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The major species of deciduous trees grown in Wyoming include quaking Aspen, silver poplar, balsam poplar, Lombardy poplar, cottonwoods and willows, which all belong to the Populous family of trees. Unfortunately, they are susceptible to several major diseases. Early recognition and identification of the problem are key to effective management.
First, we need to distinguish between environmental factors and diseases. Warm winter days and then freezing temperatures damaged or stressed most of our trees. Then hot dry weather or hail can also stress our trees. Leaf scorch results from lack of water in the leaves. This may be caused by lack of soil water, winds, high temperatures, soil compaction, trunk or root injury or shallow soil. Symptoms of leaf scorch include initial yellowing of the leaf margins followed by browning, wilting and loss of leaves. Water loss from leaves due to winds and heat must be replaced by regular and thorough watering. Often, even if the soil is saturated with water, the tree leaves lose more water than they can replace from the roots. Young transplants or trees with poorly established root systems are extremely susceptible to leaf scorch.
To avoid nutrient deficiencies of cottonwoods and aspens periodic fertilization may be needed. Lawn fertilization generally supplies major nutrients. Symptoms of iron deficiency in the leaves include light yellowing between the leaf veins. Fertilizers containing available (chelated), iron should be added to the soil on a regular basis to help prevent this deficiency. Also, lowering the ph. level of the soil toward seven, will help release the minerals tied up in the soil. This can be accomplished by adding sulfur, or gypsum.
The family of diseases called “Leaf Spots” is one of the most widespread in Wyoming. Initial symptoms can be observed in late July or early August. However, infection occurs in cool, moist weather typical of spring. Small circular spots with tar-like centers can be seen covering the leaves. The spots are tan at first, then turn brown to black as the season progresses. Severely diseased trees may lose their leaves early in the season.
Another disease called fire blight is also widespread in Wyoming and affects young leaves and stems of the current year’s growth of fruit trees. Initial symptoms appear in early summer and include angular black spots and blotches between leaf veins. These blackened areas enlarge until the entire leaf turns black and dies. The fungus then proceeds down the leaf shoot and into adjoining twigs and branches. These quickly wither and blacken. Twigs may bend downward giving a characteristic Shepard’s crook. Under optimum conditions, the fungus spreads quickly to other leaves and branches until the entire tree is blighted.
Another disease we see on these trees are cankers. This disease effects the smaller branches first and then spreads to larger branches and eventually the trunk. A close look at the branches shows small black spots (fungal spores). The only real option with this one is to prune these branches out.
Controls for all fungal-incited leaf diseases of trees, unless otherwise indicated, are as follows:
1. Sanitation — the fungi causing most of the leaf spots in Wyoming survives in fallen leaves and stems. Therefore, always rake up and dispose of diseased leaves and branches. Often pruning of diseased branches, decreases subsequent spread of the fungal pathogens.
2. Fungicides can be used to treat most disease conditions or halt further spread or used on a regular basis to protect trees. For tree protection apply the fungicide early for best control. By applying them now, you are hoping to keep disease from spreading. Keep the trees well watered and maybe fertilize them this fall.
Editor’s note: Trade or brand names used in this publication are used only for the purpose of educational information. The information given herein is supplied with the understanding that no discrimination is intended, and no endorsement information of products by the University of Wyoming Extension is implied. Nor does it imply approval of products to the exclusion of others, which may also be suitable. The University of Wyoming is an equal opportunity/affirmative action institution.
Scott Hininger is with the Sheridan County Extension office.