Management: If a tree has been infected, much can be done to prevent its re-infection. Rake up all the leaves from the tree and prune any infected twigs. Dispose of all diseased tissues by burning or putting them in closed bags and sending them to a landfill. Dense stands of poplars or aspens, or individual trees that are full and bushy should be thinned or pruned to allow better air movement in the stand and among the leaves. This may reduce the disease incidence as dense stands and bushy trees incur greater Marssonina infestations. Avoid watering the lower branches and leaves of poplars. Wet leaves and twigs, especially if they do not dry off quickly, encourage infestation by Marssonina spores. Fungicides can be used in the landscape to reduce spring and summer infestations, but with good sanitation this is usually not necessary. Several effective fungicides that contain chlorothalonil1, chlorothalonil plus thiophanate, mancozeb, triforine or ziram are often recommended. Follow labeled instructions precisely to be effective and safe. Apply the fungicide at bud break and every 7 to 10 days thereafter, as needed.

1. Use or nonuse of chemical names does not constitute an endorsement or criticism of a product containing the chemical.

Authors: Dr. Wayne S Johnson Erin Post

We acknowledge the assistance of Colorado State University Cooperative Extension Service, Colorado State Forest Service

Marssonina Leaf Spot & Twig Blight

Marssonina populi

A Common Disease of Aspen, Poplars and Cottonwoods

University of Nevada Cooperative Extension Offices

Battle Mountain 815 N Second St., 89820-2334 (775) 635-5565

Las Vegas 2345 Red Rock St., 89146-3160 (702) 222-3130

Caliente 360 Lincoln St. P.O. Box 728, 89008-0728 (775) 726-3109

Logandale 1897 Moapa Valley Blvd. P.O. Box 126, 89021-0126 (702) 397-2604

Carson City 2621 Northgate Ln., Suite 15 89706-1619 (775) 887-2252

Lovelock 810 6th St. P.O. Box 239, 89419-0239 (775) 273-2923

Elko 1500 College Pkwy., 89801-3347 (775) 738-7291

Minden/Gardnerville 1329 Waterloo Ln., Gardnerville P.O. Box 338, Minden 89423-0338, (775) 782-9960

Ely 995 Campton St., 89301-0210 (775) 289-4459

Pahrump 1651 E Calvada Blvd. 89041 (775) 727-5532

Eureka 701 S Main St. P.O. Box 613, 89316-0613 (775) 237-5326

Reno 5305 Mill St. P.O. Box 11130, 89520-0027 (775) 784-4848

Fallon 111 Sheckler Rd., 89406-8951 (775) 423-5121

Tonopah #1 Frankee St. P.O. Box 231, 89049-0231 (775) 482-6794

Incline Village 865 Tahoe Blvd. P.O. Box 8208, 89452-8208 (775) 832-4150

Winnemucca 1085 Fairgrounds Rd. Fairgrounds, 89445-2927 (775) 623-6304

Yerington 504 S. Main St.

P.O. Box 811, 89447-0811 (775) 463-6541

Fact Sheet 01-27

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MARSSONINA LEAF SPOT & TWIG BLIGHT, Disease of Aspen, Poplar and Cottonwood Trees

Many broad-leaved trees are attacked by Marssonina blights. Fortunately, Marssonina populi that attacks aspen, poplars and cottonwoods, is the only Marssonina blight of any consequence in Nevada. However, it is common throughout the Great Basin and the Intermountain West on native and landscape trees of the genus Populus—aspen, poplars and cottonwoods.

Spread: Marssonina populi is a fungus that over winters on leaves and infested twigs. The primary infestation occurs in the spring. Spores released from leaf litter on the ground or from nearby infected twigs are carried by wind and rain to the newly emerging and developing leaves. All summer long, healthy leaves are attacked by spores from blighted leaves adjacent to them and by season’s end, all the leaves on the tree become infected.

Infection and Symptoms: The blight affects leaves and small twigs of broad-leaved trees and causes them to be unthrifty, to loose their leaves and in severe cases, their small branches die. Infested trees rarely die outright unless they have been defoliating for many years and as a result, are extremely weak. Repeated infestations make the tree

susceptible to branch dieback and much more likely to be attacked by other life-threatening diseases and insects. Commonly, winter injury—dehydration, sunscald and damage from cold—occurs in continuously infested trees as well. Marssonina infestations are also more prevalent on trees under stress from adverse environmental and cultural conditions—drought, waterlogged soils, irregular irrigations, heat, and poor nutrition. This leaf spot and twig blight occurs more frequently following wet weather or where sprinklers wet the foliage.

Marssonina disease of Populus begins as dark brown or black flecks scattered over a leaf. The spots enlarge with time to form black blotches usually with yellow to tan borders. As the brown spots come together to form larger blotches, they are slow to cross over veins. Eventually, the entire leaf may be blighted. Because infected leaves produce less food from photosynthesis, they are smaller than normal and if the infestation is widespread, the vigor of the tree is less. From a distance, affected trees have a bronze cast

and a weak, almost see through appearance. Large trees are most affected in their lower crown while all the leaves of small trees are infested. In all cases, the blighted leaves drop early, sometimes several weeks earlier than uninfected trees. Early defoliation weakens the tree more, as less food is stored in the trunk and roots. Infestations are cyclical and may be more severe from one year to the next.

Prevention: Prevention of Marssonina Blight in poplar and aspen is difficult because so many of these trees are present both in landscapes and natively. Consequently, the fungal spores are present where Populus species grow. Some clones of these trees are less susceptible to the disease than others. It is difficult to know when selecting a tree in the nursery whether or not it is susceptible to Marssonina Blight, as nurserymen do not label trees as to their susceptibility to this blight. If a tree has never shown signs of having the disease, then it could be used for vegetative propagation as poplar species are easily produced by root and shoot cuttings. The trees produced from a resistant tree would not be susceptible to the disease.