Defence Against Plant Diseases
The main concern of plant pathologists in New Zealand is to gain an understanding of our diseases and their significance in the country's economy and to minimise the losses they cause, generally in one or more of the following three ways: (1) By preventing the introduction of diseases not already here; (2) By eradicating those not fully established; and (3) By control of those which cause damage with chemical or cultural methods or by the development of resistant varieties.
Some serious plant diseases now confined to other countries, if established here, could cause losses amounting to millions of pounds annually. Hence plant quarantine regulations are enforced. Plants and seeds not known to carry diseases or pests which could be dangerous to our crops may be imported without restriction but are inspected at the port of entry. Others require an entry permit, must be grown for one season in a specified area and kept under observation by officers of the Department of Agriculture who certify them as disease-free before distribution. The introduction of plants of a third category is prohibited except in small numbers under special licence. These are grown under strict quarantine conditions at the Plant Diseases Division, Mount Albert, and released only after they have been declared free from disease.
Although eradication has been attempted on a number of occasions, it is rarely practicable because a disease is usually well established by the time it is recognised. An attempt to eradicate fire blight (Erwinia amylovora) failed but others have been gratifyingly successful. Citrus canker (Xanthomonas citri) has been eliminated from the large orchard areas at Kerikeri and Otumoetai, but there are still small pockets of infection in Taranaki, while an outbreak has recently been located in Auckland. Again, though onion yellow dwarf, a serious virus disease of onions, was well established by 1939 in the onion-growing area of Marshland, Christchurch, it has not been observed there for the last 20 years. Regulations enforcing sanitation and prohibiting the growing of onion-seed crops, and alternative hosts brought about the eradication of this disease from the district. Even if not completely eliminated from the country, onion yellow dwarf has been successfully kept out of the major onion-growing areas. Attempts to eradicate Verticillium cinerescens of carnations, and the virus diseases Moorpark mottle of apricots and plum fruit crinkle appear to have been successful. Hot water treatment of barley, introduced during the 1920s, together with the production of mother seed and certification of seed crops, has eliminated the serious diseases of loose smut (Ustilago tritici) and covered smut (Ustilago hordei) from our malting barleys.
There are many ways of keeping diseases in check. Their incidence and severity may be greatly influenced by cultural factors such as soil type, date of sowing, size of plantings, crop rotation, sanitation, etc. Knowledge of these factors is used to reduce losses. Perhaps the most satisfactory way is to use resistant varieties, to the selection and breeding of which much attention has been given, with marked success in increased production of field crops and vegetables.
Heavy losses can occur through the use of diseased stocks of vegetatively propagated plants. A seed potato certification scheme introduced in 1928 has done much to increase potato yields. Similarly, the health of fruit-tree nursery stock has been greatly improved by selection of disease-free root stocks and scion wood.
Work begun by New Zealand pathologists in the 1920s contributed much to knowledge of seed-borne diseases and their control. Since that date the general application of seed treatment to many field and vegetable crops has been of considerable benefit to the country's economy.
There are some serious soil-borne diseases particularly dangerous in glasshouses. Successful investigations on soil treatment have led to control of many of the more important of these, thereby achieving much heavier yields of tomatoes and other crops. Treatments have also been evolved and successfully applied for control of soil-borne organisms in the field.
Control of the majority of fungous and bacterial diseases is achieved by the application of therapeutant sprays. A great deal of research work has gone into finding the most effective material for specific diseases, the optimum concentrations, and the times and number of applications needed. As materials of high quality are essential for best results, the extent of control was greatly improved in 1938 by the introduction of a therapeutant certification scheme; but with increasing numbers of therapeutants becoming available it was not possible with limited scientific resources to maintain this. In 1959 it was replaced by an approval and registration scheme administered by an Agricultural Chemicals Board.