BROOM OR CLOVER RAPE (OROBANCHE MINOR).

New Zealand Journal of Agriculture, Volume VI, Issue 2, 15 February 1913, Page 147

BROOM OR CLOVER RAPE (OROBANCHE MINOR).

A. Hughes.

There are several species of broom-rapes, but the only one naturalized in New Zealand is the lesser or clover broom-rape, Orobanche minor. As I have had several inquiries about it, and have seen it in paddocks of clover in the North Canterbury district, it may interest farmers to know something about it, or enough to identify it if found in their fields. This parasitic plant is destitute of greencolouring matter, and so, unlike green plants in general, is impotent to manufacture sugar and starch from carbon-dioxide and water to meet its food requirements. Broom-rape is wholly a parasite, and becomes a robber of other plant-life. The seed is sometimes sown accidentally with clover, and when it germinates becomes attached to the roots of the crop. To get command of the food in the plant to be robbed the parasite has a special tool called a sucker, which is inserted into the root of the host plant. Owing to this peculiar mode of attack it is commonly called a root parasite. Once the robber gets command of the food resources of a plant it immediately turns them to good account, for it transforms them into a corpulent underground body called the tuber, ’ from which proceed the roots, or suckers, that attach themselves to the clover. The stem is erect and fleshy, bearing small colourless pointed scales instead of leaves. It is from 8 in. to 12 in. high, and is at first a yellowish-brown colour, turning darker in shade as it grows older. . At the free end it bears flowers of a dirty- colour. These- produce oblong capsules full of numerous minute seeds of a dark colour. To prevent the attack of the broom-rape the seed of the red clover must be absolutely free from the seed of the parasite. If any spot in the clover-field shows signs of the parasite the plants there should at once be dug out and burnt, to prevent seeding. On no account should broom-rape plants be allowed to seed.

Note by

A. H. Cockayne.

The broom-rape (Orobanche minor) is, in Europe, looked upon as a very' serious clover-parasite, and is said to cause wholesale destruction in fields that are devoted exclusively to clover-growing. It is especially detrimental in those clover crops that are grown for seed.

Where present it is said to seriously diminish the yield of seed and to lower its quality.

In New Zealand it has been naturalized for many years past, and in certain localities is excessively abundant. There are fields in the Auckland District where nearly every' clover-plant appears to be affected, and the whole ground is thickly studded in the early summer with the thick erect brownish-red asparagus-like shoots of the parasite. After the broom-rape comes into flower the stems, which hitherto have been fleshy, dry up, and all the aerial portions. die away, fresh shoots being again developed in the following, year. It is very peculiar that no complaints have ever been received that the broom-rape injures the vitality of the clover-plants .to any appreciable extent. I have specially studied this parasite in the Auckland District, where it appears to be spreading rapidly, and is now everywhere to be found, but have ' not noticed any appreciable difference in the vitality of affected and unaffected plants. When it is considered that the whole of the "'food-supply necessary to produce the broom-rape, which is quite a large -plant compared with the size of the host, is derived from the plant on which it is parasitic, it would appear that the drain on the affected plants should result in a very serious diminution in .I their vitality. In general, however, affected clover-plants appear tojbe quite vigorous, and in no cases have plants been seen whose death could be attributed to this parasite. Nevertheless it seems incredible that clover-plants can be made to provide all the food necessary for the development of the broom-rape without considerably impairing their yield. Sweet-peas attacked by broom-rape have been frequently received, and in all- these cases a serious diminution in growth had resulted from the attack. Again, last year in the Wairarapa a crop of affected parsnips came under my notice, and the development of the roots of all the affected ■ plants was seriously interfered with, finally resulting in considerable distortion. From the above it can be seen that the question of the seriousness of broom-rape in New Zealand is a debatable one, and the opinions of farmers who may have had experience of this parasite in their pastures would be especially valued. No attempt is made in New Zealand to prevent the seeding of this weed parasite—a matter of great importance if it is shown that it really causes any harm. Distribution in New Zealand. Broom-rape is exceedingly abundant throughout the whole of the Auckland Province. It is also common in Taranaki, but further south is comparatively rare. In Canterbury it has been naturalized for many years in odd localities, but does not appear to be spreading.

No specimens from Otago or Southland have as yet been received by me. •• ' , Host Plants in New Zealand. Broom-rape 'has ■ been recorded on the following plants in New Zealand: Red clover, alsyke, white clover, birdsf trefoil, trefoil, burr clover, sweet-peas, parsnips, hawkweed, dandelion, and cats-ear. Seed-dispersal. In many agricultural books it is suggested that broom-rape is regularly distributed in clover-seeds. This is very unlikely, as the seed is so small that it is almost impossible for any to remain in clover-seed even if this seed is very imperfectly dressed. Perceval records finding broom-rape in clover-seed, but the seed-station at Zurich during thirty years never noted it in any commercial ’ samples, iln our own seed-testing no broom-rape has ever been found in any samples that have been submitted for examination. The seed lis exceedingly minute,, being amongst the smallest of any seeds belonging to the flowering-plants. In consequence, it can be easily transported great distances by the wind. I hold that animals must play a very important part in the disposal of broom-rape, as the seeds can easily be carried about in the wool and hair of animals that graze on areas where this plant is at all abundant.