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I planted a Janet Blair (type of Azalea) last year and since that time it has not been doing well. New leaves are coming out but soon half the leaves have some discoloration. I don’t know what is it. One side of the plant seems to have died but the other side is still getting new leaves but catching that disease. [enter image description hereenter image description here

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I have two guesses. Based on your pictures and my favorite azalea diagnostic guide from Washington State University, I think it's either marginal leaf necrosis or fungus leaf spot. They both cause the plant's leaves to die from the tips and edges back

Marginal leaf necrosis isn't a disease so much as the plant being too stressed. There are a lot of things that can cause that! The linked guide lists all of these as possible causes:

  • Cold damage occurs when temperatures dip to near or below the hardiness limit of the plant. May be accentuated by wind and drought, especially in eastern Washington.
  • Drought, especially while the plant is in active growth or the foliage is in soft growth, and on newly established plants.
  • High amounts of salts in the soil caused by excessive use of soluble fertilizers. Very common close to the house where the eaves protect soil from rain, and along the dripline of the house where salt (fertilizer) concentrations are high when the plant is in soft growth.
  • Root damage due to poor drainage, planting too deep, physical injury to root system, or disease.
  • Girdling due to weevil feeding on bark and/or roots.
  • Nutrient deficiency

If this azalea was planted a year ago and has been struggling since then, I suspect there could be something wrong with it's environment. I see on this Janet Blair guide that they want partial shade and well drained, acidic soil. Is the plant getting that? Watered and fertilized appropriately? Is the soil fertile and of good quality, and do you know that it is (soil test!)? The necrosis also tends to be slightly quarantined and bordered by the leaf's veins. These example images show that:

Image of two leaves with brown borders and living centers Image of azalea plant showing necrotic tips on several leaves

Fungus Leaf Spot is a fungal infection. The above linked guide gives us more specific guidance on this diagnostic:

Symptoms: Spots are irregular in size and color (Fig. 8). Some have red-brown borders with silvery gray centers. Very small black dots (fruiting structures of fungi) are sometimes visible in the center of the spot or in concentric rings. These fungi commonly enter through wounds. Cause: The fungi Phyllosticta, Septoria, Pestalotia are commonly the causal agents of this malady. Remove and destroy affected leaves. Spray with a registered fungicide after flowering and repeat at 10–14 day intervals until dry weather begins. It is usually advisable to apply a spreader-sticker with the fungicide to hold the fungicide on the leaves during rains.

To me, the whiteness on the leaves in your images look like sun bleaching after the tissue has been dead for a while. However, you can take a close look to see if it does appear to be a fungus. A good magnifying glass can help with this. I've also found that I can 'clean' some of the fungus off of the leaf by rubbing it with a wet rag. It's not enough to help the plant but helps you establish that there's some white stuff on the leaf (fungus), not the leaf itself turning white (bleaching).

Image of two azalea leaves turned halfway brown with shite splotches from the tips and edges towards the petiole

Good luck with your Janet Blair!

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    Will try some options. Not a slam dunk. The Azelea (not Janet Blair) next to it is doing fine. Could be stress or soil related i suspect.
    – JStorage
    Jul 14, 2023 at 21:39

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