The leaves have suddenly turned yellow with the veins remaining green. Zone 5b full sun. Dry. Loam
Thank you for writing to the Toronto Master Gardeners.
Leafs turning yellow between the veins (interveinal chlorosis) is a symptom common to many different problems including nutrient deficiency, root damage, temperature extremes, herbicide misapplication, too much light, too little water or too much water, insect feeding, or disease pathogens. Without more information or photographs, we hesitate to offer a diagnosis. The following article from the Royal Horticultural Society on chlorosis offers good general advice.