The exposure to what sounds like lots of summer sun may be the main factor. While bigleaf hydrangea can contract a few leaf-spotting diseases, the symptoms pictured are not due to infection (though those may turn up later, since they are so common). Are the neighbor's plants receiving a bit more shade, or are they in soil with different characteristics (more moisture-retentive or higher in organic matter)?

The yellower-than-usual tone to these leaves suggests they are either in more sun than they would prefer (the reddish blush can be a "sun stress" indicator, though by itself the reddening isn't harmful), or that they are in soil conditions that are not ideal (too dry, not acidic enough, or too nutrient-poor). While normally established plants should not need fertilization, perhaps the site has low organic matter and thus is not retaining nutrients well, or the foundation is making conditions more alkaline than the plant wants. If this planting spot faces south or west, the accumulated and reflected heat from the wall so close to the plant could be exacerbating its stress, at least during periods of hot weather.

Bigleaf hydrangeas grow best in sites with a mix of sun and shade, even though we have seen decent-looking specimens in fairly sunny locations in the mid-Atlantic. Usually such places have abundant air circulation, though, to help cool the plants, and the soil probably also days more evenly moist. (Although nursery retailers can display bigleaf hydrangeas in the sun, and nursery growers tend to grow them in lots of sun, they are irrigating often and the pots tend to have time-release fertilizer in the container to help compensate for the nutrients being flushed-out by the frequent watering.) If you do submit a soil sample for testing, we can help to interpret the results if desired. If the acidity isn't where it needs to be for plant vigor, amending the soil with garden sulfur should help the roots absorb what they need. Nitrogen is one of the nutrients that helps keep foliage a rich green, but iron, magnesium, and manganese also play a role and a deficiency in any one of them might cause chlorosis yellowing. We do not suspect a phosphorus deficiency, as the leaf reddening is more likely tied to sun intensity. (Some cultivars also redden more readily than others. In fact, breeders have recently created cultivars whose foliage is reddish all summer long.)

For now, just monitor the plants for watering needs as we enter periods of hot, dry weather; consider top-dressing the soil lightly around the roots with compost (no more than an inch in depth, and underneath any mulch used) if you suspect poor soil conditions, and hold off on fertilization until a soil test reveals which nutrients (if any) are low enough to benefit from supplementation. If leaf spot infections do develop later, you can clip off the worst of the affected leaves, but otherwise a fungicide probably won't be of much help. Maintaining good air circulation through crowded stems, plus not watering in a way that wets the foliage (or at least watering early enough in the day to let the foliage dry by nightfall) will help keep leaf surfaces less vulnerable to fungal or bacterial spore infection.

Miri

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