Art. XXIV.—On the Naturalized Dodders and Broom-rapes of New Zealand.
[Read before the Wellington Philosophical Society, 6th July, 1887.]
While the subjects of this paper are of exceptional interest to the botanist, as parasitic flowering-plants which obtain the whole of their nourishment from the unfortunate plants which they use as hosts, they are often the cause of serious injury to the farmer, who may find himself compelled to witness the destruction of his crops of lucerne, clover, and flax at the period when he is expecting to reap his richest profits. It is chiefly from this economic point of view that it is intended to regard them in this paper.
The seed of the dodder germinates in the ground, and developes a reddish or purplish thread-like stem, destitute of leaves in all stages of its growth; when the stem comes in contact with a suitable host-plant, it developes minute suckers, which may be termed root-suckers, as they enable it to grasp the plant so firmly that it is impossible to detach it without killing both host and parasite. When suckers are developed the true root dies, having served its purpose; thread-like stems are produced with amazing rapidity, so that the host often appears to carry masses of threads; each short stem developes new suckers, and adjacent plants are speedily involved. If left unchecked the pest will soon destroy the entire crop, as its myriads of root-suckers rapidly extract the greater portion of the assimilated nutriment of the host, which gradually becomes impoverished and dies.
After a longer or shorter period, flowers are produced, sometimes in vast profusion. They are developed in small, globose fascicles, and give rather a pretty appearance to the unfortunate hosts. The corolla is entire, consisting of five petals, alternating with five stamens. At the base of each stamen is a curious scale, which may be lacerated or fringed, or ciliated, and which affords a useful means of distinguishing the species.
Dodders belong to the Convolvulaceœ, but differ from their congeners in being destitute of cotyledons, and ultimately parasitic.
The Lucerne Dodder.
Cuscuta hassiaca, Pfeiffer.
This species was first observed on lucerne in the Canterbury District. The seed had been imported from California. In some parts of the field it was most abundant, and had extended its operations to the sorrel, knot-grass, and other weeds which grew sparingly amongst the crop. The following season the
lucerne was very weak and thin, but there was very little dodder; subsequently the crop died out. The lucerne dodder has been observed in several localities in the Canterbury District.
The Clover Dodder.
Cuscuta epithymum, L., var. trifolii.
This was first observed on red clover in the Waikato, in 1870, and still makes its appearance in clover fields in that district, but does not cause so great an amount of damage as in Europe. It has also been noticed in Southland, and in all intermediate districts except Taranaki and Westland. In Southland its ravages are of a serious character, though happily limited to a small area. It is the most destructive species which the agriculturist has to encounter, and is sometimes designated “Devil's guts.”
The Flax Dodder.
Cuscuta epilinum, Weibe.
I have been informed that this species was observed in the Canterbury District about three years ago, but have seen no specimens.
Dodders are introduced into cultivation with the seeds of lucerne, clover, flax, and other plants. As the seeds of dodder may easily be recognised, a careful examination of the clover, lucerne, or flax seed by a practised eye will easily determine its presence or absence. It is only fair to say that English-grown seeds are remarkably free from dodder. Seeds from the European Continent usually contain a large percentage. I never saw a sample of Russian flax that was free from dodder, and the same must be said of Californian alfalfa.
When dodder is once established it is not easy to eradicate it. I only know one plan likely to be successful. Mow the affected portions close to the ground, and burn it at once, taking care to char the stumps so as to destroy vitality.
Two species of Cuscuta are indigenous to New Zealand: one, C. densiflora, is rare, being found only in Marlborough and Otago, in both cases growing on Fuchsia excorticata.
The other, C. novœ-zealandiœ, grows on small herbaceous plants, grasses, fern, and low-growing shrubs. Like its imported congeners it may become a pest to the farmer, as it occasionally kills small patches of grass by exhaustion.
These are erect parasitic plants with succulent simple stems, the upper portions being crowded with sessile flowers; the whole plant being of a lurid brown hue. They are leafless,
although a few brown scales are found at the lower part of the stem, and a bract at the base of each flower. The seeds are very small, and produced in great abundance; when germinating they do not rise above the soil, but at once attack the root-fibres of a suitable host plant, and soon become parasitic on the root. When shrubs are attacked the Orobanche may be perennial, as the shrub is but slightly affected in proportion to its bulk; but herbaceous plants are speedily exhausted, even red clover being unable to sustain the constant drain upon its resources beyond the second year.
Orobanche minor, Sutt.
This appeared on red clover, near Cambridge, Waikato, in 1885, and was seen last year in much greater quantity. I observed from three to six stems, 9 to 12 inches in height, springing from a single root of clover, so that it is easy to form an idea of its exhausting effects.
In Europe it is of less frequent occurrence than the clover dodder, and its ravages are less dreaded, but in some seasons they attain serious proportions.
Its appearance in the Colony is to be deeply regretted.
Orobanche picridis, F. Schultze.
This species was first observed at Whangarei, about 1867, growing on the roots of cat's-ear (Hypochœris radicata); curiously enough it is more robust in this Colony than in Europe, and varies in height from a few inches to upwards of 2 feet, and occurs in great abundance. It is often the most prominent plant on grass lands about Whangarei, and begins to flower in October.
Unlike the preceding species, its effects are decidedly beneficial, as it invariably destroys the useless cat's-ear, and allows its place to be occupied by grasses or clovers.
Orobanche picridis has been observed in several parts of the Auckland District and in Nelson, but not, so far as I am aware, in other parts of the Colony; it is nowhere so abundant as in the Whangarei District.
It is a singular fact that in Europe this species does not attack the cat's-ear, but is restricted to Picris hieracioides. The Picris is not uncommon in the north of New Zealand, but appears to be exempt from the attacks of the parasite, which confines its attention to the unhappy cat's-ear.
There is reason to think that this species might be used as an esculent, if cooked and served in the same way as asparagus. Two species are eaten in this way in Central Asia and Afghanistan.
Although the seed is fine, almost dust-like, its presence amongst other seeds may be detected by a careful observer, although not apparent to the untrained eye. When any species of Orobanche becomes established, I believe it might be easily destroyed by prompt mowing close to the soil, if the operation be performed before the seed-vessels arrive at maturity; a careful watch should be kept, and any new shoots that make their appearance cut down at once.
Probably Orobanche ramosa, the broom-rape of the hemp, would withstand this process, but I know of no other species possessing any great power of resistance.
I may close this short paper by advising agriculturists to purchase their seeds from seedsmen who give a guarantee of the purity of their stock, as in the case of some English houses. At present, comparatively few seeds are grown in New Zealand, but I am sure that any qualified persons, taking up the business of seed-growing with proper means and appliances, would obtain a fair annual return without other protection than the cost of packing, freight and insurance, especially if prepared to give a guarantee of the purity and vitality of their seeds.