The dynamics of leaf surface wetness of sorghum seedlings in relation to resistance to the shoot fly, Atherigona soccata

In quantitative measurements of leaf surface wetness (LSW) of the central whorl leaf of sorghum seedlings in August (rainy season) and November (post-rainy season), the highest amount (6.29 mg of water) was recorded in August in the shoot fly Atherigona soccata (Diptera: Muscidae), susceptible sorghum genotype CSH 1, while the lowest (0.07 mg) was recorded in November in the resistant genotype IS 18551. Studies on diurnal fluctuation revealed that LSW was lowest at sunset, was highest between 02.00 and 04.00 h (closely corresponding with hatching of shoot fly eggs) and dropped before sunrise. This fluctuation was associated with the evaporation of water from the plant during the night. More LSW accumulation occurred during the main crop season (June-October) than in the post-rainy season (November-April). Annual fluctuation of LSW followed trends similar to the population dynamics of shoot fly and crop infestation and were correlated with rainfall, temperature and relative humidity. Measurements of leaf temperature and the vapour pressure gradient between the leaf and the air indicated that leaf surface water originates from the plant. This was further supported by the different amounts of LSW on susceptible and resistant cultivars with similar microclimatic conditions.