Black Leaf Spot (fungus – Gnomonia ulmea): Small, yellow spots appear first on upper surface of leaves, then gradually develop a shiny black appearance. Heavy spotting causes leaf yellowing and early defoliation in wet seasons. Usually defoliation does not occur much before normal leaf fall so control is not warranted. If trees have been affected seriously in previous seasons, fungicidal sprays applied when leaves are unfolding, when they reach full size, and again two weeks later will help prevent serious defoliation. Raking and burning fallen leaves will reduce inoculum for future infection.
Other Leaf Spots (fungi – Gloeosporium sp., Cercospora sp., Phyllosticta sp., and others): Dark, elongated spots develop on midribs, veins and margins of leaves, or spots of various shapes and colors may develop on any portion of leaf surface. Destroy fallen leaves and control as for black spot.
Wet wood or Slime Flux (bacterium – Erwinia nimipressuralis): Chronic bleeding of sap from crotches, wound or other weakened areas of trunk, with unsightly discoloration of bark in affected area. Sap frequently is sour smelling. Bleeding or fluxing is most pronounced during spring months or during wet weather. The problem results from fermentation processes of the causal bacteria creating pressures up to 60 pounds per square inch within the tree. Tapping directly into the trunk just below the affected area to provide an outlet for abnormal sap and gasses will relieve internal pressure and may aid in recovery. Drill a small hole (one-half inch diameter or less) directly below the bleeding site and slightly upward into the center of the trunk. Install a tight fitting drainpipe in the drilled hole making sure the end of pipe extends far enough outward so that sap does not fall on the tree.
Dutch Elm Disease (fungus – Ceratocystis ulmi): Symptoms may appear on one or more branches on any part of the tree in contrast to phloem necrosis where tops of infected trees show first abnormalities. Leaves on individual branches wilt and turn yellow; in some instances leaves wilt very rapidly, dry out, then fall while still green. Twig terminals of affected branches sometimes become curved to resemble a shepherd’s crook. As a further diagnostic aid, twigs when cut across, show discoloration or browning of water-conducting tissues in the sapwood. Tree defoliation may occur rapidly or take place over an entire season. Likewise infected trees may die in a single season or live for several years. The disease is spread by elm bark beetles infested with the causal fungus. Development of this disease has been limited in Texas.
Elm Leaf Scorch – (bacterium) – A rickettsialike bacterium has been associated with this condition. Vascular bundles are plugged to the point where water movement in the tissues is impaired. No control is known.
Powdery Mildew (fungi – Phyllactinia guttata, Uncinula macrospora, Microsphaera alni): Powdery whitish to gray growth on both sides of leaves. Affected leaves may be cupped, stunted, and show yellowing. The disease usually occurs so late in the growing season that chemical control is not necessary.
Mistletoe (parasitic plant – Phoradendron flavescens): (See Mistletoe in Plants that Grow on Other Plants)
Cotton Root Rot (fungus – Phymatotrichum omnivorum): (Chinese Elm is highly susceptible) (See section on Cotton Root Rot)
Mushroom Root Rot: (See section on Mushroom Root Rot)
Verticillium Wilt: (See section on Wilt)
Wood Rot: (See section on Wood Rot)
Root Knot Nematodes: (See section on Root Knot Nematodes)