Early blight of tomato caused by Alternaria solani is an economically important disease-causing significant yield loss at all stages of development in tomato. It is a major constraint for tomato production in Himachal Pradesh both under open field and protected cultivation. The prevailing conditions of climate change and shorter winter spells have further aggravated the disease severity and consequent economic losses. In this study, field experiments were carried out to examine the development of early blight in tomato with respect to weather conditions during kharif and zaid seasons in 2021 and 2022 at CSKHPKV (Palampur, H.P.). The effect of plant spacing and date of transplanting was also studied. Tomato seedlings transplanted in the 1st week of April (2nd April) and at wider spacing (60x60 cm) resulted in higher fruit yield and less disease severity as compared to late transplanted crop (12th April) and close spacing (30x45 and 45x45cm) under both open field condition and polyhouse conditions. Early blight severity was found to be significantly positively correlated with maximum temperature and morning RH, while it negatively correlated with rainfall. Temperature was observed as the most important environmental factor in the development of disease.