What is rust fungus?

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Puccinia graminis, the common rust of wheat and oats, is familiar to everyone. The uredospores, or summer spores, known as the red rust, and the winter spores, known as the black rust, are found in unfortunate abundance, but the aecidium stage on the barberry is not necessary for the vigorous development of rust in the United States, and it is not nearly so prevalent as the red- and black-rust stages. When found, it may be so abundant that most of the leaves of the barberry are spotted with the cluster cups. It is a curious fact that wheat and oats may be quite free from the red and black rust in localities where the aecidium stage is very abundant; and that the rust stages may be most destructive where there are no barberry bushes. But no one doubts that the aecidium stage on barberry belongs in the life-history. The aecidium on barberry cuts easily in paraffin. If the aecidium stage is not easily available, there are various aecidia which are just as good, or even better, for morphological study. The aecidia growing on Euphorbia maculata (spotted spurge) are abundant and are very easy to fix and cut. The infected plants are also very easily recognized, normal plants having the prostrate habit, while infected plants become erect and the internodes become greatly elongated. Aecidia growing on Arisaema triphyllum (Jack-in-the-puplit) are also easy to cut. The Aecidium on Hepatica has large nuclei and affords particularly good views of the intercalary cells.

It is rather difficult to get good sections of uredospores and teleutospores of Puccinia graminis, because the leaves of wheat and oats are refractory objects to cut. For illustrative purposes, soak the leaves, scrape off the spores, and study without sectioning. For sections, select species growing on less refractory hosts.

Everyone who studies the rusts should attempt to germinate the uredospores and teleutospores. For this purpose the hanging-drop culture may be employed. The uredospores germinate readily all summer, but in most forms teleutospores will germinate only in the spring following their maturity. However, the teleutospores of "lepto" species, like Puccinia xanthii on Xanthium canadense (cocklebur), will germinate as soon as they ripen, and will serve equally well for study. If a particularly good specimen is secured, it may be preserved by the method previously described for desmids, except that in this case it might be worth while to attempt staining with Mayor's haem-alum or with eosin.

Gymnosporangium, which is rather common on Juniperus virginiana (red cedar), forms its basidia in the "Cedar- apple" stage. Bring the cedar apples into the laboratory in late winter or early spring and put some into a dish of water. The yellowish, gelatinous strands with the germinating teleutospores may appear within 24 hours. The various stages are easily recognized under a low-power dry lens without even crushing the gelatinous masses. Fix in 10 per cent formalin for 24 to 48 hours. The material maybe left here indefinitely. In chromo-acetic acid the gelatinous substance goes to pieces. Stain in iron-alum haematoxylin and follow the Venetian turpentine method; or, when the thick glycerin is reached, mount in glycerin jelly. The material spreads out better and fewer basidia are torn off from the teleutospores in glycerin jelly than in the Venetian turpentine.

The interesting nuclear conditions in the life-history of a rust which has uredospores, teleutospores, and aecidiospores are not difficult to demonstrate. After the uredospore or teleutospore stage has been fixed in chromo-acetic acid and washed in water for an hour, treat with 10 per cent hydrofluoric acid in water for an hour and then continue the washing in water for 24 hours. If fixed in alcoholic corrosive sublimate and acetic acid, rinse in 50 per cent alcohol, then treat with 10 per cent hydrofluoric acid in 50 per cent alcohol, wash in two or three changes of 50 per cent alcohol. In either case, imbed in paraffin. Cut about 5( in thickness and stain in iron-alum haematoxylin. A rather strong stain of orange in clove oil will make it easier to trace the mycelium in the host.

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