If the leaves are uniformly yellow, then it is caused by your low nitrogen numbers. If the leaves are yellow with green veins, then your nitrogen isn't too bad, but you have an low-acidity/phosphorus problem. When to fertilize: For most garden situations the old rule "once before they bloom and once after they bloom" is still a sensible approach. Actually the fertilizer timing has nothing to do with the time the plant flowers, it simply means once in the early spring, probably April then again in June. Never fertilize after mid-summer. Over-fertilizing is worse than not fertilizing at all. Established azaleas often do well with no fertilizer at all. Nutrients are slowly released by any organic mulch that you use, so rely on this as the primary source of nutrients. Excess nutrients may promote larger than normal populations of azalea pests like lace bugs and azalea whiteflies. It´s very easy to burn up the fine roots. Fertilizing after late June in a northern climate promotes tender growth in the fall, which doesn't harden off before the first frosts of winter. This gets killed by the frost. This growth may have the buds for next year's flowers on it, which would also get killed by the frost. Research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved. For acidity, flowers of (powdered) sulfur or iron sulfate are best. Do not use aluminum sulfate since aluminum builds up in the soil and is toxic to most plants eventually. For flower bud production and hardiness, super phosphate is best. Around the base of each plant I use a tablespoon of dry sulfur and a tablespoon of dry super phosphate when a plant shows signs of problems. For a general fertilizer for rhododendrons and azaleas, Holly-tone is preferred by many growers. It is an organic 4-6-4 fertilizer with powdered sulfur, minor elements such as magnesium, iron and calcium, and trace elements also. Try to use an organic product since chemical fertilizers reduce the number of mycorrhizae in the soil and the mycorrhizae are what make phosphorus available to the plant. When fertilizing only fertilize once in the spring and at half the rate on the package. Some people fertilize once before blooming and once after blooming, but only fertilize at half the rate on the package. Here is a link that might be useful: How to care for rhododendrons and azaleas.

If it is lace bugs the undersides of leaves are covered with dark brown to black, sticky spots of excrement. The fact that it could be lace bug is the main reason for not spraying. Lace bugs are normally controlled by beneficial insects. Sprays kill the beneficial insects that naturally control the lace bugs. The best long-term control for lace bug is to move the plant to a shadier area. Also, the National Arboretum warns: "excess nutrients may promote larger than normal populations of azalea pests like lace bugs and azalea whiteflies." Here is a link that might be useful: Common Problems and Their Solutions.

Lace bug thrives in sunny locations on certain varieties of azaleas. You apparently have one of these varieties. If you move this plant to a shadier location and stop spraying, the problem will probably go away. There are other bugs that thrive in shadier locations such as spiders that will naturally control the lace bug. Spraying destroys the beneficial bugs as well as the lace bugs. Azalea Lace Bugs, Stephanitis pyrioides: Adults are about 1/8-inch long. The body is pale yellow. The lacy wings (very distinctive) are held flat over the back and are transparent with two dark spots present. The nymphs are black, spiny and smaller than the adults. The eggs over-winter partially embedded in leaf tissue. The eggs hatch in May. The nymphs mature into adults in June and lay eggs during late June and July. The second generation of nymphs appears in August. The over-wintering eggs will be laid when these nymphs become adults. adults and nymphs feed on the undersides of leaves by piercing the leaves with their mouthparts and sucking the plant juices. This causes a mottled, silvery or white discoloration, known as stippling, on top of the leaf where the chlorophyll has been removed. The undersides of leaves are covered with dark brown to black, sticky spots of excrement. Plant azaleas in partial shade to maximize the activities of beneficial insects.

This is an excellent time to spray horticultural oil on your azaleas (underneath the leaves) for lacebug. It's also a good time to take soil tests and turn them into your local extension office. It would be far better to find out a little bit about your soil (pH, especially) before adding anything to the soil. Right? In the meantime, you can freshen up the mulch layer around your azaleas. Here is a link that might be useful: Soil testing