Aerial photography in the control of crop disease.

Aerial Photography in the Control of Crop Disease

Aerial photography can be used to supplement and extend ground survey; information can be gathered quickly, and over large areas. Integration of data obtained from the ground or the air can then be used by photointerpreters to identify cropping problems and their causes, assess their commercial impact, and help farmers achieve better crop performance.

Aerial photography has been providing valuable information in support of all aspects of advisory and development work by the Ministry of Agriculture, Fisheries and Food (MAFF) in England and Wales for over 25 years. The Agricultural Development and Advisory Service (ADAS) aerial photography unit originated in the late 1950s when aerial survey was first used to study the spread and development of potato blight (Phytophthora infestans) in the fens of East Anglia, with a view to improving disease control methods.[1] This followed the pioneering work of Taubenhaus in the USA, where areas of cotton root rot infection (Phymatotrichum omnivorum) were recorded on black and white aerial photography.[2] Colwell, working on cereal rust in the USA, later developed the use of black and white infrared film to record disease-related crop stress.[3]

Aerial photography continues to be the most widely used remote sensing technique, despite the more recent developments in satellite technology. Satellite imagery gives a greater synoptic view but does not yet offer the fine resolution needed to differentiate between diseases and other causes of crop variation, eg soil type, crop husbandry problems, weed infestation or crop damage following agrochemical use.

Materials and methods have changed little over the past 25 years and, by careful selection of film type, filters, timing and scale, aerial photography is able to provide detailed information to ground investigators, giving them a plan view of large areas of land. This saves time on ground survey by highlighting study areas. Four film types are available -- panchromatic, black and white infrared, true colour and colour infrared (false colour) and, for crop disease studies, a film type must be chosen which maximises the contrast between healthy and diseased crops. The resulting crop pattern, recorded in changes of tone or texture on the photograph, helps the photointerpreter to determine the mechanism of disease spread (ie whether air-borne, seed-borne or soil-borne), the extent to which neighbouring crops are affected, and the effects that current farming methods are having on disease expression.

Very few cases have been reported where disease has been recorded on aerial photography before it was visible on the ground, so aerial photography cannot be considered as an early warning system. Instead it is used to study the epidemiology of diseases. Operationally, ADAS uses this information to continually revise and update crop protection advice, aiming to avoid …