Transactions of RHASS Volume 1940 - Page 067
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| Year | 1940 |
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122 AGRICULTURAL RESEARCH IN sCOTLANn IN 1939. contained in the berry or plum. Nevertheless, abnormal seedlings occur with greater frequency among the offspring of diseased parents than among those of their healthy counter- parts, and for this reason it is undesirable to use virus-affected plants as parent stock for breeding. The possibilities of breeding potato varieties immune from or resistant to virus diseases are dependent upon there being such qualities already present among cultivated potatoes or among those ‘wild’ species of potato which are capable of being used as parents in conjunction with the cultivated types. The primary search for varieties having these qualities has been made by an examination in greenhouse and field Of the behaviour towards infection of many old and new, British and foreign, cultivated varieties. In the course of this work the widespread occurrence of some viruses, the unsuspected presence of others, and the presence of hitherto obscure viruses as active agents of disease have been revealed. In addition, there has been brought to light a number Of varieties which are more resistant than the majority to the entrance of certain viruses into their tissues; others which though not resistant to the entry of certain viruses are not markedly affected by their presence; and yet others which can readily be infected artificially but which in field culture are virtually immune from infection with certain viruses. Each of these qualities has been shown, through appropriate tests, to be heritable. The foundation of a vigorous policy directed towards improving the potato as regards resistance to some of the common virus diseases has thus been laid. The most promising lines of investigation with regard to leaf roll and the mosaic diseases caused by virus Y have been provided by the identification of varieties which offer positive resistance to infection and positive resistance to the disease after infection has taken place. In the case of two obscure non-commercial varieties the resistance to leaf roll has proved to be of a very high degree and has not broken down under intensive conditions Of infection in field or labora- tory. Furthermore, their seedlings in the first generation are maintaining this high standard Of resistance. Many potato varieties when infected with virus X, either naturally or artificially, show mosaic symptoms. Other varieties when seen in the field are never affected with mosaic diseases due to virus X. These latter varieties, when infected artificially in the laboratory, however, are killed with this virus. Thus there are varieties which remain free from virus X in the field, yet succumb to the virus in the laboratory. It has been found that this apparent contradiction in behaviour is actually an expression of the same phenomenon. Under field conditions of infection the virus enters and cauSes the .w F‘I . avg :5: Age Lip-i: hear in: flint A, . -. -. :62“ A141: .;. 4 rr‘ AGRICULTURAL RESEARCH IN SCOTLAND IN 1939. 123 death of the tissue immediately surrounding the point of entry. This dead tissue acts as a barrier against further progress of the virus into the plant. The virus is thus localised at the point of infection and the plant remains, in effect, healthy. Varieties which behave in this fashion towards viruses X, A, B or C, that is, are killed with one or more of them in the laboratory but remain free from general infection with them in the field, are said to be hypersensitive to or field immune or virtually immune from the virus or viruses eoncerned. Hypersensitiveness is thus a valuable property, and varieties possessing it are of particular economic importance to Scottish agriculture, since the most common of all the virus diseases in Scotland are those caused by virus X either alone or in combination with viruses A or B. In illustration of this value it may be pointed out that in 1937 over 70 per cent of the total acreage of varieties hypersensitive to virus X was certified Grade A by the Department of Agriculture for Scotland. By contrast only 7 per cent of the total inspected acreage of other varieties attained Grade A standard. The types and extent of virus diseases in potato crops are dependent not only upon the varieties grown, as indicated above, but also upon the conditions under which they are grown. A survey of diseases throughout Scotland has revealed that leaf roll is more prevalent in the East than in the \Vest, and in the South than in the North. Severe mosaic due to virus Y, on the other hand, is chiefly a disease of the South- West. Both these diseases are spread through the agency of sucking insects, chiefly aphides or green-fly. A primary ex- amination Of the aphis populations in the South-West and South-East has been made therefore with the object of throwing light on the distribution of the two diseases. It was found that not only were the aphis populations of the South-East much greater than those of the South-West, but also that the constitution of the populations differed between the two areas. Contrary to the distribution of the aphis-borne diseases, the distribution of mild and severe mosaics due to viruses X and A was found to be largely independent of locality and greatly dependent on variety. ' THE WEST OF SCOTLAND AGRICULTURAL COLLEGE. (a) MILK PRODUCTION DEPARTMENT. bpmuted Maize for Dam] 00ws.—In this investigation sprouted maize was compared with mangolds as a source |
| Title | Transactions of RHASS Volume 1940 - Page 067 |
