Cytospora Canker (fungus – Cytospora chrysosperma): Primarily affects weak trees and occurs mainly on stems. Cankers first appear as slightly sunken areas on the smooth bark of branches and trunks. Cankers generally develop in an elliptical pattern and enlarge until stems are girdled and killed. Diseased part becomes discolored and sapwood is reddish-brown and watersoaked. During moist weather, reddish threads of fungus spores ooze from pimple-like fruiting bodies on cankered areas. Infected limbs should be removed, as the fungus moves down the stem and invades larger branches or even the trunk. Tree surgery to remove cankers in the main trunk may prolong the life of the tree. Sterilize pruning tools. Avoid wounding trees. Simon and Lombardy varieties are extremely susceptible – Rio Grande is resistant.
Dothichiza or Branch Canker (fungus – Dothichiza populea): This disease mainly attacks young planted and nursery trees. Dark, sunken, cracked cankers form on twigs, trunks and branches. Callus tissue may form in old cankers. Sapwood is discolored. Wounds are primary source of entry for the fungus and should be avoided. Lombardy is by far the most susceptible species. No effective control measures are known. Pruning of diseased parts does not help control this disease, in fact, it spreads it.
Septoria Canker (fungus – Septoria musiva or Mycosphaerella populorum): Cottonwood and Lombardy varieties are susceptible. Leaf infection precedes twig or stem infection. Canker is not easily distinguished from Cytospora canker. The canker on highly susceptible young stems appears sunken, with smooth bark and raised, irregularly concentric rings of unbroken bark. Some cankers are flat, others are swollen at the margins. Identification usually depends on finding little black fruiting structures (pycnidia). Remove infected limbs. Avoid wounding trees and sterilize pruning tools. Preventative fungicide sprays may control the leafspot stage.
Powdery Mildew (fungus – Uncinula salicis): A white fungus grows on both sides of the leaves. Damage is usually not serious enough to warrant control measures, but fungicides can be used on valuable trees.
Heart Rot (fungus – Fomes spp.): Primarily a disease of heartwood. Discoloration and soft rot of heartwood. Distinguishing feature is the shelf-like fruiting structures or conks formed on the main trunk. Trees with conks may live for several years. Remove only trees that are structurally unsound or completely dead. Avoid wounding healthy trees.
Wet Wood (bacterial): Occurs in Cottonwood and Lombardy Poplar. This disease is caused by a bacterial infection. (See the section on Wet Wood in the Elm Disease section for details and control)
Rust (fungus – Melampsora medusae): Small, yellowish-orange pustules on lower surfaces of leaves. Pustules later turn dark brown or black. Generally, most severe during cool fall weather, but not severe enough to warrant control.
Fungal Leaf Spots (fungi – Cercospora populicola, Septoria spp., Marssonina populi): Irregular to round lesions, from brown to gray in color. Spots may coalesce and cause defoliation with severe infections. The most common leaf spot is caused by Marssonina populi. Large brown spots with darker brown margins cause premature defoliation. Twigs are also invaded and killed. Foliar fungicides should be used on a regular basis.
Crown Gall (bacterium – Agrobacterium tumefaciens): This organism has been reported to cause branch and root galls on cottonwood in Texas.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Cottonwood and Lombardy Poplar are moderately to highly susceptible. (See section on Cotton Root Rot)
Mistletoe (Phoradendron tomentosum subsp. macrophyllum): (See Mistletoe in the Plants that Grow on Other Plants Section)
Chlorosis: See section on Chlorosis