Increased spacing between plants in a row, or between rows of plants or between commercial crops susceptible to the same pathogen will reduce disease spread from a seedborne source. This strategy can be very effective where pathogens are splash-borne (e.g. bacteria, pycnidial fungi) and are spread over relatively short distances. For example, spacing between crops was considered necessary in Australia to prevent splash spread of the halo blight bacterium from the newly introduced tropical pasture legume siratro bean (macroptilium atropurpureum) tos commercial bean (phaseolus uulgaris) seed crops produced in proximity to one other (Johnson, 1970). No distance is suggested by Johnson (1970), but taylor (1972) minimized the spread of halo blight in the United Kingdom by placing bean plots at distances of 46-64 m apart.
Separation of crops also may reduce the spread of certain seedborne aphid-transmitted non-persistent viruses which are retained for brief periods by the vector and are distributed over limited distances. With other viruses, depending on the type of vector, different strategies may be required including the use of insecticides, oil sprays, repellants, alarm pheromones and antifeedants (Tomlinson, 1987).
Separation of crops is less useful where the inoculum is air borne and disseminated over considerable distances. Combined strategies are probably necessary in such circumstances because the variation in range and concentration of air-borne inoculum make crop spacings which will exclude organisms very difficult to establish with confidence. For example, distances of more than 50 m (Kublan, 1952), not less than 100 m (Oort, 1940) and probably not less than 150 m (Strass, 1964) are spacings between crops suggested for the limitation of transfer of wheat and barley loose smut spores. However, such distances do not completely exclude inoculum and that which is transferred my still constitute a threat to the developing crop. Hewett suggested that even greater isolation distances, similar to those used to protect brassicas and beet from cross-pollination, i.e. 1000 m for basic seed production, might be necessary to protect susceptible barley cultivars from infection bridging by loose smut spores. This was part of a tripartite strategy which also included seedtreatment and the setting of tolerance limits for seedborne inoculum (hewett, 1968). This problem of cross-infection in the production of smut-free barley seed was resolved in canada by seed treatment and the compulsory isolation, backed by special legislation, of some 300 farms in a seed control area from other field sources of loose smut.
Air-borne conidia of alternia brassicicola infective at 1000 m downwind from the source, created similar problems for the safe siting of brassica (B. oleracea) crops for seed production in an area of continuous cropping in the united kingdom. However, the siting of crops at least 150 m upwind of an infected maturing crop gave considerable reduction of seedborne infection in the new crops. In this example the treatment of all basic seed, the siting of new crops for seed production upwind of maturing seed crops and fungicide spraying of seed pods was the combined strategy evolved to control the problem (humpher-son-jones and maude, 1982).
An alternative from spacing is the grouping together of crops with similar problems into isolated blocks to dilute the effect of air-borne inoculum. This is a form of spatial rotation which may be suitable for growers with large areas of land at their disposal. But this method would be difficult to apply to crops commanding large acreages such as cereals and oilseed rape (maude, 1988a).