Symptoms develop during the growing season, but usually appear in July and August.
Verticillium wilt is caused a soil-borne fungi which is common in many soils and affects several hundred plant species including Ash, Dogwood, Plum, Azalea, Redbud, Barberry, Elm, Rose, Boxwood, Honeysuckle, Lilac, Catalpa, Linden, Smoke tree, Cherry, Locust, Spirea, Magnolia, Sumac, Maple Viburnum, and Oak. Verticillium wilt spreads internally within the plant and can kill it.
Chronic symptoms include small, yellow foliage, leaf scorch (marginal browning), slow growth, abnormally heavy seed crops, and dieback of shoots and branches. Often the foliage on one or more branches wilts quickly.
Acute symptoms include leaf curling, drying, an abnormal red or yellow color of leaves or areas between leaf veins, partial defoliation, wilting and branch dieback. Often one branch or one side or section of the plant is affected.
Conditions Favoring Disease Development: Presence of Verticillium wilt fungus in soil, susceptible plant (see partial list above), fertile soil with low or moderate potassium and low nitrogen, and plants with moderate to severe water stress (lack of water).
First, confirm that the symptoms are indeed caused by Verticillium wilt. Fungicides will not cure infected trees. Trees with recent wilt symptoms should not be removed immediately as they may “recover” and perform fairly well. Water regularly during the growing season and provide fertilizer with low potassium to promote plant growth.
Trees showing general and severe wilt cannot be saved and should be replaced with a non-susceptible species (e.g. Apple, Hawthorn, Oak (white and bur), Arborvitae, Hickory, Pear, Beech, Honeylocust, Pine, Birch, Juniper, Larch, Spruce, Fir, Sycamore, Ginkgo, Mountain Ash, Walnut, Mulberry, and Willow). Fertilize properly to promote vigorous growth and water regularly during the growing season. Remove dead and weak branches. This does not remove the fungus from the tree, but prevents infection by other fungi. DO NOT use the chipped wood as a mulch unless it is properly heated in a compost pile.