Biological Control of Air-Borne Pathogens [and Discussion]

Some pathogens are partly controlled by microorganisms that occur naturally on aerial surfaces of plants, and many attempts have been made to improve control by applying selected antagonists to such surfaces. Antagonists often compete for nutrients with the pathogen, and antibiotics may be formed that reduce germination of its spores and subsequent growth. Hyphae of fungal pathogens may be killed on contact with the antagonist or by direct penetration. The plant's defences may be stimulated before challenge by a pathogen. Apart from killing the pathogen, an antagonist may reduce its reproductive capacity. The examples given illustrate the operation of these different mechanisms in the control of a wide variety of diseases. For diseases of foliage, flowers or fruit, glasshouse crops offer more attractive possibilities for control than field crops because the population level of antagonists is easier to maintain. In some cases plants can be protected by inoculation before transplanting them to the field. Foliage and canker diseases of forest trees present problems too intractable for successful control, but in orchards the prospects are better; for example, methods are available for combining pruning with application of inoculum. Similarly, in some circumstances tree stumps can be inoculated to prevent colonization by a pathogen. Where biological methods are as effective as chemical ones and comparable in cost, they are to be preferred on environmental grounds. In some cases they can be combined with advantage; for example a lower concentration of fungicide may suffice if applied with an antagonist.