The temperate, moist climate of most of New Zealand, where temperature and rainfall do not vary markedly with the seasons, is ideal both for the growth of fungi and bacteria and for the increase of many of the insects responsible for dissemination of plant viruses. Numerous endemic fungi occur in the Dominion, some causing damage to our native pasture plants and forest trees, but none is of major economic importance probably because few of our native plants are cultivated as commercial crops. Endemic plant bacteria appear to be rare as only one bacterial disease, a leaf spot of the forest tree Dysoxylum spectabile caused by Pseudomonas dysoxyli has been recorded and is of no importance economically. Phormium yellow leaf of New Zealand flax (Phormium tenax) spread by a native plant hopper (Oliarus atkinsoni) is the only known endemic plant virus – one which has caused widespread death of large areas of phormium, a valuable fibre plant.
The development of fungous and bacterial diseases is determined by the presence of pathogens, susceptible host plants, and suitable climatic conditions. Many of the fungi and bacteria which thrive in temperate climates have been introduced to New Zealand and most of these have become established. Surprisingly, however, others which might be expected to thrive under our climatic conditions have not been found here. These include important fungous diseases like potato wart (Synchytrium endobioticum), tobacco blue mould (Peronospora tabacina), apple rust (Gymnosporangium juniperi-virginianae) and hop downy mildew (Pseudoperonospora humuli), and serious bacterial diseases such as potato ring rot (Corynebacterium sepedonicum), bean blight (Xanthomonas phaseoli), bacterial blight (X. translucens) of cereals, and Stewart's disease (Bacterium stewartii) of maize and sweet corn.
Less dependent on climatic conditions are the plant viruses which often owe their existence to the presence of insect vectors. Aphids which have found the climate favourable have established themselves, together with many of the viruses they carry. Leaf hoppers, however, have been established less readily and we still do not have some of the viruses they transmit such as aster yellows, beet curly top, lucerne dwarf, and western-X virus of stone fruits.
Common in some localities are trace-element deficiency diseases due to lack of boron, manganese, zinc, copper, and molybdenum. Although shortage of these elements may result from a low content in the parent rock, it is often brought about by a relatively high rainfall leaching the minerals from the soil.
Diseases of major economic importance in New Zealand include the fungous diseases dry rot (Phoma lingam) of swedes and turnips, late blight (Phytophthora infestans) of potatoes and tomatoes, verticillium wilt (Verticillium albo-atrum) which is most serious in tomatoes but also causes losses in potatoes, tobacco, stone fruits, gooseberries, raspberries, straw-berries, and a wide range of ornamental plants, and the rusts (Puccinia coronata and P. graminis) of cereals and grasses which greatly restrict the growing of wheat and oats in the North Island and reduce pasture production through their effect on ryegrass. Bacterial diseases of particular importance are blast (Pseudomonas syringae) of stone fruits, tomatoes and many other hosts, black rot (Xanthomonas campestris) of brassicas, and soft rot (Erwinia carotovora). The latter causes heavy losses in brassicas and also attacks the storage organs of many plants including potatoes, onions, and carrots. Of major importance amongst the virus diseases are cauliflower mosaic of swedes, turnips, rape, cauliflower, and cabbage, cereal yellow dwarf of wheat, barley, oats, and grasses, tomato spotted wilt which is particularly serious in tomatoes and also causes losses of lettuce, peas, and many ornamental plants, and the endemic virus phormium yellow leaf which has prevented the use of New Zealand flax for developing a major fibre industry.
The study of plant pathology in New Zealand was begun in 1893 with the appointment of T. W. Kirk as Biologist to the newly formed Department of Agriculture. Apart from undertaking a considerable amount of work on the identification of diseases, Kirk did much to educate farmers and fruitgrowers to appreciate what diseases were and how they could best be controlled.
In 1911 when Kirk became Director of the Horticulture Division, A. H. Cockayne was placed in charge of the Biological Laboratory, which was moved from Wellington in 1915 to the Department's Central Development Farm at Weraroa, near Levin. The development of the Laboratory was restricted by the First World War.
The post-war years saw the beginning of a new era in plant pathology in this country. G. H. Cunningham, who joined the staff in 1919 as mycologist, began a systematic survey of diseases attacking fruits, field crops, vegetables, and flowers. His work on the classification of fungi was also begun about this time. In 1925 he published the first New Zealand book on plant diseases – Fungous Diseases of Fruit Trees in New Zealand.
The Biological Laboratory returned in 1920 to Wellington where it remained until 1928 when the staff was transferred to Palmerston North to form the nucleus of the Plant Research Station, an organisation administered jointly by the Departments of Agriculture and of Scientific and Industrial Research. Within the new Station, Cunningham was placed in charge of a Mycological Laboratory. The next eight years was a period of considerable development and expansion in plant pathology. Investigations which until then had been mainly concerned with fungous and bacterial diseases were widened in scope to include virus and physiological diseases, techniques of spray application, and evaluation of therapeutants used for disease control.
In 1936, following a reorganisation of agricultural research in New Zealand, the Department of Scientific and Industrial Research was made responsible for plant research and the Plant Research Station disbanded. As a result of the reorganisation, the Plant Diseases Division was established as a branch of the Department of Scientific and Industrial Research with G. H. Cunningham as Director. The Division moved in 1939 to permanent quarters at Mount Albert, Auckland, where modern laboratory and glasshouse facilities had been provided on an area of 16½ acres.
Plant Diseases Division, the centre of plant pathological work, has a permanent staff of 70 including 24 professional plant pathologists. Of the latter, 19 are stationed at Mount Albert where they are grouped in teams working in the fields of systematic mycology, economic mycology, bacteriology, virology, and therapeutant control of diseases, each group being responsible for both basic and applied research in its own specialist field. The remainder are stationed at substations, disease problems of field crops being studied by three professional officers at Lincoln in the South Island, while diseases of pasture plants are investigated by an officer at Palmerston North, and tobacco diseases by one at Nelson.
In addition to those employed by the Plant Diseases Division, there are about 12 other plant pathologists practising their profession. Within the Department of Scientific and Industrial Research, the Crop Research Division employs a pathologist to investigate virus diseases of potatoes, the Fruit Research Division takes part in the investigation of fruit tree viruses, and a nematologist is attached to the Entomology Division. To assist with plant quarantine and advisory and extension work, the Department of Agriculture also employs several plant pathologists. In addition to their servicing work, these officers undertake research on diseases of vegetables and berry fruits and on chemical control of diseases. Research on diseases of forest trees is carried out by the plant pathologists employed by the Forest Research Institute of the State Forest Service. At Massey and Lincoln Agricultural Colleges, students are trained by qualified scientists who also undertake investigation of plant diseases. At least one plant pathologist is employed by a chemical-manufacturing firm.
Scientists in other fields also contribute to work in plant pathology. Thus plant breeders at the Crop Research and Grasslands Divisions of the Department of Scientific and Industrial Research are concerned with the breeding of disease-resistant varieties of field crop, vegetable, and pasture plants, and chemists of the Dominion Laboratory assist with studies on therapeutant chemicals.
An extension and advisory service concerning plant diseases is provided by the Farm Advisory and Horticultural Divisions of the Department of Agriculture, whose officers make substantial contributions to plant pathology with their field observations and by carrying out plant-disease surveys and conducting field trials on disease control. They also assist pathologists by drawing to their attention new disease problems, collecting disease material for identification and study, and by assisting in the selection of disease-resistant varieties. The Department of Agriculture also provides a plant quarantine service and administers seed certification schemes which have as one of their aims the production and distribution of disease-free seed.
Introduction and use of control measures have been successful in greatly reducing losses caused by plant diseases, but in spite of this and the fact that hundreds of thousands of pounds are spent annually on disease control, direct losses greatly exceed £10,000,000 and are probably considerably in excess of £20,000,000 a year. Also of major importance are indirect losses brought about by the inability, because of disease, to grow economically certain crops, such as brassicas and cereals, in many parts of the country. At a moderate estimate the combined ravages of fungous, bacterial, and virus diseases reduce the value of our brassica crops, which are widely used as supplementary stock food, by £5,000,000 a year, while diseases of fruit crops take an annual toll of more than £1,000,000. In some seasons losses in cereal crops also amount to more than £1,000,000, and diseases in vegetables regularly cause losses of the order of millions of pounds a year. No crop grown in the Dominion is unaffected.
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.
New Zealand has a comprehensive literature on plant diseases and their control. Most of the earlier papers appeared in annual reports of the Department of Agriculture and in the New Zealand Journal of Agriculture. For the past 25 years, however, the more technical papers have been published in the New Zealand Journal of Science and Technology and, more recently, in the New Zealand Journal of Agricultural Research. Popular papers have continued to appear in the New Zealand Journal of Agriculture and also in the Orchardist of New Zealand, the New Zealand Commercial Grower, and other growers' journals. Articles on the classification of fungi have been published, mainly in the Transactions and Proceedings of the Royal Society of New Zealand, and some papers have appeared in overseas journals. Articles on plant pathology are too numerous to be cited but a few of the more important books and bulletins are given here.
by Edward Edinborough Chamberlain, D.SC., Director, Plant Diseases Division, Department of Scientific and Industrial Research, Auckland.