The fungus lives from year to year in the bark of living pines (Fig. 57), finally fruiting and developing a crop of seciospores which blow to Ribes (currant and gooseberries) near by. In pines the organism may be shipped thousands of miles. It is not known whether the fungus hibernates on the Ribes or not, although there is some evidence that such is the case. In the spring from early April to June (chiefly in May), the seciospores are blown to young Ribes leaves. These spores are produced over a period of about two weeks. They apparently are never carried more than a few hundred feet. Should any of these spores fall on pines, infection will not result; the fungus cannot pass from pine to pine; it must first go to the currant or gooseberry, if it is to grow. In the presence of moisture the seciospores germinate and infect the Ribes leaves. Cold weather inhibits the rapid progress of the fungus in the currant leaves. With a favorable temperature the fungus soon establishes itself, and after two weeks a new crop of spores is produced. These are always produced on the lower surface of the Ribes leaf. They are distinct from the seciospores which come from the white pine, and are known as uredospores. The uredospores are capable of infecting other currants and gooseberries. This stage is sometimes called the summer or repeating stage (Fig. 55). This repetition may proceed throughout the growing - season, a new generation of uredospores being produced every two weeks if conditions favor. Thus the fungus spreads rapidly and over great distances, assuming, of course, that Ribes are present.
Fig. 57. - European Currant Rust; aecia on white pine.
From the latter part of July until leaf - fall, spores of another kind are formed on the currant and gooseberry leaves (Fig. 56). They are recognized as groups of three to twelve stout threads, measuring a quarter of an inch or less in length. The spores of this stage are called teliospores. The teliospores differ from the uredospores in that they cannot reinfect the currant and gooseberry. They germinate in a peculiar fashion: a short tube is produced on which small spores, called sporidia, are developed. The sporidia are extremely light and are blown easily by the wind. These blow to the white pines, germinate, and infect the bark of the same. The mycelium resulting spreads through the bark for several inches, but there may be no external evidence of the disease for several years (one to six or more). Finally the bark begins to thicken, and the first external evidence of the fungus itself consists of minute bodies known as pycnia; these appear from June to September. Closely following the development of the pycnia are the aecia. Spores from the aecia blow back to the currants and gooseberries.
From the point of view of the people at large the removal of the less important host would involve the currant and gooseberry. But in some cases the commercial grower of bush-fruit should destroy the white pine only in those cases where but a very few trees stand in close proximity to the berry plantation. The safety-zone may be put at five hundred feet; that is, the two hosts, white pine on the one hand, and currants and gooseberries on the other, should not be allowed to grow within a distance of less than five hundred feet of each other. It is safer at present to destroy those Ribes plants which are persistently diseased; for there is a slight possibility of their carrying the blister rust fungus over winter.
Considerable effort has been made to date to control the disease with reference to the five-needle pines. So far, however, the work has not been wholly successful. The difficulties involved are these: (1) There has never been any attempt previously, in Europe or America, to eradicate a tree-fungus of this sort; so there is no past experience on which to base the procedure. (2) The fungus incubates, or lies dormant without showing itself externally, for a period varying from one to six or more years; thus inspection has been inefficient. Diseased trees have been overlooked. (3) Neither the federal nor the various state officials who must carry on this inspection have the power to destroy the currants and gooseberries found within the danger zone. (4) The people generally do not realize the seriousness of the blister rust disease, and therefore cooperation has not been unanimous.
With the knowledge of the disease now available it seems likely that rational measures may be employed even in the absence of previous experience. On account of the long incubation period of the fungus in the pine, annual inspections are necessary, unless, of course, the whole group of pines be destroyed immediately on discovery of the disease. Special emphasis is laid on the destruction of wild currants and gooseberries, for the uredospores may spread from plant to plant for many miles during the summer. There is great need for adequate state laws. Inspectors should be regarded as friends of the people rather than as personal enemies to the fruit-grower or the forester. Their presence should be taken as an index of precaution against calamity rather than of any ill-influence of a supposedly antagonistic citizen. Such officers not only need cooperation, but, unfortunately, as experience has shown, they need legal power to compel concerted action in the eradication of diseased plants. In most cases the removal of currants and gooseberries, whether healthy or diseased, is essential. The situation is critical, and unless the proper measures are taken at once the disease will have reached a range beyond any possible control. The fruit - grower must face this problem: that, irrespective of his innocence and his personal feelings in the matter, the total destruction of his currant and gooseberry bushes should not be delayed when he has the official information that such should be done. In a preliminary way the grower can cooperate by making inspections of Ribes, beginning about July first.
Spaulding, P. The white-pine blister rust. U. S. Agr. Dept. Farmers' bill. 742:1-15. 1916., Spaulding, P. The blister rust of white pine. U. S. Agr. Dept. Plant Indus. Bur. Bul. 206: 7 - 78. 1911. Spaulding, P. European currant rust on the white pine in America.
U. S. Agr. Dept. Plant Indus. Bur. Circ. 38: 1-4. 1909. Stewart, F. C. An outbreak of the European currant rust. New York (Geneva) Agr. Exp. Sta. Tech. bul. 2: 61 - 74. 1906. Atwood, G. G. Emergency bulletin on the blister rust of pines and the European currant rust. New York State Agr. Dept. Hort. bul.
2:1 - 20. 1909. Stewart, F. C, and Rankin, W. H. Does Cronartium ribicola overwinter on the currant? New York (Geneva) Agr. Exp. Sta. Bul.
374:41 - 53. 1914.