Of recent years the most troublesome disease has been that of Silver leaf due to Stereum purpureum. The fungus, one of the Basidiomycetes (see FUNGI), enters through a wound and through the mycelium remains in the stem. The leaves become affected and turn a curious ashen colour, something of the appear ance of lead. The valuable Victoria plum is particularly attacked.
The disease is a notifiable one in Great Britain. (X.) Cultivation in the United States.—In European countries, varieties of three, or at most four, species of plums are under cultivation, but in North America representatives of at least 12 quite distinct species are commonly cultivated, 2,000 forms of which have been named and described. It is hardly too much to say that of all drupe fruits plums furnish the greatest diversity. Species and varieties give a great range of colours, forms, kinds, sizes, flavours, aromas and textures. The plants are quite as diverse in America as the fruits ; some plums are true trees with stout trunks and sturdy branches, while others are slender shrubs; some species have thin, delicate leaves; others, coarse, heavy foliage ; in some the plums are large and attractive, in others, small and unattractive with a disagreeable odour.
In geographical distribution in North America, wild and culti vated plums cover almost the whole temperate zone, the several species and varieties being adapted to .a great diversity of soils and climates. In Europe nearly all of the varieties of plums be long to the species Prunus dornestica. This species does not hold the same relative position in America that it does in Europe for the reason that it does not possess in a high degree the power of adaptation to trans-Atlantic environment. The feature of environ ment most uncongenial to European plums in America is the climate. The plums thrive best in an equable climate like that of Eastern Europe and Western America, and do not endure such extremes of heat and cold, wet and dry, as are found in America east of the Rocky Mountains.
The temperate zone in North America, however, is a natural orchard of wild species of plums, scarcely any part of the United States lacking in one or more species of this fruit growing in the wild. At least ten of these wild species have been more or less domesticated with orchard representatives of several species com monly grown.
The plum is comparatively easy to suit in the matter of soils, and orchard exposure. The chief requisite for the genus in general
seems to be a good drainage. Given this condition, some sort of plum can be grown in almost any soil in the United States not wholly prohibitive to plant growth. In western America several plums are commonly grown for the making of prunes. Any plum that can be cured, without removing the pit, into a firm, long keeping product is a prune. The growth of the prune industry on the Pacific coast is one of the most remarkable industrial develop ments of American agriculture. The first commercial orchard of prunes was planted in California about 1870. In 1930, the output per annum was valued by the producers at over $1,500,000.
About 1,500 varieties of plums have been described in horti cultural literature in America. Of these perhaps 1,000 are of the common European domestic type; perhaps ioo are damson plums; and 400 sorts may be distributed among the 10 or 12 species of wild native plums now under domestication. In addition to the European and native species, an Asiatic plum, Prunus salicina, has been introduced from Japan, of which there are prob ably 200 sorts, if hybrids with native species be included.
The leading varieties of plums cultivated at the present time are in order of ripening:— Beauty, a splendid large, very early Japanese plum, yellow, juicy flesh ; Abundance, a large, juicy, sweet, Japanese plum ; Burbank, a productive, red, early Japanese plum, home or local markets; Formosa, Japanese plum recommended for productiveness and large fruits; Santa Rosa, new Japanese plum, tree and fruit characters surpass Abundance and Burbank ; American Mirabelle, larger than the Etropean Mirabelle, splendid for dessert and culinary purposes; Grand Duke, very large, dark purple, long oval in shape, low quality ; Hall, good, dark purple, large, well flavoured, desirable for roadside stands or city markets ; Stanley, prune shape, large, dark blue, freestone, excellent quality ; Imperial Epineuse, purplish red, prune shape, excel lent in quality ; Reine Claude, yellow, roundish oval, good in quality, late, desirable for canning ; Italian Prune, purplish black, freestone, excellent quality for dessert or canning ; French damson, the largest and best of the damsons, excellent for preserves ; Albion, purplish black, large, flesh golden yellow, clingstone, good, very late.