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.