1Centre for Research in Botany, Saraswathi Narayanan College, Madurai 625 022, India; 2Department of Chronobiology, School of Biological Sciences, Madurai Kamaraj University, Madtirai-625 021, India

Background and objectives
The role of timing in any critical epidemiological event haS received little emphasis in plant pathology. The branch of study that deals with the potential importance of chronobiology in pathology of plants is designated chronophytopathology [1].

Materials and methods
In the present study, paddy (Oryza sativa L.), variety Co.43, grown under controlled environmental conditions (LD cycle of 12:12 h providing 72.5 Wm-1 of light and temperature at 26+/-1C) was inoculated with the fungal pathogen Helminthosporium oryzae, the cause ofbrown leaf spot disease. Inoculations were at different times of the day, such as 0600h, 1000h, 1400h, 1800h, 2200h, 0200h. Studies under continuous light and different photoperiods were also made to determine their effect on time of pathogen inoculation on the incidence of disease.

Results and conclusions
The maximum number of brown leaf spots and disease severity was observed after inoculation at 0600h, indicating the most successful time of pathogen entry into the host. The results of our studies were also correlated with in vitro and in vivo germination of conidia and diurnal changes in stomatal resistance and leaf temperature in host plants. In the inoculated plants, the appearance of brown leaf spots was mostly confined to the periods of light. A similar pattern of appearance of brown leaf spots also persisted in continuous light. Plants inoculated (0600 h) under different photoperiods exhibited differential susceptibility of host plants and maximum susceptibility in the LD cycle of 12:12 h. Observations on changes in time interval between the peaks and troughs of closely related biochemical rhythms show evidence of delinking of metabolic events and serve as a prelude for early diagnosis of disease. Our studies suggest the relation between biological rhythms and diseases [2] in the advancement of agricultural productivity.

1. Pauly JE, Scheving LE, 1987. Advances in Chronobiology, Part A, Alan R. Liss, Inc. pp 95-103.
2. Hayes DK, Pauly JE, Reiter RJ, 1990. Chronobiology: Its role in Clinical medicine, General biology, and Agriculture, Part B, Wiley - Liss, Inc. pp 867-881.