A Practical Treatise on Olive Culture, Oil Making and Olive Pickling/Chapter 5
CHAPTER V.
DISEASES.
The olive tree is so robust by nature, and its bark, leaves and fruit are so bitter that in consequence of those diflferent advantages it is less exposed than other trees to the ravages of insects and animals, especially when it is planted on hills and mountains, on light and well drained soils, for, no one can ignore the fact that fruit trees in general are so much more exposed to the many pests that endanger their existence and check their bearing capacity as they are planted in low and moist lands.
The most dangerous enemy of the olive tree seems to be the black scale. This insect has a marked preference for the orange tree, as well as for the laurel tree, which are generally planted in rich soil and very seldom on high elevations or meagre and well drained lands.
Under the shape of very small shells of a dark brown color, these insects fasten themselves very closely to the branches, leaving after them a trail of a blackish dust formed by the sap they extract from the tree mixed to their leavings, the whole of which is covered by the dust spread over it by the wind.
Riondet tells us that during the winter, when the young insects are still under the calapash of the mother already dead, under which they remain yet protected from the cold weather, a large number of them can be crushed by rubbing the branches with a hard brush dipped into vinegar.
Lardier recommends to rub with lime water; Reynaud says to sprinkle the tree with that same preparation, and Du Breuil affirms that such an energetic rubbing and spraying will cause the disappearance of both the insects and their black trails.
In California where the black scale has pretty generally appeared and caused great havoc among the orange trees of the southern part of the State, the trees when young should be sprinkled with a whisk broom dipped in a bucket containing a mixture in equal parts of sulphur and whale oil dissolved in hot water; and if any olive grower is ready to act the moment those insects begin to appear here and there on any of his trees, he will easily prevent their spreading to others.
Other preparations in which enter kerosene, or infusions of tobacco, absinth leaves, etc, are also recommended, but while I fully appreciate their merit, the one just given brings about very satisfactory results; it can be prepared very easily and will not cost over ten cents a gallon, which quantity will be amply sufficient for the sprinkling of fully one hundred young trees.
But neglect and ignorance are capital sins in most things of this world, especially in arboriculture, and I have seen olive trees, in rich soil, adjoining orange, lemon and laurel trees, none of which had ever been cleaned and which were covered all over with black scale. How can any one expect a product from trees that are in the clutches of death? If, however, in spite of their leprous condition, they give a little fruit, it can be but in very small quantity, and if allowed to grow so, their weak and sickly condition will delay, if not preclude their propensity to bear.
We can thus say with reference to this that an ounce of prevention is worth ten pounds of cure.
There are other insects of a secondary importance which attack the olive tree, but provided one keeps his eyes opened to face the enemy when its vanguard puts in appearance there is but little to be feared.
Moreover, the ever provident nature comes oftentimes to the rescue of human carelessness. If the insect known under the scientific name of Hylesinus oleae bores the young twigs to the heart, causing some of the branches to break down under a heavy wind; and if a fly called the dacus oleae deposits its eggs on the berry and feeds afterwards on its flesh, the ant, and other insects belonging to the carnivorous class come and feed on them.
But the best preventive is, first, to prune carefully, so as to give free access to light and air in all parts of the tree; then to rub energetically the trunk and largest branches with lime-water, or with the whale oil and sulphur preparation, and these enemies will be kept at bay; only a small portion of them, if any, will survive, to become the prey of their own enemies, which nature provides to finish the work of man, that is: of the man who knows how to help himself.
In Europe, says Du Breuil, the most dangerous enemy of the olive tree is the excessive cold of certain winters, when the thermometer runs down as low as 12° and 10° Fahr.
Looking back over a century, it has been found that the olive trees have been frozen, on an average, every nine or ten years. Among the winters most disastrous to them were those of 1740, 1745, 1749, 1766, 1770, 1789, 1795, 1811, 1820, 1830, 1837, 1843, 1859 and 1866. No more recent data is at hand.
Is it, then, surprising that this culture rather tends to decrease than to increase in the best oil regions of Europe, and that they should raise the olive tree mostly from the seed? It is true that it can thus cope more successfully against those severe winters which come at frequent intervals; but the period of bearing is considerably delayed thereby. It is for such reasons as these that many European agriculturists are deterred from engaging in this culture, the result being that the production of pure olive oil is far from being equal to the consumption of the world; hence the many adulterations in which cotton-seed, linseed, cocoanut, lard oil, etc., play such an important part, and tend to keep up a natural prejudice against an article whose bad taste is so far from the exquisite delicacy of the genuine.
But similar dangers are not to be feared in California, where the thermometer falls very seldom below 30° Fahr., and will reach only very exceptionally 28°, or one or two degrees lower; while the Olive tree will stand without danger as much as 15°, and even 12° and 10°. It is thus that what is a serious check to its development in Europe, is the very reason why its culture should be adopted fearlessly and extensively under the temperate climate of California, for which Providence has been so lavish in its beneficent gifts.