SILVER BLIGHT.

New Zealand Journal of Agriculture, Volume II, Issue 2, 15 February 1911, Page 73

SILVER BLIGHT.

RECENT WOBURN EXPERIMENTS.

[By

A. H. Cockayne, Biologist.

In the Twelfth Report of the Woburn Experimental Fruit-farm* is a very important paper on the cause of silver leaf, a disease which is said to be identical with the silver blight of New Zealand. As the results of the experiments conducted at Woburn are distinctly at variance with the general ideas prevalent in New Zealand regarding this disease, some account of the investigations will be valuable to New Zealand fruit-growers.

General Description of the Disease.

Silver blight has been long known to fruit-growers and gardeners in New Zealand. The foliage of affected trees becomes light and silvery in appearance, and diseased trees can be at once distinguished, even at a considerable distance, from healthy ones by the silvery sheen of the affected leaves. In certain cases only individual branches become affected during a season,, the other portions of the trees remaining perfectly normal.'' In other cases the whole , tree becomes affected, and no normal foliage may be present. During the winter on , deciduous trees the presence of the disease cannot be detected when once the leaves have fallen, but on evergreens, such as the Portugal laurel (Cerasus lusitanicus), the silvered leaves are conspicuous during the whole of the year. Silvering of foliage is, however, not always to be attributed to silver blight. Thrip-attack will often produce a silvering of the upper surfaces of leaves, especially on rhododendrons and -the giant laurustinus ; but in the case of true silver blight the leaves attain their characteristic hue when no insects are present. In general, affected trees die after a certain lapse of time, but this may not occur for several years, and, while suffering, a considerable amount of growth may take place and a small quantity of fruit be produced. At times silvered trees appear to throw off the' disease and become perfectly healthy again, but definite information on this point is scanty. It is, however, certain that affected trees do occasionally completely recover, even when no treatment is adopted. ! ■

When a diseased branch is cut there will, generally be seen a discoloration of the wood, which seems to extend down the stem below the point of silvering. This discoloration may be .merely a streak, but at times a considerable area of the wood from the bark to the pith may be affected.

In peaches and plums this discoloration is not always- present, but in all the cases of apples that I have examined the wood appears affected. Nearly all members of the plum family seem to be liable to attack,- together with certain other plants such as currants ; but the main fruit-trees affected in New Zealand are peaches, plums, apricots, apples, nectarines, and occasionally pears. The Portugal laurel, which is badly .attacked in England, frequently becomes attacked here, but appears to be able to live for -a good many years after the trees become first affected.

Suggested Causes

For many years the cause of silver blight was a matter for much conjecture. It was but natural to think that the disease, whatever it might be, would first gain an entrance through the leaves; This idea held for many years, not because any definite causative organisms were ever discovered in the leaves, but, as the leaves were the only organs outwardly affected, it was thought they must naturally contain the organism causing their diseased appearance. In 1902 Professor Perceval* attributed the cause of silver blight to the activity of a fungus known as Stereum purpureum, in the wood of affected trees. The' fruiting portions of this fungus develop on the bark of dead trees only, and are found on the dead wood of many trees that have never shown any signs of silver blight, such as horse-chest-nut, sycamore, hornbeam, pines, willow, birch, and elm.f In England it also commonly fruits on the dead. portions of trees affected with silver blight. In New Zealand, Stereum has been collected by me on apple-trees, plum-trees, and Portugal laurels that have died from silver blight, but I was under the impression that it had only gained an entrance after the trees had died. I have never noticed Stereum on the bark of affected peach or apricot trees ; but, as badly attacked trees are generally cut down and burnt before they entirely die, this may account for the apparent scarcity of this fungus in orchards suffering from silver blight. Professor Perceval’s conclusions have by no means been generally accepted, and the following extract from Massee’s “ Diseases of Cultivated Plants and Trees ”J well sums up the views .of his opponents : —

This [silvered] appearance is v due to the presence of ’air^cavities under the cuticle, formed by the more or less complete separation of the cells of the epidermis, due to the splitting-apart of the vertical walls. In other respects an affected leaf appears to be perfectly —there is no stunting, and the chlorophyll is present as usual.. In some instances a single branch is attacked, which usually dies. during the second season after showing the disease, the rest of the tree .'remaining perfectly healthy. This is more especially the case with trees grown under glass.’ When growing in the open the whole tree is not infrequently attacked, and then usually succumbs within two or three years —in some instances, however, lasting much longer.

Trees, when once attacked, as a rule do not recover. The disease appears to be much more prevalent at the present day than in past times, or perhaps it is more frequently noted now. Professor Perceval claims to have proved that a fungus (Stereum purpureum) is the primary cause of the disease. . This, however, I have not been able to confirm ; I have examined hundreds of diseased trees without finding a trace of the fungus, either on the surface or in the tissues.

Another view is that the injury is due to excess - of nitrogenous matter* in the soil; but the disease often breaks out on a large scale in places where no such excess of nitrogenous matter exists. Besides, this idea could not account for the disease attacking a single branch of a tree. I consider the effect to be the outcome of some physiological disturbance, resulting in the production of a ferment capable of dissolving the pectic substances present in the;- middle lamella, and thus allowing the cells to break away from each other.

The Woburn Experiments.

Ever since 1904 the Woburn Experimental Fruit-farm has been experimenting on the . cause of silver leaf, and the . results are fully embodied in the Twelfth Annual Report of that institution. That they appear to fully confirm Perceval’s contention that silver leaf is due to Stereum purpureum is shown by the following quotation. . .

Independently of his results, however, those described in the present report would be quite sufficient to place the matter beyond doubt; for the inoculation of a tree with a piece of a fungus has nearly always been found to produce silver leaf,whilst neighbouring trees which were not thus inoculated remained quite healthy. Further, trees which have died partially or entirely after an attack of silver leaf so often develop -the Stereum on the dead wood that it is impossible to avoid connecting the one with the other, especially as no single instance is on record of the appearance of the fungus on a tree which has been known to have shown no signs of silvering whilst alive.f

For the purpose of inoculation it is only necessary to insert a small portion of this fungus, about twice the size of a pin’s head, in a cut made in the bark of the tree, binding up the cut so as to prevent, the fungus from being washed out. Previous to use, the fungus should be kept in a warm, moist atmosphere, so as to insure its being in an active condition.

The first sign of infection, consists of a gradual appearance of silvering of the leaves, which, when the tree is in active growth, may occur in. a .week or two after the inoculation, whilst the whole tree may become seriously affected within four or five weeks. In many cases it is noticeable that the infection seems to spread from the point of inoculation : if one branch only has been inoculated, that branch is the first to suffer, and if one side of the stem has been inoculated it is on that side that the silvering first appears.

The outward manifestation of disease is no doubt due to a poison formed during the growth of the fungal threads —probably this is so with all fungoid and bacterial diseasesfor no evidence of the presence of the fungus itself -—i.e., its myceliumcan be found in the leaves. The silvery appearance

of the leaves seems to be due to the cells composing them .‘'having become' partially disconnected, owing to changes brought about in the nutrition of the plant. <. "

Turning to the actual experiments on the subject : These were started early in 1906, when a plantation of 112 two-year-old trees was made, consisting of twenty-eight specimens each of Monarch, Czar, Rivers’s ■ Early Prolific, and Victoria. The trees were planted in rows of eight, two of each variety, at a distance of 4 ft. between each tree, except in certain cases, where, for special reasons, still . closer planting was adopted ;; generally, each row was treated in some particular way, and formed a separate experiment. . ■ ’

Forty-eight of these trees were inoculated early in the year with Stereum obtained from a silver-leaved plum-tree, and, of these, thirty-nine, or 81 per cent., showed silvering before the end of July, four of them having killed outright by that time, and two others being half-dead. ■ Besides the dead trees, eight of the others were silvered as to the whole of their foliage, whilst in the remainder which suffered, the silvering was only partial. In contrast with these results, it is' found that, not one of the sixty-four uninoculated trees showed any signs of silvering, and, it may be . added, none of them, , unless subsequently, inoculated, has shown any signs of it during the succeeding years. These results are in themselves ample proof that the disease is a consequence of Stereum purpureum. But, beyond this, of the inoculated trees, nineteen died during the first two years after planting, and all of these developed Stereum on the dead wood, whilst none of the uninoculated trees died, or showed any signs of the fungus. ’ '.

Many experiments were,. conducted on. various portions of trees — branches, and roots —with the result that stem-infection produced by far the largest amount of silvering. Different periods of the year were selected for the inoculating, to ascertain whether there was any correlation between the season and infection ; but the results indicate that the date of innoculation has no material influence. This is an important point,-and-greatly increases the difficulty of. control-methods. There were, also many valuable experiments conducted to show the action of Stereum obtained from one variety 'of tree on other varieties, such as Stereum from plums used on apples, pears, laburnums, and laurels ; the susceptibility of different varieties of plums ; the effect of vigour on infection ; and the reinoculation of recovered trees. The experiments, are carefully tabulated to show at a glance the results of the inoculations. All these experiments seem to conclusively show that silver blight is entirely due to the infection, through wounds or cut surfaces, of the spores of purpureum. , s . The Spread of the Disease. ’ . It .has been generally accepted ,in New -Zealand that silver blight can be communicated to healthy trees by pruning them with the same knife that has just previously been used on affected trees. Mr. T. W. Kirk states,'* “ It may be communicated from an affected to a healthy .tree by

means of a knife,” and in consequence makes the following recommendation : “ Use two knives, the first for pruning until the sound wood is reached, and the second for cutting off a piece of sound wood; or else sterilize the knife before making the final cut.” This point has been carefully investigated at Woburn, and the following statement appears in the report : —-

Whether the infection can be carried from tree to tree in the' ordinary operation of pruning is another matter of practical importance. It is evident, of course, that if the pruning-tool happens to get impregnated with the spores of the Stereum itself it will be liable to infect the next sound tree on which it is used ; but is it possible to infect such a tree by a tool that has been used merely on a silvered tree with none of the fungus actively fructifying on it - ? To definitely investigate the matter, two rows of eight plumtrees each were taken ; one of these rows was inoculated, and all the trees in it became infected. The other row was not inoculated. In pruning these trees, one cut was made on a silvered tree, and then one on its sound neighbour, the whole of the trees being pruned in this alternate manner ; but not one of the sound trees became infected, although the process was continued for two seasons. These two rows of trees, also, were planted only 2 ft. apart, so that in many cases their branches were actually rubbing each other. That there is no power of communicating infection by anything present in the silvered leaves was shown by making a pulp of such leaves and inserting it into a wound in the stem of a sound tree. In four instances where it was tried no results whatever followed.

The above experiment appears to be fairly conclusive ; but it must be remembered that the affected trees used were ones that had been artificially inoculated. I should like to see a. similar experiment conducted where the affected trees were naturally silver-blighted. Root-pruning of affected and healthy trees with the same implement also gave negative results. Experiments were made to graft infected scions on to healthy stocks ; but in no cases was a successful juncture formed, although the operations were tried in two successive seasons. The complementary experiment of grafting healthy scions on diseased stocks also proved a failure. That heavy pruning seems to increase the amount of silver blight in New Zealand has been noticed from time to time. If the Stereum theory is accepted, the argument would be that heavy pruning, in exposing many wounded surfaces, renders the trees more liable to infection by air-borne spores. This, however, is hardly feasible in orchards where affected branches are all cut out and destroyed before any fungus has fruited. In such cases the presence of any large number of Stereum spores would be unlikely.

The Use of Sulphate of Iron.

The digging-in of sulphate of iron round the roots of affected trees has been recommended in New Zealand for many years, and amounts up to 12 lb. per tree have been used. The effect of this material has been carefully investigated at Woburn. Two sets of experiments were conducted, one on trees already suffering from silver blight, the other on healthy trees that were inoculated with Stereum, and which had an application of iron-

sulphate directly afterwards. ■ The amount of material used varied from Jib. to 1 lb. per square yard. The results .were quite unsatisfactory, and no good effects were demonstrated, either in reducing the disease on the already silvered trees or in stopping the infection of the healthy ones. The experiments are summed up thus : “ Treating the ground with’ iron-sulphate must therefore be regarded as being without avail.”

Treatment.

The only treatment that is suggested in the Woburn report is the drastic one of uprooting and burning any affected trees, so as to stop the formation of any of the fructifications of Stereum. In the case of trees that- are slightly affected, cutting of the branches below the point of silvering is suggested, and the success that has attended this practice in New Zealand is quoted. No sterilization of the pruning-knife is recommended, except in cases where dead wood that is liable to be carrying spores of Stereum is being cut out. In such cases the tool should be disinfected by dipping it in kerosene or a solution of carbolic acid.

In New Zealand the cutting-out of affected branches is largely practised, but the new wood frequently develops' silver blight, and it cannot be said that this method has been at all successful in controlling the disease.

Of late years the view has been spreading that the application of certain materials to the soil gives good results. Iron-sulphate has been largely recommended, but according to the Woburn experience is- useless. The use of sulphate of ammonia applied in varying quantities, from lb. to 10 lb. per tree, dug in round the roots of affected trees, has recently been said to be effective, but definite experiments under this head are lacking. This remedy is distinctly at variance with one of the European theories,* that the use of nitrogenous manures increases the virulency of silver-blight attack. Dusting the leaves with lime has also been tried by many growers ; but the results have not been successful enough to warrant its use.

Summary of Woburn Results.

The Woburn experiments seem , to have demonstrated, so far as England is concerned, the following points : — 1. The disease is caused by- infection of the tree with the spores of Stereum purpureum, the spores gaining an entrance through wounds and cuts on the stems and branches. 2. The spores of Stereum are likely to infect trees at any period of the year. . . ' . .. •, 3. The disease is not spread from tree to tree by pruning living affected and healthy trees with the same knife. , ■ 4. The use of sulphate of iron at the rate of 1 lb. per square yard has no effect in controlling silver blight. . ' '

The Need for Definite Experiments.

From the above it would appear that silver blight is undoubtedly due to infection of the living tree by the spores of Stereum purpureum. I must, however, confess that I have great difficulty in accepting such to be the case in New Zealand. The Woburn experiments conducted by Spencer Pickering, an able and careful investigator, must, however, be accepted rather than opinions based . on supposition alone, and until disproved by careful and accurate experiments they must hold good. As silvering is said to take place within a few weeks of the actual inoculation, it should be a simple matter to prove conclusively whether Stereum is the real cause ■of silver blight in New Zealand. When once the aetiology of the disease is thoroughly established there is some hope of remedial measures being formulated. At present , the experiments that have been conducted for its •control in New Zealand have been largely guesswork, and as such are not likely to lead to any satisfactory result.

Specimens of Stereum purpureum from ver-blighted trees are necessary before any definite experiments can be taken in hand. Can any readers of the Journal furnish me with this material ? Stereum from affected peach and apricot trees is particularly required. The fungus itself forms flat, roundish discs on the bark of the trees often two or three inches in diameter, and is generally of a purplish or pink colour, except when grown ■on the shady side of the tree, when it may. be almost pure white.

■ * Twelfth Report of the Woburn Experimental Fruit-farm. By the Duke of Bedford and S. U. Pickering. 1910.

* Jour. Linn. Soc., 35, 390. 1902. f W.’ G. Smith. British Basidiomycotes. ■? P. . 405. a a ■> j Masses, G. : “ Diseases of Cultivated' Plants and Trees.” 1910. P. 66.

* In . New Zealand certain nitrogenous manures have been recommended for the control of silver blight. .. •. \ t This must refer to fruit-trees only, as ■ the fungus" regularly appears on " the dead wood of many forest-trees that have never shown any signs of silver blight when alive.

* Annual Report, N.Z. Dept, of As., 1901, p. -318.

* Massee, loc. cit.