Cocoa Research Unit, The University of the West Indies, St Augustine, Trinidad and Tobago, West Indies

To relate weather patterns to plant disease, weather variables may be averaged over different periods of time. These periods should be considered in relation to the response of pathogens to various stimuli. Daily values of air temperature and humidity, rainfall and wind speed are commonly available from standard meteorological records. More frequent (hourly) records can be obtained from automatic weather stations or from simulations, by interpolating between daily maximum and minimum values. In some circumstances, weather factors averaged over several days or weeks may be appropriate for disease prediction.

Temperature commonly affects different parts of pathogen life cycle in different ways. Three cardinal temperatures, the minimum, the optimum and the maximum, normally define temperature response curves. These can be used to estimate the rate of development of pathogens, percentage spore germination, infection and sporulation. The interpretation of disease development in relation to hourly temperatures may be appropriate in some circumstances, while daily values are commonly adequate for disease prediction.

Humidity strongly affects the majority of plant diseases, yet relatively few studies have reported relating pathogen activity to humidity. For some pathogens, spore germination, germ tube growth and host penetration can occur when the air is close to saturation but the host surface is dry. Others require surface wetness for infection. Sporulation is often associated with high relative humidity but, again, surface wetness may be a requirement. In the natural environment, high relative humidity in the atmosphere commonly coincides with times when the vegetation in crop canopies is wet but there may be exceptions.

Many plant pathogens require surface wetness to infect the host, but the duration of wetness needed for infection varies greatly for different diseases. In inoculation experiments, relationships between the amount of disease and surface wetness duration are normally described adequately by an exponential asymptote: This allows the definition of a minimum wetness period for infection and an estimate of the period necessary for maximum disease expression. Adaptation to natural environments has resulted in a wide range of wetness requirements for pathogens to infect the host. The minimum wetness period for infection ranges from less than 1 ;h to about 20 ;h. Pathogens requiring prolonged wetness for infection are commonly able to withstand drying and may be adapted to respond positively to intermittent wetness patterns. Surface wetness periods for sporulation often exceed the requirements for infection. Careful interpretation of disease/weather relationships has the potential not only to improve the efficiency of disease control in integrated disease management systems, but also may optimixe techniques to screen for disease resistance.