In the spring of the year a slight hoariness is observed upon the branches of many of our apple trees. As the season advances this hoariness increases, and towards the end of the summer the undersides of some of the branches are invested with a thick downy substance, so long as at times to be sensibly agitated by the air. Upon examining this substance, it will be found that it conceals a multitude of small wingless creatures which exude it from their bodies, while at the same time they are busily employed in preying upon the limb of the tree. This they are well enabled to do by means of a long beak, or proboscis, terminating in a fine tubular bristle, which, being insinuated through the bark and the sappy part of the wood, enables the creature to extract, as with a syringe, the sweet vital liquor that circulates in the plant. The sap-wood, being thus wounded, rises up in excrescences all over the branch and deforms it; the limb, deprived of its nutriment, grows sickly; the leaves turn yellow, and the tree perishes. The insect which is productive of so much mischief is a species of the Coccidæ—order Homoptera—Coccus (or Pseudo-coccus) adonidum, otherwise the Aphis lanigera, or woolly plant louse, popularly called the American Blight. It was first observed in England in 1787, but it is uncertain if it was, as has been supposed, accidentally imported there from America. Some entomologists say it came from France. At all events, there is little doubt its original habitat was a warmer climate than that of Britain. It has, however, found its way hence from the latter country.
The wonderfully rapid development of the Aphis has thus been described by a popular writer:—
“It produces in the course of a season eleven broods of young. The first ten are viviparous, or brought forth alive, and consist entirely of females. These never attain their full development as perfect insects; but being only in the larval state (the larvæ are active and resemble the perfect insect, but are wingless) bring forth young, and the virgin aphides thus produced are endowed with singular fecundity. But at the tenth brood this power ceases. The eleventh does not consist of active female larvæ alone, but of males and females. These acquire wings, rise into the air, sometimes migrate in countless myriads, and produce eggs, which, glued to twigs and leaf-stalks, retain their vitality through the winter. When the advance of spring again clothes the plants with verdure the eggs are hatched, and the larva, without having to wait for the acquisition of its mature and winged form, as in other insects, forthwith begins to produce a brood as hungry, as insatiable, and as fertile as itself. Supposing that one aphis produced 100 at each brood she would at the tenth brood be the progenitor of one quintillion (1,000,000,000,000,000,000) of descendants.”—Paterson, “Science Gossip,” 1865.
It will thus be seen what very formidable foes these insignificant-looking little pests are to our orchards, and two questions naturally arise—first, what is the cause of their being so? and, second, where can a thoroughly radical remedy be found against their ravages?
There are some particular kinds of apple trees more attractive to these insects than others. Whether this may be attributed to the particular colour of the bark, a deficiency of lime, the presence or absence of certain juices in the sap, or to over cultivation or climatic influences creating an abnormal condition of the tree, and consequently rendering it more susceptible to parasitical disease, it is hard to say; but the blight now treated of is evidently more destructive in semi-tropical climates, such as Australia and New Zealand, than in Britain, owing in a great measure to the effect which the frequent hot sunny days succeeded by the cold frosty nights of early spring have upon the circulation of the juices of the tree, unduly stimulating their flow in the day time and abruptly checking their current at night, by which they burst their vessels and become the food of such insects as have been already described, the insects being often mistaken for the cause of the disease, while they are really the effect of it.
That the action of the American Blight, the Scale Blight, and the Cicadæ on our apple trees is to a great extent the effect of the last described condition, there cannot be much doubt.
Assuming, therefore, such an hypothesis to be correct, it is clear that, in place of the various nostrums or specifics—such as the preparations of carbolic acid, corrosive sublimate, kerosene, lime, or sulphur, which are recommended for washing the diseased trees, or the plastering of the infested parts with moistened clay, all of which are very transient in their effects—a non-liable stock to disease should be selected on which to graft any liable species the grower may desire to cultivate. That there are such stocks proof against blight, there are several authorities for stating, and, moreover, there is a member of this Association, Mr. Lightband, senior, living in our midst, who has succsssfully treated the disease by grafting an anti-blight tree, using a species of winter apple, on diseased ones.
In Darwin's book, “Animals and Plants under Domestication,” Vol. II., chapter xxi., “On Natural Selection,” he says:—
“From some unknown cause the Winter Majetin apple enjoys the great advantage of not being infested by the coccus.
“On the other hand, a particular case has been recorded in which aphides confined themselves to the Winter Nelis pear, and touched no other kind in an extensive orchard. * * Liability to the attacks of parasites is also connected with colour.”
Considerable controversy has lately been carried on in the pages of the
“Australasian” on remedies for the American Blight, and much has been said in favour of using stocks of the Majetin apple as a sure prevention of the disease. A Mr. Wade, of Pomona Place, Launceston, Tasmania, in a communication on the subject to the same paper, says:—
“That on his arrival in Tasmania he devoted especial attention to the check and prevention of apple blight, and one of his first ideas was to raise stocks from the seeds of those sorts not affected by blight. He chose the seeds of the Siberian Bitter Sweet, and the result was success far beyond his most sanguine expectations, for barely one per cent of stocks raised from those seeds were in the least affected by blight, while some alongside, raised from promiscuous seeds, were destroyed by it; and that he has continued the system for several years with the same unvarying success.”
Mr. Lightband's operations, above referred to, have been most successful. The juices of the fresh graft after a while permeate the whole of the diseased tree, infusing as it were a new life and fresh vigour into it. The aphides avoid infesting it, the leprous bark exfoliates, and a clean sound bark takes its place; the tree continuing to bear two kinds of fruit—that of its original stock as well as of the anti-blight graft. These, however, will no doubt in time merge their respective types or qualities, the one with the other.
From these circumstances it is not too much to say, there are good grounds for assuming that, in the first place, as a prevention of the disease the selection of an anti-blight stock on which to graft the particular kind of apple desired to be grown, will be the best means of insuring a healthy fruit-bearing tree; and in the second, as a cure for trees already affected with the blight, the inoculation process of Mr. Lightband is the most rational plan that can be adopted.
It is not the apple tree alone, however, that such parasites persecute. The pear, the peach, the apricot, and the nectarine, as well as the smaller fruits, also the grape vine and the hop plant, are all, more or less, infested by a species of one or other of them; and those who desire to derive both pleasure and profit from their fruit-gardens or hop-grounds, should not fail to seek for and apply proper remedies in good time.
In conclusion, it may not be out of place to advance a few words on the bearing which the theory of natural selection, or the survival of the fittest, has upon the subject now under consideration. It is obvious that without an operating cause—one, doubtless, amongst many, such as the parasitical influence of aphides on fruit trees in enforcing, as regards the latter, the necessary process of renewal by stimulating horticulturists to adopt improved methods of cultivation, as well as, instinctively as it were, to select such stocks as will prove the fittest against the destructiveness of these pests—with little exception, many fruit-bearing trees would be left untended, and, as a natural
consequence, would inevitably degenerate, and eventually dwindle away. In this will be recognized one, perhaps, of the many purposes designed for these tiny insects by that Providence who hath numbered the very hairs of our heads, and without whose knowledge a sparrow doth not even fall to the ground.
The Vice-President having placed his microscope at the service of the meeting, the author was enabled to illustrate very clearly the peculiar nature of the American Blight, as well as the Scale Blight. He also exhibited several branches of apple trees from Mr. Lightband's garden, showing the curative effect of that gentleman's anti-blight grafting treatment.
A discussion ensued, in which several members took part.
Mr. Elliott mentioned that when on a visit at the Hon. W. Robinson's in the Amuri three years since, while walking through the orchard one morning he discovered a tree affected with the American Blight, much to the disgust of the gardener. The trees in this orchard were at the time ten years old. No fresh tree or graft had been imported into it for at least seven years, and the nearest fruit garden was twelve miles distant.