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Skip the fall feeding and only use HollyTone at half strength in the spring. Yellowing of leaves surfaces, often with brownish burned areas, occurring on leaves that are more exposed to sun, is caused by more sun exposure than the plant is able to tolerate. Some varieties need shade, while all plants that have been protected from direct sun will be tender until hardened off by gradual exposure to sun light. Possible solutions are to give the plant more shade or move it to a more protected site. This damage will never recover but the new leaves looking good means that the condition was apparently a transient condition in the past. Look at the undersides of the leaves. If you see insects or different problems, here are some possibilities: 1) Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of Rhododendron Lace Bugs, Stephanitis rhododendri, and 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. 2) Thrips are characterized by a silvery white discoloration on the leaf's upper surface and silvery on the bottom with small black frass specks. The damage is similar to that from lace bugs. Thrips are a problem in warm and dry climates like California and New Zealand. Most of the standard insecticides and insecticidal soaps can be used. Control for thrips whether an insecticide or insecticidal soap is a contact control and must be applied on the adults. This is most likely done in May in California when they are visible. Locally, biological controls may also be available. 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). One of the puzzling aspects of this fungal problem is the fact that two different affected rhododendrons vary in appearance. Rhododendron cultivar 'Unique,' for instance, shows almost no upper leaf changes, other than occasional very faint lighter yellowish areas, while the underside of the leaves will be completely covered in brown spots. A deep green leaf may begin to show lighter green patches, and these areas will gradually become more yellow. Another cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves. 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 rhododendrons, 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. Here is a link that might be useful: Rhododendron problems and solutions.

Do you have anything else you could add to your description, or even a photo? Causes could be several, including powdery mildew, but also - grayish mottled leaf surface, dark brown varnish like spots on undersurface: lace bug yellow mottled leaf surface, larvae on undersurface: white fly white stippling on leaf surface, dried or burned appearance: spider mite

I doubt you have a disease per se - plant diseases tend to be rather specific to individual plants rather than widespread throughout a garden. For example, rhododendron problems generally will not affect laurels, madrone diseases are not common to magnolias, etc. And ivy is virtually disease-free!! It is far more likely that there is some deficiency or other problem with the soil itself. Poor drainage, chemical contamination, excessive compaction, lack of fertility - any of these can create sufficient stress in otherwise healthy plants so that they perform poorly, display off-color foliage and/or become susceptible to insects or common fungal issues that are secondary to the primary problem. Do any of these things srike a chord? Have you had a soil test done? Anything different with the garden beginning late last year - a different type of mulch used, a spraying service, perhaps the use of weed 'n feed products on your lawn, neighbors using pesticides, construction activity that could disturb roots or alter drainage? Unfortunately, soil issues are not easily diagnosed just from photographs, although may provide some indication. You might be better served by hiring a consultant to view your garden personally and evaluate the situation. A local nursery may have a CPH on staff that could offer such a service or direct you to someone with these qualifications or you could inquire at your local extension office. Feel free to email me the photos if you like (email address on member page) and I'll give it my best shot, but based on what you describe, I truly think a personal, onsite evaluation is your best bet for getting to the root (no pun intended!) of the problem.