Marssonina salicicola (Bres.) P. Magn. on Salix.
The most serious disease is caused by the fungus Marssonina, which, though it occurs on both species of Salix, is chiefly found on S. babylonica. The parasite attacks the tree with great vigour, causing considerable injury and loss, and throughout the Nelson district practically no tree of this species has escaped infection. Many have lost branches and are dying.
(In April, 1925, since this paper was in print, specimens of the disease on Salix babylonica were collected in Ward, Marlborough.)
The disease was also noticed by the writer in a mild form on weeping-willows at Mitcham, South Australia, in January, 1924, and at Aldgate, South Australia, in January, 1925.
Pathological Changes in Salix babylonica.—The effect of the parasite on this host is most, noticeable. In trees which are only mildly attacked the leaves become covered, principally upon the lower surface, with pale reddish-brown spots which have a purple tinge when fresh; a few brown cankers may be seen on the twigs. When badly diseased the tree almost completely loses its “weeping” habit; the leafy branches, instead of being long and pendulous, are much shorter and tend to grow more upright, until, at a distance, the tree might almost be mistaken for one of the upright varieties (Plate 8).
This effect on the host is due to the shortening of the internodes in the diseased “weeping” branches, the average distance between any adjoining two of the ten nearest the tip in a number of diseased branches being 0.8 cm., as compared with 2.4 cm. in an equal number of healthy specimens. This leads to an increase in the number of twigs, which are produced closer
together and are shorter. Weeping branches may be 1 m., 1.5 m. long, or more, while on badly attacked trees there may be no true weeping branches, but in their place a number of small twigs, a few to 30 cm. long, bearing leaves at fairly crowded nodes. The twigs become distorted with cankers, and tend to curl upwards.
When the twig is first infected a small black elliptical spot appears upon it, which rapidly spreads (Plate 9, fig. 1). Upon this spot whitish acervuli are produced beneath the epidermis of the host, which they raise and finally rupture, setting free the hyaline two-celled spores. As the infected tissue of the twig dies, it dries, cracks, and is broken off by another layer of spores developing below. In this manner several layers may crack off in succession. The canker spreads along and round the twig, and may join other cankered areas, until a large part is involved (Plate 9, fig. 2). The infected leaves become covered with spots, described on pages 60 and 61 (see Plate 9, fig. 3). On badly diseased twigs the leaves, all of which are often infected to some extent, are usually very much smaller than those on healthy branches, averaging 5.9 cm. in length as compared with 12.5 cm. Instead of hanging pendulous against the twigs, they tend to curl and grow more at right angles to it; this is often due to infection of the petiole, causing it to blacken and shrivel. The midrib may also be infected, when elliptical or elongated black spots appear similar to those on the young twigs and petioles. The stipules are often attacked as the leaves unfold, when they drop off at a very early age, while normally they remain at the base of the leaf long after it is mature. Eventually the leaves become yellowish, and fall prematurely. It was observed in early spring that the young leaves were infected immediately on the opening of the leaf-buds (Plate 9, fig. 4), infection probably being caused by spores from cankers upon the living twigs of the tree, from bud-scales (Plate 9, fig. 5), and from dead leaves which had been diseased the season before and remained on the ground. Examination of these during the winter and early spring showed numerous acervuli still producing a large quantity of spores.
Spots similar to those on the leaves, bearing typical acervuli and spores, were also found on the catkins of the weeping-willow.
Pathological Changes in Salix fragilis.—The same fungus as the above occurs on the crack-willow, but much more rarely, and then with less disastrous effect. Blackened areas may be formed along the young twigs, and cankers are sometimes produced on the older ones; but the main effect is seen on the leaves, which become thickly covered on the upper surface with dark-brown or black circular or irregular spots, which differ from those on the weeping-willow by being smaller and more numerous. On the lower surface of the leaves of the crack-willow faint small reddish-brown irregular spots may sometimes be seen; these at times become confluent. The spots are more numerous on the upper surface of the leaf than on the lower. They produce acervuli and spores similar to those on the weeping-willow.