Leaf spot diseases can be difficult to diagnose with precision since there is a lot of overlap in symptoms. Virginica Sweetspire can contract fungal infections like Phyllosticta leaf spot and Anthracnose (Colletotrichum), but we are not certain which (if either) may be responsible here. Sometimes abiotic stressors -- those not caused by a pest or pathogen -- can also give plants leaf spot look-alike symptoms or reddening of the foliage. Overall, the plant looks good, so this does not appear to be concerning damage and can be left alone and monitored in case it worsens or changes symptoms. Fungicides can't cure existing disease and their use (when they work) only suppresses infection of still-healthy growth, but their residues also risk harming other organisms like pollinators, so we generally don't recommend their use when not needed to save a plant from decline. You can rake-up and dispose of the Sweetspire leaves later this autumn once they have fallen, in case this helps to reduce overwintering infectious spore abundance, though it's not a foolproof way to completely prevent infections in a future year. Fungal and bacterial outbreaks are also highly weather-dependent, so this condition might not return or be as prominent next year. If you can't avoid wetting the foliage when irrigating, try to do so early enough in the day so that leaves can dry by nightfall, since wet leaf surfaces are more vulnerable to infection.

For now, just keep monitoring the plant for watering needs and feel free to send us photos if the symptoms worsen or change drastically.

Miri

While Sweetspire (Itea virginica) is fairly trouble-free, they can contract one or more types of "leaf spot" in the right conditions. Leaf spot is a collective term for an array of fungi or bacteria that cause foliage spotting symptoms, not all of which are identified in horticulture literature as to the exact organism responsible. Despite the eyesore they can cause, plant health is not usually significantly impacted.

Since you mentioned that you haven't been watering beyond rain water, it sounds like the plants are drought stressed. Many plant this year are displaying early leaf drop from the hot weather and lack of rain. We also had such a dry spring so the plant wasn't able to receive sufficient water before the hot dry part of summer. 

You may want to supplement irrigation to help the plants absorb water and have healthy roots before winter. While Itea can handle some dry conditions, they tend to prefer more evenly moist soil. You can refer to our watering trees and shrubs webpage for further guidance on how to irrigate plants. There are also some links on there that refer to drought stressed plants. 

You can also add mulch (or replace it with fresh, if some is already there) over the surrounding soil to help suppress future spore-splashing from the soil surface onto the leaves. A layer only a couple of inches thick is sufficient, and keep the mulch off of the plant crown (where the stems emerge from the soil) so it gets good air circulation. Affected leaves do not resolve their symptoms even if disease progression halts. The leaves that are yellowing and dropping could be that they are the leaves that were stressed from leaf spots earlier. 

Let us know if you have further questions. 

Emily