I can think of three possibilities:

1) Small bright yellow spots is a symptom of Azalea Rust, Puccininastrum vaccinii, a fungal disease. Orange-red pustules containing spores form on the lower surface of the leaf. This rust is only sporadically severe and typically infects deciduous azaleas. Control by avoiding those hybrids and species which are very susceptible to rust. Good air circulation is helpful. Triadimefon seems to help, but may not be registered for use on azaleas.

2) Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of Azalea Lace Bugs, Stephanitis pyrioides, small insects with transparent wings on under-surface of leaves. This insect hatches early in spring as the new foliage begins to mature and its numbers may build to damaging levels with successive generations. Lace bugs reach their peak in late summer and do their worst in sunny, exposed sites. Spiders are important predators of lace bugs and since they shy away from sunny, hot places, plant your azaleas where there is some shade. Insecticidal soap, horticultural oil, or a systemic insecticide may spare your azaleas the damage if applied in spring when the first hatchlings are noticed. Care must be taken to spray the lower surfaces of the leaves where the lace bugs live. Moving a plant to an area with more shade may alleviate the problem. Lace bugs are more prevalent on certain varieties. The following azalea cultivars have resistance to azalea lace bug: Dawn, ÂPink Star, ÂEreka, ÂCavalier, ÂPink Fancy, ÂDram, ÂSeigei, ÂMacrantha, ÂSalmon Pink, ÂElsie Lee, ÂRed Wing, Sunglow and ÂMarilee.Â

3) Light green or yellowish patches on leaves sometimes accompanies by brown spots on the back side of leaves is a sign of Powdery Mildew (Microsphaera azaleae). A deep green leaf may begin to show lighter green patches, and these areas will gradually become more yellow. This common disease is named Powdery Mildew despite how little the symptoms resemble the familiar fungal disease often seen on roses and azaleas. Usually the disease doesn't produce the familiar white powder-like spores, although late in the summer some may become visible. The disease manifests instead as color changes in the leaves, followed by defoliation toward the end of the growing season. Many azaleas, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms. Last year's leaves, once they have been hit by the disease, will always have it, with symptoms persisting from year to year until the leaves drop off. High relative humidity at night and low relative humidity during day with 70-80 F (22-27 C) temperatures is ideal for the disease to flourish.
* Keep azaleas healthy to help them manage this problem. If you notice symptoms on last year's leaves, consider protecting the new growth with a fungicide . Apply it now to the new growth as it expands, before symptoms appear on this spring's leaves. Fungicides won't get rid of the existing problem on old leaves. A new fungicide, 'Remedy,' which is a potassium bicarbonate (made by Bonide Company), is registered for the problem. Thorough leaf coverage is necessary with all fungicides. Fungicides containing sulfur (such as Safer Garden Fungicide RTU) are also registered. Others are Funginex (sold as Ortho RosePride Funginex Rose and Shrub Disease Control Concentrate.) Be very careful to read all label instructions, and wear protective goggles and gloves. Funginex can be corrosive to eyes. For more information see the section above on 'azalea powdery mildew'. The symptoms are different, but the organism and control are the same.

Ditto on what Rhodyman said. Good luck with your plants.

Will apply fungicide to new growth. Thankyou You