Plant Bacteriology In New Zealand.
The study of bacteria in relation to plants is a comparatively recent endeavour and in New Zealand our first bacterial disease was recorded by T. W. Kirk in 1905. From that time until 1919 further diagnosis of bacterial plant diseases was based on symptoms described in overseas literature. In 1919 R. Waters introduced more accurate diagnosis by studies of morphology, physiology and pathogenicity of the causal organisms. Control measures for economic diseases, however, still followed overseas practices.
In recent years more attention has been given to fundamental taxonomy and to more detailed experimental work on control methods suited to New Zealand. In an institution concerned largely with economic diseases of agriculture and horticulture it has been necessary to develop these phases of work simultaneously, for in dealing with the practical application it has been more and more necessary to consider basic work on morphology and physiology of the organisms involved. Likewise, in evolving suitable control measures, it has been increasingly important to know more of the dissemination and pathogenicity of the organism and susceptibility of host plants.
Plant bacteriology has included studies on nodule bacteria of legumes. For some years culture for inoculation of lucerne has been distributed to farmers and this past season sufficient for approximately 200,000 lb. of seed was used. This method of inoculating lucerne crops is highly successful provided soluble fertilisers are not mixed with inoculated seed. The same method has been applied to white clover in an attempt to improve establishment of this legume, but satisfactory results have not been achieved. In New Zealand strains of whiteclover
nodule bacteria vary widely in their ability to accumulate nitrogen, but it has not been possible to induce the “best” strain to give efficient results over all districts. At present one strain is efficient in Otago, a second in Canterbury, a third in the Manawatu, and a fourth in Auckland. The cause of this is not known, though we have found that the nitrogen efficiency of these organisms can be changed by environment, e.g., we have found that repeated transfers on media rich in or deficient in soluble calcium increases or decreases the power of the organisms to accumulate nitrogen in the host plant0.
Classification of Plant Bacteria.
Morphological, physiological and pathogenicity studies of our plant pathogens show they agree in most characters—but not necessarily in all—with those described in other countries. Similar technique and materials are employed, but discrepancies regularly occur, particularly in the fermentation of carbon compounds. It would appear that the present “standard” methods of determining physiological reactions are not reliable or that cultures from different sources give slight variations in reaction. In the classification of the nodule bacteria a similar, but less confusing, position occurs. In this case six or more species of Rhizobium, determined by this ability to form nodules on different groups of legumes, have been described. We have found that these “species” are cross inoculable to hosts of other species and that in reality only one species exists.
Control of Bacterial Pathogens.
The majority of plant diseases caused by bacteria in New Zealand are also known overseas. There are three exceptions, namely, grease-spot of Passiflora edulis, a disease of the endemic forest tree Dysoxylum spectabile and one affecting Hakea saligna.
Since 1903 the Department of Agriculture, through the operation of the Orchard and Gardens Diseases Act, has attempted to prevent entry of infected plants, but unfortunately we now possess the majority of serious diseases known to affect our introduced plants. Within New Zealand the Act also serves to prevent spread of serious plant diseases in that it requires the destruction or treatment of certain diseases in orchards, gardens and nurseries. The obvious advantages to be derived from the Act are dependent on thorough inspection and collaboration of growers. Some degree of success has resulted, chiefly in that incidence of many diseases has been greatly reduced, but there are many instances of serious diseases evading the net.
The methods used for control of bacterial diseases are similar to those employed for fungi and include (a) use of disease-free soil, (b) selection of disease-free seed and plants, (c) use of resistant varieties, (d) eradication of infected plants or parts of plants and (e) application of therapeutants. One or more of these may be used for a disease, the method depending largely on whether the disease is disseminated by soil, by seed, or by rain, insects, etc.
Control by disease-free soil. Soil-carried diseases are not common in New Zealand, the only ones of importance being crown-gall, which affects a variety of host plants, soft rot of cruciferous crops, and occasionally canker of tomato. These diseases have been avoided by refraining from growing susceptible crops in an infected area. Treatment of soil by sterilisation methods is possible, but with our present knowledge is practicable only on limited areas as in glasshouses, in seed-beds, or for special horticultural crops. Crown gall is as yet not severe in New Zealand except in some nurseries and gardens and control appears to be dependent on maintaining the natural low pH (5.5) of our soils. Soft-rot diseases, which are widespread and often serious, are linked with injuries to host plants, and good control can be attained by protection of crops against unnecessary damage by stock or implements or by use of resistant varieties. Tomato canker occasionally destroys the whole or part of a crop and is then usually spread with irrigation and deep cultivation. Control, however, is primarily dependent on use of disease-free seed and soil followed by early diagnosis and removal of infected plants.
Selection of disease-free seed. Seed-carried diseases are common in this country and two methods of control are adopted: (1) use of seed from disease-free crops, and (2) treatment of seed to destroy pathogens present. The former is applied for halo-blight of beans, grease-spot of passion vines, canker of tomatoes, blackleg and brown-rot of potatoes and black-rot of crucifers, though use of resistant varieties or of therapeutants is sometimes more practicable. In regard to potato
diseases the seed-certification system of the Department of Agriculture, wherein seed-tubers are taken from clean crops, has appreciably reduced the incidence of bacterial diseases of potatoes. Bacterial-blight of peas, Rathays disease of cocksfoot and halo-blight of oats are not common and most lines of these seeds are disease-free. The second method, seed treatment, is not used extensively and when carried out is supplementary to selection of seed from crops that appear disease-free. Acid treatment of seed pulp is employed in harvesting of seed of tomatoes and passion fruits and dusting of pea seed is recommended as a means of reducing seed rotting in wet periods.
Control by resistant varieties. The growing of resistant varieties has in recent years provided a satisfactory alternative method for control of some bacterial diseases. All runner types and white-seeded dwarf beans are virtually immune to halo-blight infection and have become the predominant varieties for commercial and private production of green beans. The New Zealand grapefruit (Citrus sp.), under orchard conditions, is rarely infected with citrus-canker and growers now prefer to replant with this variety in areas where freedom from citrus-canker is not assured [ unclear: ] ieties of swedes such as Whilemsburger and Vilmorin and Choumoellier (Brassica) are resistant to fungous, virus and bacterial pathogens and have largely replaced the more Susceptible varieties for winter stock food. English plums and a few Japanese plums are resistant to bacterial-spot, but as yet this is not widely known among growers, though one of the largest nurseries has stopped propagating varieties known to be highly susceptible.
Control by eradication. Within established orchards some diseases are best controlled by eradicating infected plants. The Citrus Canker Regulations require the destruction of any infected tree together with all citrus within a radius of 36 feet. Eradication has been efficacious in eliminating the disease from the Keri Keri district, but to date has not proved so successful in Tauranga, where a few leaf lesions are found each year. For want of a better control, cutting out of infected wood is still the standard method of treating fireblight, and as long as inspections are regular and pruning is thorough outbreaks are few and have little effect on trees or fruit production. Pruning out of diseased parts of plum trees affected with bacterial-spot does not reduce incidence of disease, though in this case spray treatments have done so.
Control by therapeutants. During the last four years the use of therapeutants has been developed as a new technique for control of bacterial diseases. Results from repeated trials show that Bordeaux and copper-oxychloride give efficient and practical control of halo-blight of beans and grease-spot of passion vines. Bordeaux also reduces to a negligible amount the incidence of bacterial-spot of plums, and citrus-blast is controlled by the same spray material. Growers have now adopted the use of Bordeaux for control of grease-spot, bacterial-spot and citrus-blast. As yet trials have been limited to treatment of the above four major diseases and to preliminary work with walnut-blight. Though success has been achieved with copper sprays only, it is possible that further work will provide other therapeutants equally suitable. Cut cabbages and stored carrots can be protected against soft-rot infection by application of Bordeaux.
Other bacterial diseases of plants are present in New Zealand including pathogens of lettuce, tomato and cauliflower, but these diseases are of slight economic importance and have not been investigated.