ORCHARD LOSSES

New Zealand Herald, Volume LXXI, Issue 21899, 7 September 1934, Page 5

ORCHARD LOSSES

BROWN ROT INFECTION PROTECTING STONE FRUIT CAWTHRON INSTITUTE BULLETIN BY K. M. CUBTIS, M.A., D.1.C., D.SQ. No. 11. In some stone fruit a considerable period of resistance or even immunity is associated with the developing fruit under average seasonal conditions. This holds especially in the peach and the apricot. In plums, infection of the half-ripe fruit sometimes occurs, though in this case penetration is usually effected through skin weaknesses due to excessive rain or accident. Cherries on the other hand are definitely susceptible to green-fruit infection without any accidental agency assisting the fungus. lij certain seasons in New Zealand it is not rare for up to three-quarters of the cherry crop of particular trees to consist of brownrotted fruit infected when green. ' Spread of the disease from fruit to fruit in the cherry is exceptionally rapid, owing partly to juxtaposition of the fruit in the cluster, and partly to the texture of skin and flesh. With the approach of maturity all stone fruit enter upon the second period of high susceptibility to brown rot. Early conidia from blossom and shoot infection will by then have been largely distributed by wind and rain over all parts of the tree, including the ripening fruit, and secondary conidial crops will have developed. It requires little more than a day for a conidium lying on the surface of a ripening peach to produce its germ tube, for the germ tube to grow through the skin into the flesh, and for the numerous branches of fungal tissue thereupon developing to ramify through the flesh and reach the stone. In softtissued fruit such as cherries and nectarines the time taken is even shorter. Protection of Skin In some stone fruits the skin offers a limited protection against penetration by the germ tube. The more hairy types of peach and plum varieties possessed of a tough skin, and certain apricots, resist skin penetration more than smooth-skinned peaches and soft-skinned plums, nectarines and cherries. Once within the skin the fungus does not encounter any great resistance in the flesh, although in apricots and other firm-fleshed fruits the rate of progress is somewhat slower than in those with a juicy flesh. Of the fungicidal sprays applied during the season for brown rot control on the average stone fruit two are essential, at least one early pre-blossom spray (usually Bordeaux mixture), and some type of sulphur 'spray at pre-maturity of the fruit. The Bordeaux is usually applied at bud, movement and is followed by a lime sulphur at early pink. Sulphur sprays after the blossoming period are required at petal-fall and thereafter at intervals of three weeks to a month, the final, important, prematurity spray being applied 10 to 14 days before the fruit ripens. Recommendations as to type of sulphur spray have undergone frequent changes in late years. Ordinary lime sulphur was in general use at one time and is still advocated for the early pink. Then dry-mix lime sulphur was considered the best, and now colloidal sulphur has come to the fore. Colloidal sulphur 'has a better spreading power and less caustic action on the foliage than lime sulphur, and present recommendations advocate a combination of the two for all sprayß after petal-fall. Harvesting the Orop Strict cleanliness should be observed in methods of handling, in the utensils for holding the fruit, and in the packing shed itself. No worker should pick both infected and healthy fruit. If infected fruit happens to be on the tree at picking time it should be left untouched until the close of that day's picking, as its removal will release a shower of conidia into the air, on to clean fruit, and on to the picker's hands. Re-used containers should have been previously dipped in a sterilising liquid such as formalin solution (3 per cent) or bluestone (lib. in 60 gallons). The packing shed should be kept, scrupulously free from brown-rotted fruit. Ordinary air currents set up by draughts are sufficient to bear the conidia from one end of the shed to the other, and a case of brown-rot rejects in one part of the shed can easily prove a source of infection to fruit exposed elsewhere in the shed for packing. , ' ... The necessity of avoiding bruising of the fruit is too well know to need emphasis. What perhaps is not so generally known is that grades of bruising sufficient to break down the protective action of the skin exist and are yet not readily visible. A little excess pressure during picking or handling, the slight bump of fruit against fruit when a picking bag is used instead of « rigid container the slow cumulative effect of the weight of fruit on fruit in a deep case —are all sufficient to cause the collapse of the thin-walled cells lying close beneath the fruit skin. As a cell bursts, its sap is released and seeps out on to the skin, and even a fine film or droplet of juice is sufficient for the germination of any conidium on the moistened area. For'jt is safe to assume that if there is much brown rot in the orchard practically every seemingly healthy fruit bears conidia on its surface when picked. The Picking Receptacle The picking receptacle should be relatively shallow and rigid-sided. Some advocate the use of a pad on the bottom. Ideally the layers of fruit even during the picking process should be single. In many orchards the exigencies of the picking-packing-despatching system give little choice in the time of picking. Nevertheless, picking when fruit is warm during the heat of the day should be'avoided jf at all possible. This applies particularly to peaches and nectarines. The packing shed should be cool ana well-ventilated to allow the temperature of fruit warmer than is desirable to fall before being packed. When fruit is packed warm there is danger of sweating, and sweating provides moisture in which the conidia on the surface will germinate. The best pack in commercial use at the present time is probably that of the Continental special-quality fruit supplied to Covent Garden Market, London. The case is a single-layered tray, has a pad of cotton wool and the fruit (peaches and nectarines chiefly) are spaced so that none touches either another fruit or the tray's side. Local prices in New Zealand do not warrant these details, but each feature is rytht in principle and can be copied with modification, that is, shallow packing and avoidance of basal and lateral contacts. Lateral separation can be achieved without waste of space by wrapping the fruit. Moreover, wrapping is of marked assistance in preventing the spread of brown rot through the case. But the wrapping of fruit when warm is to be avoided, as the confined air-space between fruit and paper may become charged with moisture through the high rate of transpiration of such fruit.