BLACK-SPOT OF APPLES.

New Zealand Journal of Agriculture, Volume XV, Issue 2, 20 August 1917, Page 98

BLACK-SPOT OF APPLES.

W. H. Taylor,

Horticulturist

The black -spot” disease of apples is commonly known in scientific nomenclature as Fusicladium dentriticum, but that' is really the conidial or summer form of Venturia inequalis. It is important that both forms of the disease should be understood, as the effective control of the Fusicladium form is to a very great extent dependent on the destruction of -the Venturia or winter form of the disease. . Intelligent pruning and timely spraying, must go hand-in-hand or there cannot be effective control of the disease. Fungoid pests are. most troublesome in wet seasons because that condition fosters their growth. It therefore happens that measures that, were apparently quite successful one season fail to control the disease in a season when there is more rain, dry weather in the former case having acted as an auxiliary force against the disease. We cannot predict the weather, so we should prepare for the worst, even if it entails a little more expense. As a matter of fact it does not do so ; all that is required is to act promptly at the proper time and do the work thoroughly. -

VENTURIA INEQUALIS.

Young shoots are the chief points of attack ; the infection takes place during summer, and the disease winters in the' shoots. More rarely it attacks older branches, entering through wounds or insectpunctures. . Young shoots are attacked because their bark is tender and easily pierced. The disease first shows as dark-coloured patches; If at this time a strip of bark were cut off, the wood would be seen to be discoloured. Eventually the bark becomes ruptured and torn, or completely thrown off, the ruptures being particularly noticeable at the base of the shoots. In early spring the diseased shoots are easily distinguished. At this.time the blackish patches are densely covered with the conidia of the second form known as Fusicladium dentriticum. The conidia are carried by wind and rain on to the young leaves and blossom-stalks. On the leaves the disease first appears in the form of small, roundish, dark-coloured spots, mostly on the upper surface. The spots soon. increase in size and run into each other, forming large irregular-shaped blackish-olive, blotches. At first the mycelium spreads in the leaf under the skin, which is eventually

ruptured, exposing numerous Fusicladium conidia, similar to those on the shoots. The conidia formed on the leaves, also from the shoots, are in turn conveyed to the young fruit, which becomes infected, and the result is “ apple-scab.” As previously stated, the disease also attacks the stalks .of the blossoms, its effect being ,to disable them and cause the fall of the young fruit.-

CONTROL MEASURES.

It is of the utmost importance to remove as far as possible all sources of infection. These include diseased shoots, dead leaves, and mummified fruit, on all of which the disease winters. When it is recognized that the disease cannot be killed by spraying during winter, and -that it attacks young leaves . as soon as they unfold and frequently blossom-stalks immediately the cluster bud opens, there should be no need to emphasize the importance of .cutting off and burning diseased shoots and burying dead leaves in order to prevent the spread of conidia from them to the trees. The reason why winter spraying is ineffectual is because at that time the disease is under the bark of shoots .or between the tissues of leaves. This at once explains why spraying may be without apparent effect in checking the disease during summer. The disease quickly gains entrance beneath the rind of the fruit or the skin of the leaves, where the fungicide cannot reach it. It is evident that in order to attain perfect control of the disease it is . absolutely necessary to apply the fungicide before the disease penetrates the skin of ’the. leaf or the rind of the fruit. The case may be stated thus : The means of prevention are easy and sure, but to effect a cure is'extremely difficult, if not impossible.

The first spray should be Bordeaux, 8-6-40, applied just before the buds burst. The second should be half the latter strength, and be applied when most of the cluster buds have opened but before the flowers —when, in fact, they are in the state spoken of as “ showing pink.” The third spray should be given when most, but not all, of the petals have fallen ; but owing to the danger of scorching the tender skin of the young fruit the formula should be reduced to 2-3-40, or slightly below the ordinary summer strength. In a normal season little further spraying will be ‘ required, but more should be given at discretion if the season be a wet one. The copper-sulphate mixture (Bordeaux) is undoubtedly a more effectual fungicide than lime-sulphur, but owing to the russeting effect it has on the fruit, particularly some of the . more tender-skinned varieties, its use in the orchard in the summer should be restricted as much as the exigencies of the season will allow, substituting lime-sulphur at the ratio of 1 part to 100 or 120 of water. • .. * .

BLACK-SPOT OF PEARS.

This is known as Venturia -pinna, but, although the name is different, there is practically no difference between this disease and that affecting apples. The sequence of development on twigs, leaves, and fruit is the same. Cracking of the fruit is caused by the disease hardening the skin, which ceases growing, and the cracking is caused by internal pressure of the developing fruit. Cracking is more prevalent in pears than in apples, but this is due to the physiology of the fruit, not to the effects of the disease. The treatment required is the same as for apples.

Peanuts.— Messrs. King Bros., Papakura, write that without any previous experience or knowledge they planted last season a few peanuts in a small patch, and had an excellent return, the nuts clustering thickly to the roots. The nuts were sweet and crisp. Their place is on the shores of Manukau Harbour, where the climate is mild.

The Minister of Customs has granted general permission for the exportation of New Zealand hemp to the United States of America. No application or permit will be required while the general permission is in force, but a declaration of ultimate destination must be produced to the Collector of Customs before shipment.

The English Board of Agriculture has made arrangements with the leading banks for lending money to farmers on a Government guarantee for the purchase of seeds and manures for growing wheat, oats, and potatoes. . When the grant of credit is approved by the County Committee the applicant will be able to order the seeds or manures, and the bill will be paid through a bank on the applicant signing a promissory note undertaking to repay the amount within nine months with interest at the rate of 5 per cent, per annum.