Minimum Tillage Irrigation

exposure of the inoculums to desiccation by the sun. Modern agriculture has moved away from regular cultivation of soil between crops towards a system of minimum tillage, or even no-tillage. Minimum tillage is a method of planting crops that involves no seed bed preparation other than opening the soil to place the seed at the intended depth. Usually the soil is not cultivated during crop production and chemicals are used for weed control. Minimum tillage reduces injury to the roots of crop plants caused by mechanical tillage or hand weeding, reducing the opportunities for opportunistic pathogens to infect. It also reduces the spread of pathogens by tillage practices. However, it may favor the development of some diseases such as those that

Irrigation applied during dry seasons means the propagules of pathogens are not exposed to desiccation during periods of drought. Consequently, the level of inoculums increases. The seriousness of this situation is compounded in areas where, because of irrigation, it is possible to grow two susceptible crops in the same field in one year. In addition, irrigation water may contain propagules of pathogens which it carries from one place to another unless carefully treated before use. Overhead watering may promote disease by increasing the period of time a layer of free moisture remains on leaf surfaces (leaf wetness period). The longer leaf wetness periods increase the likelihood that sufficient time will be available for fungal spores to germinate, form infection structures and penetrate the plant surface to the relatively constant and favorable environment within the leaf. Irrigated crops may become a green island in an otherwise dry environment and attract insect vectors of virus diseases. In such cases, it is often better to delay sowing an irrigated crop for some time after other vegetation has dried up and the vector population has been reduced by desiccation. On the other hand, irrigation can be used as a tool to reduce the level of inoculum and to retard disease development. Alternately drying and rewetting soil encourages the activity of micro-organisms that destroy sclerotia. Overhead irrigation can reduce or inactivate airborne inoculum by washing it out of the atmosphere. Short daily watering encourage the germination of powdery mildew spores but the plants do not stay wet enough for long enough for the fungus to penetrate. Overhead sprinkling of dormant fruit trees reduces the incidence of apple scab because the short-lived ascospores are released in response to temperature changes whilst the tree is dormant and cannot survive until leaves are present.