Abstract: The difference in PBND incidence in the rainy and dry seasons in Thailand has led to a hypothesis that temperature and relative humidity might be the causal factors. The objective of this study was to investigate the effects of temperature and relative humidity on the effectiveness of mechanical inoculation of PBNV on peanut. Two experiments were conducted in the rainy and dry seasons during 2004 to 2005, using the susceptible peanut genotype Tainan 9. The first experiment evaluated the effect of low temperature on the effectiveness of PBNV transmission by mechanical inoculation. The second experiment compared two temperature and relative humidity conditions in the rainy season and in the dry season. The results showed that exposing the PBNV-inoculated plants to low temperature (25°C, 90% RH) during a 12 h light period and 20°C, 90% RH during a 12 h dark period) for all the time during the experimental period did not increase the infected plants. Two climate conditions representing the dry season (daytime, 32°C, 44% RH, night-time, 22°C, 65% RH) and the rainy season (daytime, 36°C, 46% RH, night-time, 25°C, 70% RH) in Thailand also showed no difference in the infected plants. These results indicated that low temperature or relative humidity did not affect the transmissibility of PBNV. The difference in disease incidences in the rainy and the dry seasons in Thailand could not be explained by the difference in climatic conditions but could possibly be accounted for by the difference in vector infestation.