This disease, known also as currant felt rust and white pine blister rust, is caused by a fungus which was introduced into the United States from Europe a few years ago. The reforestation movement had created a market for young white pine stock, and since American nurseries have not been able to supply this demand these plants have been imported from Europe. Whatever else may be said of the result of this wholesale importation of foreign white pines, it is to be regretted that the blister rust pathogene has thus been brought into the United States.

So far as the currant is concerned the disease is of little economic importance, since it does not destroy whole bushes. Gooseberries are also affected, but the condition is similar on that host. The chief importance on these plants (Ribes) lies in the fact that the pathogene lives on them, from which it spreads to the five - needle pines. Here young trees are killed in the nurseries and in plantations; older white pines are also severely affected, sometimes being killed by the disease.

The disease is known in practically all Europe, except in the Balkan and Hispanic peninsulas. It is especially prevalent in Germany, Denmark, Belgium and Holland. Just when the parasite was introduced into the United States is not definitely known. It was found in Kansas in 1892, but at that time it was not recognized as the pathogene here under consideration. How it reached Kansas is not known. It became known in 1909 that several millions of young white pines had been previously imported from Germany, with which the fungus unquestionably was introduced. Later shipments brought it from France. Although Americans were warned of the appalling injury by the fungus to white pines in Europe, yet it became established here without attracting government consideration. In 1912 the federal government enacted The Plant Quarantine Act, which provides for the regulation and quarantine of diseased nursery - stock; this, however, came too late, as the pathogene had by that time spread to several states in the Union. The disease has been found in Indiana, Ohio, New York, Pennsylvania, New Jersey, Connecticut, Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Wisconsin and Minnesota. It is also found in Ontario, Canada. It is now widespread on wild and cultivated Ribes in Connecticut, Rhode Island, Massachusetts, New Hampshire and Maine. In time the disease will doubtless be found in all parts of temperate North America, although the federal and state governments are cooperating in an attempt to prevent such a calamity.

Symptoms

On currants (Figs. 55 and 56) and gooseberries the disease is found from June first to leaf-fall. Rust pustules one-sixteenth to three-eighths of an inch across are formed on the lower sides of the leaves. These are scattered or dense, depending on the intensity of the infection (Figs. 55 and 56). If severely affected, the leaves die and fall prematurely. The rusty mass on the leaves has given rise to the names Currant Rust, felt rust and currant felt rust.

The disease may first be observed on the white pine (Fig. 57) in the spring from April to early June. The stem or branches are often girdled and the portion above dies. Most young trees die in a relatively short time; others live for some time, but even old stems finally succumb, the tree eventually breaking at the lesion. The disease in its early stages shows peculiar fusiform, or spindle-shaped, swellings which taper upward. These are usually found on wood at least three years old. When a swelling is first noticeable, small transparent spots appear which develop into blister-like pustules surrounded at the base by a translucent spot, whence the name blister rust. These pustules break open and drops of sweet, honey-colored fluid exude. Later, spore-pustules break through the bark. These are small, scattered, and from them orange - colored masses of spores push forth through the break in the bark (Fig. 57).

Fig. 55.   European Currant Rust; uredinia on lower surface of black currant leaf.

Fig. 55. - European Currant Rust; uredinia on lower surface of black currant leaf.

After the spores escape, a whitish membrane is left about the edges of each pustule (Fig. 57); this disappears by the first of June and an empty depression remains. Affected trees are stunted: the tops have a peculiar bunched growth, the past season's growth is shortened, and finally the needles turn yellowish in color.

Cause Of European Rust

This rust fungus, Cronartium Ribicola, like many others, is heteroecious; that is, it requires two distinct kinds of hosts for its full development. And during its life-history five spore-forms are developed. The fungus attacks and lives in the bark of the white pine in one stage of its life-history; as such it is known to scientists as Peridermium Strobi. It may also occur on other five-needle pines, such as the stone-pine (Pinus Cembra), sugar-pine (Pinus Lambertiana), western white pine (Pinus monticola), and Himalayan white pine {Pinus excelsa). The white pine (Pinus Strobua) is found in North America from Newfoundland to Pennsylvania, along the Appalachians to Georgia, west to eastern Iowa and Minnesota; in Canada from Lake Winnepeg to the northern shore of the St. Lawrence Gulf and Newfoundland. In the other stage, the fungus lives in the leaves of about twenty - five different species of wild and cultivated currants and gooseberries; this stage is known as Cronartium Ribicola, the name now used to apply to any and all stages of the fungus on whatever host it is found.

Fig. 56.   European Currant Rust; telia on lower surface of black currant leaf.

Fig. 56. - European Currant Rust; telia on lower surface of black currant leaf.