. Manual of tree diseases . Trees. 342 MANUAL OF TREE DISEASES common and is found generally distributed throughout the northern states. The other two are apparently rare and are kliown only in Colorado and Utah. The leaf- rusts are impor- tant on yoimg trees because defoliation often results. Symptoms, The three rust- diseases are very similar in appear- ance. In early simi- mer the affected leaves show yellow- ish areas on which are borne small orange-yellow pus- tules. These pus- tules appear pow- dery, due to the mass of spores they contain. Later in the season on the The. Fig. 70. —Powder
. Manual of tree diseases . Trees. 342 MANUAL OF TREE DISEASES common and is found generally distributed throughout the northern states. The other two are apparently rare and are kliown only in Colorado and Utah. The leaf- rusts are impor- tant on yoimg trees because defoliation often results. Symptoms, The three rust- diseases are very similar in appear- ance. In early simi- mer the affected leaves show yellow- ish areas on which are borne small orange-yellow pus- tules. These pus- tules appear pow- dery, due to the mass of spores they contain. Later in the season on the The. Fig. 70. —Powdery mildew on willow leaves. same areas, small dark brown raised spots are formed, heavily infected leaves may drop from the tree. Cause. The common willow rust is caused by Melampsora Bigelowii Thiim. This fungus also causes a rust of the leaves of larch (see page 212). The spores (seciospores) borne in the pus- tules on the larch leaves infect the willow leaves. The orange-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rankin, W. Howard (William Howard), b. 1888. New York : The Macmillan Company
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