A FUNGUS DISEASE ATTACKING BLACKBERRY.

New Zealand Journal of Agriculture, Volume XXIV, Issue 1, 20 January 1922, Page 23

A FUNGUS DISEASE ATTACKING BLACKBERRY.

IDENTIFIED AS RASPBERRY CANE-WILT.

G. H. CUNNINGHAM,

Biological Laboratory, Wellington.

Probably the most serious fungus disease that raspberry-growers have had to contend with in New Zealand is that known as “ cane-wilt.” During last season numerous specimens of blackberry-canes apparently infected with the same disease were sent to this Laboratory from various parts of the North Island. At the same time many statements appeared ■ in northern newspapers to the ' effect that large areas of blackberries were being killed * outright. Opinions were divided as to the cause of the death of these plants, one theory being that fireblight had spread from hawthorn to the blackberries. Interest became so widespread in this supposed new disease, credited with exterminating one of the most troublesome of noxious weeds, that the writer was directed in May last to visit the localities where the outbreak had been reported as being most severe, in order to investigate the nature of the causative organism, and, if possible, to ascertain the extent of the damage done and the probable value of the organism as a controllant of blackberry.

At Hamilton I was joined by Mr. L. Paynter, Orchard Instructor for the district,' and we proceeded via Rotorua to Ngawaro and Oropi, where the disease had been reported as doing considerable damage. At Ngawaro we were informed that the most serious infection in that locality had been seen in plants immediately across the. road from the post-office. Our informant assured us that the infection in that year had been slight, but that in the previous season some dozen or more plants had had many of their canes killed back to about ground-level. Inspection showed that these plants appeared to have made normal growth during the current season, but the fact that they had been severly attacked the previous year was evident from the presence of numerous dead canes. That the current season’s infection was slight was evident from the fact that out of half an acre or so perhaps a dozen plants in all were attacked, and these chiefly on the fruitingpanicles, a feature which is discussed later in some detail.

At Oropi it had been reported that a large area of blackberry had been severely attacked by this disease, this area being situated in a bush-clearing about half a mile from the main road. Here the infection, although slight, was more general, as an area of perhaps half an acre had the greater number of plants in it more or less severely attacked. In some cases two or three of the canes were seen to have been killed back for about half their . length, but generally cane-infection was slight, the disease being .confined to the branches of the panicles, which, together with the maturing fruits, were often killed outfight.

From Oropi we proceeded to Tauranga, thence to Waihi, Te Aroha, and Morrinsville, • and < back to Hamilton. Throughout these districts odd infected plants were noticed by the roadside, in all cases panicleinfection being common, while cane-infection . was slight or absent.

Specimens forwarded to this Laboratory from Auckland, Gisborne, Napier, and Hastings all show the same diseased condition of the panicles, indicating that the disease is widely distributed in northern parts of the North Island.

THE CAUSATIVE ORGANISM.

Many specimens were gathered on the trip and brought back to the Laboratory, where they were microscopically examined. Although numerous fungi were present on older canes that had been killed in the previous year, study of these showed, them to be all saprophytes. However, Mr. Paynter continued to forward specimens, and before long the fruiting-stages of the causative organism were obtained. The fungus concerned proved to be the same as one which had some time previously been isolated from diseased raspberry-canes sent in from Motueka — Leptosphaeria Coniothyrium (Fcl.) ■ Sacc., causing cane-wilt disease. ■ '

This fungus belongs to the Ascomycetes, and possesses two stages in its life-cycle. The first or conidial stage, which has been named Coniothyrium Fuckelii Sacc., is the actively parasitic one, and the killing of canes is due to this. It consists of a simple flask-shaped receptacle (pycnidium) in which very many minute olive-coloured elliptical spores are produced. If one of these spores alights on any portion of a raspberry or blackberry plant, providing conditions are favourable, it germinates, producing a fine thread (germ-tube) which penetrates the epidermis. Once inside the tissues of the host this thread divides repeatedly, and quickly gives rise to a mass of threads (hyphae) which ramify in all directions.

Later in the season the perfect (ascigerous) stage of the fungus appears. Little is known as to what part the perfect stage plays in the reproduction of this fungus, but from a knowledge-of the behaviour of certain other related fungi it is probable that this ascus stage is the resting-stage, and is capable, of remaining viable for a considerable time, probably for three or four years. The perfect stage consists of a flaskshaped receptacle in which is borne a number of colourless sacs (asci), each containing eight four-celled ascospores. On germination an ascospore produces hyphae, which, behave in a similar manner to conidial hyphae, giving rise in turn to pycnidia. Generally conidia are produced only during the growing season, the ascigerous stage appearing only after the death of the plant or part of plant attacked.

EFFECT OF THE FUNGUS ON ITS HOSTS.

As the behaviour of this fungus varies somewhat according to the host'attacked, raspberry and blackberry, infection will be dealt with separately.

Raspberry (Rubus Idaeus L.) : Infection is more severe on this host than on blackberry. It is as a rule confined to fruiting - canes, but occurs also on young growing shoots, and, rarely, on fruiting-laterals (panicles). On canes,-the first appearance of the disease is marked by the leaves becoming reddish-brown in colour. This condition is rapidly followed by the wilting of portion or all of the canes. This wilting is due to the cutting-off of the water-supply owing to the presence of

masses of the fungous hyphae, which have penetrated to the conduction vessels (phloem and xylem) and there grown so rapidly as to completely block them up. It is thus seen that if the cane is attacked at or near its base the whole dies, whereas if infection were to occur near the apex of the cane only those parts above the point of attack would die, the portions below remaining quite healthy and maturing their fruit. Young growing shoots are killed outright, so that where infection is severe not only the present crop but the succeeding year's crop may be almost entirely destroyed. - ‘

Blackberry (Rubus fruticosus L.) : On the whole, blackberries are much less severely infected than are raspberries. Panicle-infection is by far the commonest stage, cane and leaf infection as a rule .being slight. On canes, infection, is not followed by sudden wilting, but where it has been severe, cankers appear which so. weaken the cane that it gradually dies. The first. appearance of the fungus on canes is made noticeable by the presence of minute white blisters. These are formed by the epidermis being separated from the tissues below on account of the dissolution of the latter by the hyphae of the fungus. These blisters gradually increase in size and coalesce, the. epidermis finally falling away, leaving minute irregular cankers in which. may be seen the fruiting bodies of the fungus.

. Flower and fruit infection is the common stage on blackberry, resulting in the nearly mature fruits, their pedicels, and the main axis of the panicle being killed back to the cane. Further than this the fungus rarely penetrates. This stage becomes very conspicuous where attack has been severe, as the whole panicle, including the fruits, turns a pale-chestnut colour. Infection may occur in the fruits themselves, in which case the hyphae ramify between and send branches into the drupelets, which are soon drained of their contents.' The fungus then works down the pedicel and up into other fruits, or else by attacking the lower portion of the peduncle may cause the death of most of the fruits by cutting off. the water and food supplies. The fact of this organism being the cause of the death of the panicles is often obscured by the presence of numerous saprophytic fungi which make their appearance on the dead fruits. Of these the most common is a species of Rhizopus, which forms a dense black mould over the whole surface of the fruit.

Leaf-infection, which is slight both on raspberries and blackberries, results in minute dead areas being formed, which fall away, leaving small perforations.

It is noticeable in the field that certain varieties of blackberry are more susceptible to attack than others. For example, one of the most common varieties in the North is a small semi-prostrate profusely fruiting one ; wherever this plant occurs in an infected area it is invariably badly attacked. A much less common variety is somewhat erect, largeleaved, vigorous-growing, and shy-fruiting ; this . plant is usually free from all traces of the fungus. . Infection in all degrees of severity may thus be seen in the field. Observations lead one to believe that this is due to the paucity or abundance of fruits, as invariably profuse-fruiting varieties are small and stunted in their growth, and suffer much more severely than the strong-growing, vigorous, shy-fruiting varieties.

CONTROL MEASURES.

As regards raspberries, American experience has shown that spraying for cane-wilt is of little avail. As the receptacles from which the spores are produced are buried in the tissues, they are ■ unaffected by any application of a spray mixture. Moreover, as spores are produced throughout the season repeated applications would be necessary to prevent spore infection. The disease being spread from fructifications embedded in drying canes, or canes which may have been dead for two or three seasons, it is obvious that control measures should lie rather in the removal‘of all infected canes so as to prevent spore-distribution. Therefore on the disease making its appearance all dead canes, whether newly killed or otherwise, should be removed and burned. Where possible, the whole infected plant, including the roots, should be destroyed. . Regarding the use of this fungus in the control of blackberry, when it first made its appearance on blackberries it was thought by many persons that the disease would exterminate this weed. Numbers of specimens were forwarded to this Laboratory with requests for information as to what had killed them and whether the organism could be used as a controllant. It will become evident to any one reading the above notes that the fungus is much more severe on raspberries than on blackberries. In fact, although many hundreds of infected blackberry-plants were observed on the trip referred to, not one was seen to have been killed outright. Where infection is severe practically all fruiting is prevented by the killing of the panicles, but cane-injury is so seldom seen as to render this phase of the disease entirely negligible.

CONCLUSION

From the foregoing it would appear that although the Leptosphaeria Coniothyrium fungus is the cause of . heavy loss in certain seasons .to raspberry-growers it causes little or no real damage to blackberryplants, though, where infection is severe, fruiting on certain varieties may be entirely prevented. The probability therefore is that if the fungus were introduced into any locality with the object of exterminating blackberry it would do little or no damage to the latter, but would spread to a host more favourable to its full development, such as raspberry, should this plant be growing in the neighbourhood. Furthermore, there is always the danger of its spreading to hosts other than the two under discussion. In America it has been recorded on roses and apples, and in Europe on a large number of hosts, among which may be mentioned barberry, elderberry, and willow.

Mountain Grassland Top-dressing. — Recent experiments in Switzerland on the top-dressing of mountain meadows and grazing-land with commercial fertilizers show that not only was the yield and plant-composition of the pasture greatly improved, but also its quality. In a case cited chemical analysis of the fodder gave the following percentages of crude protein : No manure, ro-g ; superphosphate, 17; superphosphate and potash salt, 15-1 ; superphosphate and liquid manure, 12 per cent. '■

Noxious Weeds. Recent declarations under .the Noxious Weeds Act by local bodies comprise the following : Cook County, Waikobu County, Waihi Borough, and Birkenhead Borough—Hawthorn; Clifton County—; Featherston Borough—Gorse, broom, hemlock, and fennel. .