BROWN-ROT OF FRUIT

New Zealand Journal of Agriculture, Volume XV, Issue 2, 20 August 1917, Page 84

BROWN-ROT OF FRUIT

INVESTIGATIONS IN HAWKE’S BAY.

G. Esam,

Orchard Instructor, Hast : ngs.

Of the many pests and diseases now attacking fruits and fruittrees in New Zealand perhaps the most destructive is brown-rot. The past season certainly showed the seriousness of the disease in stone-fruit orchards. It caused heavy losses of fruit in Hawke’s Bay, and Auckland suffered even worse. Noting the serious headway the disease made in December last year, the writer spent several days with stone-fruit growers investigating the extent of infection, the preventive measures adopted, and the results. At the same time Mr. W. C. Morris, Agricultural Instructor to the Hawke’s Bay Education Board, at Hastings, conducted inoculation experiments which he has been good enough to allow me to include in this article. I propose to show how closely Mr. Morris’s experiments coincide with my own investigations of the most successful methods of control, but before describing the experiments a simple outline of the life-history of the disease will be given.

LIFE-HISTORY OF THE DISEASE.

• To the ordinary observer - the disease first attracts attention on ripening fruits. It appears on matured stone-fruits as small, brown, circular spots, which rapidly increase in size, causing the whole of the fleshy part of the fruit to turn brown. As the spots increase in size small tufts of mould, resembling the blue mould of . bread, rise on the surface. On the top of. this mould

are the fruiting-spores of the fungus, which are also known as the summer spores. Some of these spores are dispersed by natural agencies such as wind, others are carried by insects. Under certain conditions these spores germinate, one essential condition being the presence of moisture : this may either be ordinary atmospheric moisture or moisture from the fruit itself. Very often the disease first appears on fruits which are touching — moisture being held for a considerable time at the point of contact of two fruits. After germination that is, the rising of a new shoot or growth from the spore —-it enters the fruit and therein multiplies in a very remarkable manner, feeding on the plant-food. Small circular patches of decay have been observed in just over twenty-four hours, and the whole fruit will decay in three to four days. As the disease spreads in the fruit fresh summer spores are thrown out on the surface. It is recorded that these summer spores can be produced under favourable conditions in thirty hours from the time of the spore entering the fruit. With the spread of the disease on the fruit the flesh turns a characteristic brown colour; but the flesh does not become sunken until the larger portion of the fruit becomes diseased. Some of these infected fruits fall on the ground, others remain attached to the tree. The flesh ultimately shrivels and dries up, the fruit becoming mummified.

Although it is considered by scientists generally that the disease is carried over winter on mummified fruit, and that it is highly important that diseased fruit should be destroyed, it is held that the disease also winters in the wood of young shoots. That these diseased shoots or twigs are even more dangerous as a source of infection is evident from their position in close contact with other parts of the tree and with the fruit. To secure control of the disease it is absolutely necessary that all such diseased wood should be removed.

On mummified fruit two kinds of spores are produced (i) semi-dormant spores, or oospores ; (2) dormant or winter spores, or ascospores. Summer spores may also be present, but I think it is a generally recognized fact that active spores may and do become dormant or semi-dormant spores when unfavourable conditions prevail, such as when the plant or fruit which is attacked dies. It is claimed that the winter spores, or ascospores, will remain dormant on mummified fruit in the ground for two winters and then germinate. However, in the spring these spores germinate, producing fresh spores, which are liberated by various agencies. These spring spores settle on buds, flower-petals, young leaves and twigs, and even young freshly formed fruits. If the weather •conditions are suitable they germinate and enter the parts mentioned. The fungus grows in them and causes the affected parts

to turn brown and die. It - also causes shrivelling,- decay, and dropping of small , green fruits. Summer spores are produced on these affected parts. Blossoms affected by the fungus remain attached to the twigs for. the greater part of the summer, and hence, as indicated, twigs can also be a medium of carrying the disease from one season to the next.

In the Hawke's Bay District last season early peaches and nectarines suffered most damage, and Japanese plums were also badly attacked. Cherries, English plums, and quinces also suffered, and apples to a much less extent. The disease was at its worst following the week of wet, hot weather at Christmas - time. Again, early in March when the same atmospheric conditions prevailed it threatened to treat some of the late peaches in a like manner.

INOCULATION EXPERIMENTS.

The. following is a brief account of Mr. Morris’s investigations, carried out. in January and February of this year. The first part of the experiment was conducted to test the time of the development -.of the ' fungus from spore to spore. Firm but nearly ripe peaches were picked from a tree and inoculated with a sterile needle in three places with summer spores taken from a freshly picked infected peach. Twelve hours later spots of a dirty, dark colour were showing about J in. outwards from the points of inoculation. In twenty-four hours the places spread to | in. in diameter, and in thirty-six hours the infection had doubled in size, but no summer spores were showing on the surface. By . the end of forty-eight. hours summer spores were produced, and the fruit' was almost wholly rotten, and in sixty hours it was quite bad. This experiment was conducted outdoors, and demonstrates the rapid spread of the disease, taking only thirty-six to fortyeight hours from the time it first enters the fruit until it again produces another batch of spores to carry on further infection.

■ In Mr. : Morris’s second series of experiments different kinds of fruits were inoculated with two classes of spores. One kind was the same as that used in the preceding experiment, taken from a freshly infected peach the tissues of which had not collapsed. The other was of oospores, or semi-dormant spores, taken from a peach which had fallen a week or longer, the flesh being almost shrivelled up. In the first test' of the series fruits of the following kinds were inoculated with both the foregoing classes of spores, a sterile needle being used : . Matured but green pears, firm plums, green grapes, tomatoes turning colour, green beans, matured but green nectarines and ■ peaches. Both kinds of spores took, and set up decay, on all. except the beans. The summer spores taken from the freshly infected fruit set . up decay sooner than the

oospores, or semi-dormant spores, taken from the shrivelled peach. With the former spores the fruits were rotten in two to three days, while the latter spores took three to five days to set up similar decay. This test clearly shows that no matter. what kind of spores enters the fruit rot will be set up, and that even green fruit will' go bad.

In a further test the same two kinds •of spores were utilized, but ' instead of a sterile needle being ■ used for inoculation the clean fruit was rubbed against the infected fruit, care being taken not to wound the tissues. of the sound fruit. After , ten days .no result was noticed on any of the kinds of fruits rubbed. This points to the conclusion that rubbing without injuring the tissues does not set up rot, and, further, the combined results of the two tests indicate that some —either by insects, such as flies, or other agentmust first be done. Smooth-skinned nectarines also gave a negative result, by rubbing. I am inclined to suggest that the totally negative results of the last experiment may possibly be due to. the absence of moisture, the fruit being dry when the spores were rubbed on. - . '

LOCAL ORCHARD INVESTIGATIONS.

Early in. January this year Hawke's Bay growers complained of the large amount of fruit which had rotted in transit. The fruit was apparently quite sound when packed, the usual degree of ’ ripeness as in . former years being maintained when picking. The two large companies in the. district received daily the same disastrous reports about the early stone-fruits they handled and ■ despatched. Retailers would not buy in quantity,' with the result that prices declined very considerably, . even for fruit which was sound when it reached its destination. The two companies supplied me with the' names of their best suppliers in relation only to freedom from brown-rot. I interviewed these and many other growers as to the precise method they had adopted to control the disease. My inquiries were principally confined to summer control. One. orchard only was free from disease ; it ' contained peaches and pip-fruits but no plums. The peaches were sprayed three times with lime-sulphur solution, 1-130, the first time just after the fruit had set, and two later sprayings, the last one two or three weeks ■ before picking. All fallen fruit in this orchard was ■ gathered up regularly and destroyed by burying; This orchard had brown-rot the previous year. Within a stone's - throw of this orchard another grower sprayed Cardinal . nectarines four times with lime-sulphur, but with negative result, all going bad in transit. This grower also lost heavily in . early peaches. No fallen fruit was gathered up in this, orchard. Three other orchards were nearly free from disease, under 5 per . cent, of fruit being lost. One of them has quite a number of plums, but no Burbanks.

No summer spraying was done in these orchards, but all fallen fruit was gathered up at regular intervals and destroyed. In another orchard which was nearly free from disease no summer spraying was done, but all rejected fruit at picking-time was immediately destroyed. Eight o + her orchardists who had sprayed with lime-sulphur in the summer from one to three times, but who had not gathered the fallen fruits, had more or less negative results. These growers did not appear to be in any better position than those growers who had done nothing to control the trouble. This investigation suggests that by destroying fallen and infected fruit loss from infection by brown-rot is minimized.

In badly infected orchards the presence of flies on infected fruits was most noticeable —in fact, under one affected tree I estimated that flies were feeding on 75 per cent, of the infected fruit. -' I : formed the opinion, rightly or wrongly, that flies were the main carriers of. the” disease, when puncturing the skin with their beaks to secure the sugary sap of'- juice. , This assumption is also borne out by Mr. Morris’s experiments. In these, it will have been noted, infection readily took place when the skin was punctured. Professor H. B. Kirk, of Victoria University College, Wellington, -an authority on flies, was good.'enough to supply some . information in reply to a query as to whether flies . would puncture the skin of mature and ripening peaches, nectarines, and plums, and thereby infect the fruit. He states, “ The biting stablefly will, in the laboratory, penetrate the wall of bananas, grapes, and pears, and ' probably also of the fruits you mention. I have not observed it in the open, but should expect it to occur.' The same applies to mosquitoes. But, as you know, many other insects will injure fruits, as will birds. To a wound so caused . non-biting flies may carry infection from, diseased fruit on which they have settled. Blow-flies, house-flies, and the fruit-fly are all well equipped for this purpose and frequent fruit in orchard or shop.”

The later peaches and nectarines appear to have a slightly thicker and tougher skin than the earlier varieties. This might possibly be the reason why the later fruits are not nearly so badly attacked, as flies and other insects would certainly be better able to puncture the skin of the thinner-skinned earlier varieties. Norton states that most of the infection takes place through wounds caused by various means, and claims that on peaches all the disease follows insect-bites or other injury. As stated when dealing with the life-history, other authorities are of the opinion that spores germinate on and enter the skin.

SUGGESTIONS FOR CONTROL

The foregoing knowledge of the life - history of the disease, coupled with Mr. Morris's experiments and my own investigations,

has : led me to suggest the following lines of preventive treatment. Commence in the dormant season by picking off the trees all shrivelled and mummified fruit; cut off and burn all dead twigs ; and gather up and burn all prunings and mummified . fruit, including stones. This winter work is important in that it destroys most of the mediums by which the disease is carried from one season to another.. To catch and destroy the spring spores, spray with a 'fungicide just as the buds show pii*k. In the Hawke’s Bay District all growers spray for leaf-curl, so that operation will serve the double purpose. Of the four orchards mentioned .as being . practically free from brown-rot, two were sprayed at this stage with lime-sulphur and two with bordeaux --personally I prefer the latter.

Now . comes a debatable . point that of summer control. In this the following three points are worthy of. the most earnest attention of growersnamely, thinning of , fruit, degree of \ ripeness when picking, and destruction of fallen fruit. Thin judiciously by leaving good’ spaces . between the fruits,, allowing. no fruits .to touch. The point of contact of two fruits is very often where the disease first makes its appearance. In regard to picking, the riper a fruit is up to a certain stage the more subject it is to brown-rot. This knowledge is helpful. If brown is feared, pick the fruit on the green side, yet sufficiently matured to ripen properly. Avoid as much as possible picking and packing the fruit when wet, also wipe the hands after touching infected .fruit, so .as not’ to carry spores on to clean fruits. Gather up and destroy all fallen fruit, allowing nothing at any stage to rot on the ground. This will prevent infection being carried by., flies and other insects.

Summer control by spraying is admittedly a phase we know very little about at present. Quite a lot of spraying has been done with more or less indifferent results. I think summer spraying coupled with the.' destruction of fallen fruit the best means of control. I am inclined to favour atomic sulphur, with lime-sulphur next. To me there seem to be two very important periods of applicationnamely, immediately the fruit has set, . and again a fortnight or so before picking. I would also recommend one or two intermediate applications. A Henderson (Auckland) grower has stated that he had excellent results with atomic sulphur combined with arsenate .of lead. The latter evidently acted against leaf-roller and other biting-insects, preventing wounds by which the fungus enters the fruit. The Horticultural Division at Arataki Station and in certain Henderson orchards is carrying out very extensive spraying experiments, the results of which, no doubt, will give us a more definite line of control for succeeding seasons.