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Rose rust is caused by Phragmidium mucronatum, a fungus which lives in the tissues of the leaves, stems, and calyces. It appears in spring as orange or yellow spots, made up of spore masses, varying from one-twelfth to one-sixth of an inch in length. They form hypertrophies, and often cause curvatures of the affected parts. In midsummer the spores change in character and the colour of the spots alters from orange or yellow to red. As winter approaches the spores undergo a further change and develop black hairlike tufts on the under-surfaces of the leaves. The affected leaves turn yellow and fall prematurely, spreading the disease. Any visibly affected part should be burnt promptly; the rest of the plant should be sprayed with Thiotox, or one of the colloidal or wettable sulphur preparations. Rose rust is most troublesome in hot and humid areas. Adelaide nurserymen regard it as one of the major diseases affecting their young plants.