PHORMIUM TENAX.

New Zealand Journal of Agriculture, Volume XXIII, Issue 5, 21 November 1921, Page 298

PHORMIUM TENAX.

THE NEW ZEALAND FIBRE INDUSTRY.

ESMOND H. ATKINSON,

Biological Laboratory, Wellington.

VI. DISEASES AND INSECT PESTS.

YELLOW-LEAF.

The most important of’ the diseases which have hitherto attacked phormium is that now generally known as yellow-leaf.” A plant attacked by this disease first becomes conspicuous from the brightyellow colour of some or all of its leaves. It is true that yellow leaves may be seen on healthy plants, but in this case only the oldest leaves at the outside of' the fan are so coloured, yellow being the colour first assumed in the course of the natural decay of an old leaf. In a diseased plant young and old leaves are equally liable to a change in colour. There is, however, no sharp line of distinction as regards general appearance between healthy plants and those in the incipient stage of yellow-leaf ; in the later stages the leaves become brown, and shrivel up, and a closer examination will show in many cases that a number or all of the roots are dead and the rootstock decayed. A preliminary study of the disease, extending over a period of fourteen months, has been carried out .by Dr. L. Cockayne, F.R.S. The time covered by the investigation , was admittedly not long enough for a complete study to be made, so that some .of the conclusions arrived at . may need modification. A number of plants, both healthy and otherwise, were marked, and the progress of the disease in them noted. The conclusion was arrived at that yellow-leaf is an infectious disease, probably due to fungal or bacterial attack, which is . aggravated by the presence of stagnant water around the plant, and which would to a large extent be remedied by draining the swamps better in winter and keeping them moister in summer. From the marked plants above referred to it was seen that recovery, ■ even in badly affected cases, was not infrequent. Improvement was most noticeable in autumn and winter, followed by deterioration in spring and summer, the latter season being the period when the disease is at its worst. The extension of the rootstock of a diseased plant on to more favourable ground is almost always followed by an improvement in the general health of the plant.* Very few plants are killed outright, and those that have recovered are capable of producing leaf in no way inferior to that from plants that have remained uniformly healthy. The frequent occurrence of plants which, although surrounded by diseased ones, remained healthy over a long period suggests that certain races are immune, and the cultivation of such forms would undoubtedly be One of the most effective ways of dealing with yellow-leaf. A fungus from the roots of affected plants, in the tissues of which it was fructifying, has been isolated and cultured by Mr. R. Waters, of the Biological Laboratory, and was identified at Kew as a species

*Mr. A. Seifert, however, writes as follows’: “I took some phormium roots suffering rather badly with the disease from the. Makeru’a area, and planted them in my garden at Palmerston North. For a while the plants appeared to recover, but. later took the disease again and died. Mr. R. T. Bell had the same experience.’'

of Ramularia. Inoculations made with this fungus on the . roots of plants. that, as far as external examination could show, ‘ were in. all respects healthy resulted in . the formation of lesions which proved to contain Ramularia. Although no inoculation experiments have. yet resulted in the production of the actual yellow-leaf condition, a number of such experiments, chiefly upon young plants, have been instituted during the present year by Mr. Waters, and it is more than. possible that with, the approach of the seasons during which yellow-leaf is most conspicuous in commercial areas the disease may manifest itself in the inoculated, plants.

FUNGI IN GENERAL.

' Although phormium has been recorded as a host plant for a number of fungi, up to the present but little systematic work has been carried out On them, and their distribution in milling - areas and relative economic importance are not known. During the present year, however, a certain amount of preliminary work in this direction has been done by Mr. G. H. Cunningham (Biological Laboratory) and the writer in preparation for an account of the fungous diseases of phormium which it is intended to publish ■ shortly. The following are some of the more important of these fungi: . Heterosporium sp. —This causes large irregular sooty blotches on the under-surface of the leaves, the fibre beneath these blotches becoming red in colour and quite rotten. Where the leaf is badly affected it is therefore rendered quite useless for milling, and even where the attack is slight the fibre will lose many points in grading owing to its bad colour. • • •

Fusarium sp.—Causes large elliptical whitish areas of dead tissue on young leaves, ruining them for milling purposes. Phaeoseptoriasp. —Is the cause, of more or less severe leaf-spotting, accompanied by destruction of the fibre. . Septoria sp.—Causes longitudinal purple stripes on the leaf, ruining the fibre and finally killing the whole leaf.

INSECT PESTS.

As regards insect enemies, phormium affords a good example of how the “ balance of nature ” may be upset when the advent of man in a new country brings about profound changes in the conditions previously existing there. Although all the insects which now attack the plant are indigenous, it was not until the early years of the present century, when the draining of swamps first entitled phormium to a place among the agricultural crops of New Zealand, that their depredations began to assume a serious aspect.

Leaf-eating Caterpillars.

The larvae of three moths — Xanthorhoe praefectata Walk., Melanchra steropastis Meyr., and Oeceticus omnivorus Fereday — responsible for most of the damage done to the leaves, but Oeceticus at present does very little harm compared with the . other two, .of which Xanthorhoe is the more important. The name “ flax-grub ” is generally applied to the larvae of Xanthorhoe, and Melanchra. . Xanthorhoe praefectata.- —The larvae of Xanthorhoe attack phormium on the under-surface of the leaves, which they gouge out in long narrow

grooves from J in. to J in. or more in breadth and up to several inches in length. In depth the grooves extend nearly to the upper epidermis, the fibre being removed and its continuity thus destroyed. A • badly attacked leaf is quite useless for milling purposes. The caterpillars feed at night, hiding in the daytime in the tube-like rolled dead leaves. When they have reached their full size (about f in.) they enter the soil to a depth of about 2 in., where they pupate. The moths are nocturnal in their habits, and are very pretty, being pure-white in ground-colour with delicate grey markings on the wings, which measure about i| in. in expanse in the female and slightly less in the male. The species is widely distributed throughout New Zealand, and is found wherever phormium occurs. The seasonal history of Xanthorhoe is somewhat complicated, and need not be fully gone into here ; it is enough to say that there are two distinct generations, a main and an overlapping one, and that each produces two broods in the . course of the year. The maximum damage done by the larvae of the main generation is from the end of July to the middle of September, and again from the middle of December to the middle of January. In the case of the overlapping generation the maximum damage is done in April and May, and again in October and November, but there is no time ,of the year (with the exception of the early part of February) when phormium is . free from larval attack. These periods are only broadly indicative of the seasonal history of the insect, and it must be realized that they are subject to many local and climatic conditions which may considerably modify them.*

Melanchra steropastis. — The damage done by Melanchra differs widely from that caused by Xanthorhoe. The larvae attack the leaf along its edge, cutting a deep notch cleanly out, and leaving none of the tissue of the upper surface as is the case with Xanthorhoe. The lifecycle of Melanchra has not yet been fully worked out, but the larvae are known to shelter in the dead leaf-tubes in the same way as those of Xanthorhoe. As a rule , they pupate in the ground, though pupae have been found in the rolled-up leaves. Melanchra has a wide distribution in New Zealand, but, except in Southland, is of quite secondary importance compared with Xanthorhoe.

Oeceticus omnivorus.— This is the widely distributed and wellknown “ bag-moth,” the larvae being protected by strong silken cases which may be seen on a great variety of plants, hanging from the leaves or stems. The larvae are active through most of the year, and sometimes do a certain amount of damage to phormium, though they are never abundant. Large patches are eaten from the lower surface of the leaf, often extending through to the upper surface and forming more or less circular holes in the blades.

Control Methods.

Three main controlling influences occur in the case of Xanthorhoe—namely, floods, predaceous and parasitic insects, and insectivorous birds.

Flooding.— lt has already been pointed out that Xanthorhoe was of little importance before draining was introduced, and it has been

noticed that there has been little damage done by the pest, even in those districts • most susceptible to it, after the occurrence of floods. The artificial flooding of infested areas naturally suggests itself. Flooding during larval attacks is not likely to be as successful as during pupation, owing to the probability of . the larvae crawling up the leaf; tubes and thus escaping from the water. . Flooding during the period of pupation would no doubt be most effective, but owing to the complexity of the seasonal history of Xanthorhoe it is not always easy to select the most suitable time for the operation. There are, however, two periods(l) from the latter part of July to the end of September, and (2) from December to February— the greatest number of pupae from the main and overlapping generations are in the ground simultaneously, and it is at these times (subject, of. course, to seasonal variations) that the floodings would te most effective.

Predaceous and Parasitic Insects. The most important of the predaceous insects is a small hover-fly (Syrphus Ortas'), the larvae of which prey (though not exclusively) upon those of Xanthorhoe. Of the parasitic insectsthat is, those which lay their eggs in the bodies of the larvae, which are later killed by - the developing parasite—several species are known, the most important being two ichneumon - wasps {lchneumon sp. and Paniscus pro ductus). The former of these confines itself entirely to Xanthorhoe, and destroys the larvae early in their life before much damage has been done, while the latter is slower and attacks other species as well. The usefulness of Ichneumon sp. is lessened, however, by its being liable to the attacks of a secondary parasite. It is not considered that any of these insects will do much more than keep Xanthorhoe in check, the fact that . the moth, its foodplant, and its parasites are all indigenous pointing to the probability of a balance existing between them which could only be upset by the complete elimination of one or other of the controlling factors.

Insectivorous Birds. — The part played by birds in the. control of the flax-grubs is not well known. Certainly such birds as pukekos, wekas, &c., have disappeared in many areas where they were once common, the draining of swamps, followed by fires and the increase of rats, weasels, &c., being no doubt the cause of this. It is probable that in the drier parts of the virgin swamps the larvae and pupae of Xanthorhoe and Melanchra were eaten by some of these birds, but the relation between the disappearance of the latter and the increase of the former is a matter of conjecture. The large numbers of starlings which are now often seen in phormium areas probably play a part in the control of flax-grubs, but here again exact data are wanting.

Other Larva.

The larvae of a species of Porina, a large moth, have been recorded as damaging and destroying individual fans of phormium by boring into the bases of the leaves, but such damage is not frequent enough to be of any economic importance.

Scale Insects.

Several species of scale insects occur on the leaves of phormium, the following being those most commonly seen : Mytilaspis cordylinidis Mask., Leucaspis stricta Mask., Dactylopius calceolariae Mask., Coelostoma wairoense Mask., Poliaspis media Mask.

The last-named is often very abundant, covering the whole undersurface of the leaf with a white and woolly mass. Apart from the damage done by the scale insects themselves, many species exude “ honey-dew,” upon which a black sooty mould grows. The scale insect and fungus together have the effect of softening the leaves and greatly reducing their value.

( Concluded.)

* Those interested will find a full account of the life-history of Xanthorhoe, by Mr. D. Miller, Entomologist, Biological Laboratory, .in the Journal' for October, 1918, the above being simply an abstract from this and other papers by the same writer. ' •