Q. Leaves on my ornamental crabapple tree turn completely yellow, and drop off. Why? No spots beforehand.
Your crabapple tree may have contracted a disease called Scab (Venturia inaequalis) a dull, olive-green, velvety fungal growth develops on the surface of leaves and petioles in the spring. Leaves yellow and fall prematurely, giving the tree a thin, bare appearance by mid-season. Infected fruit have circular, rough spots on their surface. to help control this problem rake and destroy fallen leaves and fruit. Apply a fungicide as flower buds begin to show color (early pink) and again 3 weeks later. There are resistant cultivars/varieties to this disease (see list below).
Scab-resistant crabapples:Adams, Baskatong, Brandywine, Callaway, David, Dolgo, Donald Wyman, Malus floribunda, Henry Kohankie, Henningi, Jewelberry, Ormiston Roy, Professor Sprenger, Malus seiboldi var. zum cultivar Calocarpa, Silver Moon, Sugartyme, Malus tschonoski, Weeping Candy Apple, White Angel, and White Cascade have been observed in many locations to have only slight to moderate scab infections.
Hope this is helpful.
Courtesy of the NYBG Plant Information Service