1911 Encyclopædia Britannica/Plum

PLUM, the English name both for certain kinds of tree and also generally for their fruit. The plum tree belongs to the genus Prunus, natural order Rosaceae. Cultivated plums are supposed to have originated from one or other of the species P. domestica (wild plum) or P. insititia (bullace). The young shoots of P. domestica are glabrous, and the fruit oblong; in P. insititia the young shoots are pubescent, and the fruit more or less globose. A third species, the common sloe or blackthorn, P. spinosa, has stout spines; its flowers expand before the leaves; and its fruit is very rough to the taste, in which particulars it differs from the two preceding. These distinctions, however, are not maintained with much constancy. P domestica IS a native of Anatolia and the Caucasus, and is considered to be the only species naturalized in Europe. P. insititia is wild in southern Europe, in Armenia, and along the shores of the Caspian. In the Swiss lake-dwellings stones of the P. insititia as well as of P. spinosa have been found, but not those of P. domestica. Nevertheless, the Romans cultivated large numbers of plums. The cultivated forms are extremely numerous, some of the groups, such as the greengages, the damsons and the egg plums being very distinct, and sometimes reproducing themselves from seed. The colour of the fruit varies from green to deep purple, the size from that of a small cherry to that of a hen's egg; the form is oblong acute or obtuse at both ends, or globular; the stones or kernels vary in like manner, and the flavour, season of ripening and duration are all subject to variation. From its hardihood the plum is one of the most valuable fruit trees, as it is not particular as to soil, and the crop is less likely to be destroyed by spring frosts. Prunes and French plums are merely plums dried in the sun. Their preparation is carried on on a large scale in Bosnia and Servia, as well as in Spain, Portugal and southern France.

Plums are propagated chiefly by budding on stocks of the Mussel, Brussels, St Julien and Pear plums. The damson, wine-sour and other varieties, planted as standards, are generally increased by suckers. For planting against walls, trees which have been trained for two years in the nursery are preferred, but maiden trees can be very successfully introduced, and by liberal treatment may be speedily got to a fruiting state. Any good well-drained loamy soil is suitable for plums, that of medium quality as to lightness being decidedly preferable. Walls with an east or west aspect are generally allowed to them. The horizontal mode of training and the fan or half-fan forms are commonly followed; where there is sufficient height probably the fan system is the best. The shoots should be laid in nearly or quite at full length. The fruit is produced on small spurs on branches at least two years old, and the same spurs continue fruitful for several years. Standard plum trees should be planted 25 ft. apart each way, and dwarfs 15 or 20 ft. The latter are now largely grown for market purposes, being more easily supported when carrying heavy crops, fruiting earlier, and the fruit being gathered more easily from the dwarf bush than from standard trees.

The following is a selection of good varieties of plums, with their times of ripening:—

Dessert Plums.
Early Green-gage	e. July		Transparent Gage		b. Sept.
Early Transparent Gage	b Aug.		Jefferson		b. Sept.
Denniston's Superb	b. Aug.		Kirke's		m. Sept.
Oullin's Golden	m. Aug.		Coe's Golden Drop		e. Sept.
Green-gage	m e.Aug		Reine Claude de Bavay	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	e. Sept.
					b. Oct.
M'Laughlin's	e. Aug.		Ickworth Impératrice		b. Oct.
Angelina Burdett	b. Sept.		Late Rivers	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	e. Oct.
					b. Nov.
Culinary Plums.
Early Prolific	e. July		Victoria		Sept.
Belle de Louvain	Aug.		White Magnum Bonum		Sept.
Belgian Purple	m. Aug.		Pond's Seedling		m. Sept.
Czar	e. Aug.		Diamond		m. Sept.
Pershore	e. Aug.		Monarch		e. Sept.
Prince Englebert	e Aug.		Grand Duke		Oct.
Mitchelsons'	b. Sept.		Wyedale		e. Oct.

Diseases.—The Plum is subject to several diseases of fungal origin. A widespread disease known as pocket-plums or bladder plums is due to an ascomycetous fungus, Exoascus pruns, the mycelium of which lives parasitically in the tissues of the host plant, passes into the ovary of the flower and causes the characteristic malformation of the fruit which becomes a deformed, sometimes curved or flattened, wrinkled dry structure, with a hollow occupying the place of the stone; the bladder plums are yellow at first, subsequently dingy red The reproductive spores are borne in sacs (asci) which form a dense layer on the surface, appearing like a bloom in July; they are scattered by the wind and propagate the disease. The only remedy is to cut off and burn the diseased branches.

Plum-leaf blister is caused by Polystigma rubrum, a pyrenomycetous fungus which forms thick fleshy reddish patches on the leaves. The reproductive spores are formed in embedded flesh-shaped receptacles (perithecia) and scattered after the leaves have fallen. The spots are not often so numerous as to do much harm to the leaves, but where the disease is serious diseased leaves should be collected and burned. Sloes and bird cherries should be removed from the neighbourhood of plum-trees, as the various disease-producing insects and fungi live also on these species. The branches are sometimes attacked by weevils (Rhyncites) and the larvae of various moths, and saw-flies (chiefly Erwcampa) feed on the leaves, and young branches and leaves are sometimes invaded by Aphides. Leaf-feeding beetles and larvae of moths are best got rid of by shaking the branches and collecting the insects. Slug-worms or saw-fly larvae require treatment by washing with soapsuds, tobacco and lime-water or hellebore solution, and Aphides by syringing from below and removing all surplus young twigs.

(After Sadebeck From Strasburger's
Lehrbuch der Botanik, by permission
of Gustav Fischer)

Taphrina Pruni—Transverse
section through the epidermis
of an infected plum. Four
ripe asci, a₁, a₂, with eight
spores a₃, a₄, with yeast-like
conidia abstracted from the
spores.

st, Stalk-cells of the asci
m, Filaments of the mycelium
cut transversely.
cut, Cuticle
ep, Epidermis.