms look different from lace bug damage, which is a fine stippled lack of green. For nutrient deficiencies like iron chlorosis, the veins in the leaf stay green and the surrounding leaf tissues become yellowed but evenly-colored and not speckled. Although the leaf veins in your photo are slightly greener, this is more due to lack of lace bug feeding along the vein than anything else. The stippling is the key sign of sap-sucking insect damage. You can also inspect the leaf undersides for the bugs themselves or, if they've disappeared for now, their black fecal/egg dots.

Among the "-tone" fertilizer line, Holly-tone is more appropriate for acid-loving plants than Plant-tone. Having already used Plant-tone would not have created a pH issue, however, and soil pH changes do not occur quickly on their own. You can use a home pH test kit or send a soil sample to a lab if you wish to get a sense of pH and nutrient makeup (lab tests are best for the latter, so they're more accurate). Occasionally, nutrient deficiencies in foliage are not the result of soil deficiencies, but rather a problem with root health and their inability to absorb what they need. For moisture-sensitive plants, this almost always is a result of over-watering or poor drainage rotting roots. This does not look like your symptoms, though.
https://extension.umd.edu/hgic/topics/soil-testing

Miri

Again, thank you. I will start applying a neem oil (since I have some) - unless you recommend I purchase horticultural oil?

As for prognosis, we cannot predict how the plants will respond and recuperate, but the plants will probably recover so long as their roots stay healthy. Keep monitoring them for watering needs the rest of summer and fall (and even during mild spells in w inter), soaking when needed but letting them get somewhat dry a few inches beneath the surface in between otherwise. Make sure the mulch isn't touching the base of their stems.

Treatment for lace bug this season may be of limited benefit, but monitoring and then treating next spring will be important. Don't expect much, if any, new leaf growth the rest of this year as plants naturally shut this process down this by late summer to use their resources to start preparing tissues for winter and to grow next spring's buds instead.

Miri

Thank you. I love this service and greatly appreciate your thorough responses and feedback.

Neem may work equally well - you may have to try it and see. Although both oil-based, these two pesticides work differently, and horticultural oil may be more effective overall, including for smothering eggs. (This is harder with lace bug, though, since their eggs are already somewhat protected under their frass/fecal coating.) Here is a page describing the differences: https://extension.umd.edu/hgic/topics/fruit-insecticides. Note: product labels may simply say "horticultural oil" rather than specific sub-types, which is fine; all you want to avoid is dormant oil, as that should be used in winter as opposed to summer because its stronger concentration can be damaging to plant tissues in heat.

Miri