BLACK-ROT (PHYSALOSPORA CYDONIAE ARNAUD).*

New Zealand Journal of Agriculture, Volume XXVII, Issue 2, 20 August 1923, Page 95

BLACK-ROT (PHYSALOSPORA CYDONIAE ARNAUD).*

A FUNGOUS DISEASE OF APPLE, PEAR, AND QUINCE.

G. H. CUNNINGHAM,

Biological Laboratory, Wellington.

This disease has been, recorded from North America, Australia, and Europe. In certain parts of North America it is considered as a serious parasite of the apple, second in importance only to black-spot, but in Europe and New Zealand it is comparatively a minor disease. In New Zealand it is a common canker of apple and pear, and in North America it has been recorded on many additional hosts, among which may be enumerated elder, hawthorn, lilac, mulberry,. oak, and rose. In our earlier reports cankers caused by it were attributed to European canker, Nectria galligena Bres., but fortunately this fungus is not known to occur in New Zealand.

APPEARANCE AND EFFECT ON THE HOSTS.

Black-rot infects shoots and branches, fruits, and leaves. On the laterals and branches, but more frequently on the latter, it forms definite cankers. These at first appear as small elliptical areas, noticeable on. account of their colour differing from that of the healthy bark. Shortly after its formation the cankered area becomes separated from the normal bark by a crevice. Then the diseased bark shrinks so that the canker appears slightly sunken (Fig. 1). Usually the healthy bark at the margin of the canker becomes slightly raised, due to the development of corky tissue in this region. The diseased area then

becomes lighter in colour, and consequently more conspicuous. Growth of the fungus proceeds in a radiate manner, so that invariably in old cankers there are present numerous crevices, arranged in zones, which have been formed as successive layers around the canker. These render the canker conspicuous . and readily discernible ; further, they serve as quite a good character to separate black-rot cankers from those formed by other diseases (Fig. 2). Cankers may continue

to grow for several seasons, or until the branch is ring-barked, when the portions above die. Death is often preceded by a gradual, yellowing of the leaves, which usually fall prematurely. The bark sometimes falls away from old cankers and exposes the wood.

Sections through a canker on a medium-sized limb show that the sap-wood is discoloured for some distance beyond the visible point of infection, and microscopic examination reveals the presence of hyphae of the fungus in this discoloured area. On large limbs girdling may not occur, the fungus in such a case forming large irregular cankers,

which may attain a length of i ft. or more. Some time after . a canker has been formed fructifications of the fungus appear on the surface of the killed area.

Spots become noticeable on the leaves shortly after they emerge from the bud, and infection may occur during the whole of the growing season should conditions become favourable. At first the spots are minute, circular, scattered, and dark purple in colour. They soon increase in size,- growth proceeding in a radiate manner, so that the centre portion (the original spot) may appear surrounded by definite zones. Later this central portion changes to greyish-brown, and as the surrounding zones are darker, these spots present a characteristic appearance, which has led to the name “ frog-eye ” being applied to them. Finally, spots may lose their circular outline and become lobed and irregular in shape. In cases of severe infection the spots may become so numerous as to coalesce, forming irregular dead areas on the leaf. Severe infection may be followed by defoliation.

Fruit-infection is followed by the appearance on the surface of small circular brown areas ; these rapidly increase in size until the whole fruit becomes rotted. As these areas enlarge, zoning may occur, as in the case of leaf-infection, but this is not a common manifestation of the disease. Infected fruits do not become soft, but remain firm and spongy. Finally, the colour changes to jet-black, and the fruit gradually shrivels and becomes mummified (Fig. 3).

ECONOMIC IMPORTANCE.

Although in certain parts of the United States black-rot is a serious disease, causing an annual loss of several hundred thousand dollars, in New Zealand it is of minor importance, its chief damage being due to the cankers it forms on the branches of apple and pear trees. On leaves its effects with us are so slight as to be negligible, and on fruits it has little effect, as it appears to be confined to those which have been injured by codlin-moth or damaged during picking or packing. During the recent fireblight campaign in the Auckland District many, hundreds of cankers were forwarded to this Laboratory, and in nearly every instance these were found to be caused by fireblight, black-rot, or macrophoma-canker. In most cases it was found that the source of infection of black-rot was through some bark-injury, such as is caused by branches rubbing together, or abrasions caused by woolly aphis.

LIFE-HISTORY OF THE CAUSATIVE ORGANISM.

Black-rot is caused by the fungus Physalospora Cydoniae Arnaud,* an ascomycete having two spore stages in its life-cycle. The first or pycnidial stage, commonly known as Spha&ropsis malorum, is the parasite, the second or ascomycetous stage being saprophytic, as it appears in the dead bark of old cankers. In North America this stage

appears to be rare, but in New Zealand it is quite common in cankers that are more than two years old. This fungus is a wound parasite, for it is apparently able to infect fruits and branches only through some injury of the epidermis or bark ; but under certain conditions it is a true parasite, for in America it has frequently been demonstrated, by experiments in which spores (conidia) have been sprayed on to leaves, that the hyphae of the fungus are able to penetrate directly through the epidermis into the underlying tissues.

Spores discharged in the early spring from fructifications embedded in the dead bark of cankers and the epidermis of mummified fruits are carried by wind or other agency to leaves and injured surfaces of branches, where if moisture conditions are suitable they germinate and produce a germ-tube (hypha). This penetrates into the tissues and there branches repeatedly, the hyphae growing between the cells and absorbing from them the food substances necessary for their continued existence. As a result the host cells are killed, and turn brown. At this stage the hyphae are colourless, but after a time they become dark-coloured, and it is these black masses of hyphae . that give the characteristic colour to infected fruits. After a time masses of hyphae immediately beneath the dead epidermis become aggregated into little knots, which eventually develop into . spore - bearing receptacles or pycnidia. These are flask-shaped or globose (Fig. 5), and contain numerous one-celled olive-coloured spores (Fig. 5, d, conidiaf), which are borne on slender stalks (Fig. 5, c, conidiophores) produced from the inner surfaces of the lower portion of the pycnidia. The apices of the pycnidia at maturity pierce the epidermis ; each is perforated by a small opening (ostiolum) through which the spores escape. The spores are embedded in mucilage, and as this readily absorbs moisture the spores are forced out through the opening by the swelling of the mucilage, when they appear on the surface in olive-coloured tendrils. The mucilage is dissolved away by rain, and the spores are released, when they may be washed by rain on to lower leaves and branches, or else carried by wind and insects to adjoining trees.

If a canker lives for more than one season, and the killed bark persists, the second or ascigerous form may appear. This consists of a flask-shaped perithecium containing numerous asci in which colourless one-celled spores are borne (Fig. 6).

These spores may be discharged on to the surface and carried to 'adjacent trees, where they are probably able to infect leaves and branches, and produce hyphae, which in turn give rise to pycnidia.

whereas Hesler (1913), after carrying out similar experiments with ascospores of Physalospora Cydoniae Arnaud, also obtained Sphaevopsis malorum. Hesler was able to infect apple-branches with ascospore material and produce typical blackrot cankers. On account of this, and the fact that the New Zealand ascigerous material agrees closely with his descriptions and figures, the name . he used has been adopted. . Our species obviously belongs to the Pleosporaceae, and not to the Melogrammataceae, so that Shear’s claim would appear to be untenable in so far as the New Zealand organism is concerned.

Conidia are apparently able to remain viable for a considerable time, for in this Laboratory the writer has been able to.infect apple-fruits with conidia taken from pear-cankers which have been kept in the herbarium for over twelve months.

REMEDIAL TREATMENT.

As the mycelium of this fungus may remain alive in a canker for several seasons, and during the growing-period is capable of producing fructifications (pycnidia) bearing spores, it would appear that these cankers are the means by which the fungus is able to. carry over from season to season. This is borne out by American experience, for there it has been observed that, early in the season, in the vicinity of viable cankers, leaf-inf ection commences shortly after the leaves emerge from the buds. Furthermore, American workers have recorded the fact that infection may commence from mummified black-rot fruits, remaining o'n the trees, for they have frequently observed leaf-infection to commence in the vicinity of these fruits. On spots on living leaves spore-production is so uncommon that it is probable leaf-infection occurs throughout the season from spores produced from pycnidia in cankers and mummified fruits. Fructifications are commonly produced on leaves that have fallen to. the ground, so fallen leaves that have, escaped desiccation during the winter months are in all probability a source of infection the following spring.

Doubtless in New Zealand black-rot is held in check somewhat by spray treatment, but not entirely, for it appears to be about equally common in sprayed and unsprayed orchards. Such being the case, the eradication of the sources of infection would appear to be the only effective treatment that can be recommended. These sources are (a) cankers on the branches, (b) mummified fruits remaining on the trees and lying on the surface of the ground, and probably (c) infected leaves which have escaped desiccation during the winter months. The following treatment is therefore suggested : —

(i.) Cut out cankered branches, cutting some 3 in. below the visible point of infection.

(2.) Remove and destroy any black-rot mummies lying on the ground or hanging on the trees.

(3.) Plough in late autumn, after the leaves have -fallen, and with a spade turn under portions beneath trees „ which have been left untouched by the plough.

The writer does not suggest that any treatment of cankers be practised, for at best this is difficult and involves a great amount of work ; furthermore, as the hyphae of the fungus spread in the sap-wood some distance beyond the visible point of infection, control at best would be uncertain. As many wounds are caused by branches rubbing together, trees should be pruned in such a manner as to prevent this. Finally, all wounded surfaces should, as soon as made, be coated with coal-tar.

SUMMARY.

(1.) Black-rot forms cankers, on the branches, spots on the leaves, and causes rotting of fruits.

(2.) In New Zealand it is confined to the apple, pear, and quince, but in North America has been recorded on numerous other hosts.

(3.) On branches and fruits it behaves as a wound parasite, as it is apparently able to infect these only, through some abrasion in the bark or epidermis ; on leaves it behaves as a true parasite, infecting them directly through the epidermis.

(4.) Black-rot is caused by the fungus Physalospora Cydoniae Arnaud.

(5.) It overwinters by means of resting mycelium in cankers and mummified fruits, and probably in fallen leaves.

(6.) Remedial treatment consists in the removal of the sources of infection.

(7.) In New Zealand its economic importance is comparatively slight, but in certain States in North America it ranks second in importance only to black-spot as a disease of the apple.

literature consulted.

Arnaud, G. (1912) : Notes Phytopathologiques. Ann. L’ecole. Nat. Agr. Montpellier, vol. 12, p. 9.

Brooks, C., and De Meritt, Miss M. (1912)’: Apple Leaf-spot. Phytopathology, vol. 2, p. 181.

Hesler, L. R. (1913): Physalospora Cydoniae. Ibid., vol. 3, p. 290

Lewis, C. E. (1912) : Inoculation Experiments with Fungi associated with Apple Leaf-spot and Canker. Ibid., vol. 2, p. 49.

Scott, W. M., and Rorer, J. B. (1908) : Apple Leaf-spot caused by Sphaeropsis . malorum. U.S. Dept. Agr. Bull. 121, p. 47.

Shear, C. L. (1910): Life-history of Melanops Quercuum (Schw.) Rehm, forma Vitis Sacc. Science, n.s., vol. 31, p. 748. (1914): Life-history of Sphaeropsis malorum Berk. Phytopathology, vol. 4, p. 48.

* Synonyms : Sphaeropsis malorum Berk. ; black-rot canker; black-rot leaf-spot; body-blight; body-canker frog-eye; fruit-spot; New York appletree canker ; ring-rot.

* Considerable doubt exists in mycological literature as to the proper name that should be applied to this species. For Shear (1914) claimed that ascospores of Melanops Quevcuum Rehm, forma Fhh's Sacc. produced in pure cultures pycnidia and spores morphologically identical with Sphaeropsis malovum Berk.,

These spores are more correctly termed “ pycnidiospores ” or “ pycnospores,” as they are borne in pycnidia ; but to save unnecessary use of terms they will in this and subsequent articles be termed “ conidia.”