2.5.21
RELATIVE IMPORTANCE OF RAINFALL AND TEMPERATURE DISTRIBUTION IN A WINTER WHEAT CROP ON TAKE-ALL INCIDENCE IN WESTERN FRANCE

P LUCAS, A SCHOENY and S CARRILLO

INRA, Station de Pathologie Végétale, BP 29, 35653 Le Rheu Cedex, France

Background and objectives
Crop sequences and cultural practices such as soil tillage, sowing density, sowing date, nitrogen fertilization have been shown to affect take-all development on wheat, a disease caused by Gaeumannomyces graminis var. tritici. Some of these factors have been incorporated in models which attempt to simulate disease progress curves [1]. Nevertheless, an important part of variability is not explained by these models when their usage is extended to different years and areas of wheat cropping. The objective of the study was to analyse the influence of the climate on the disease progress curves.

Materials and methods
Data on the percentage of infected plants were collected at Le Rheu (western France) from second winter wheat crops between 1985 and 1997. For each crop, the number of disease assessments ranged from three to seven. They were compared with air temperature and rainfall during the respective cropping periods. For all sets of data, a typology of disease incidence development was made and a search for a correlation between trends in disease development and trends in climatic events was made. When possible, depending on the number of assessments, a disease progress curve was fitted to the observed data, giving estimates of parameters reflecting the importance of the primary and secondary cycles. These parameters were then interpreted as a function of climatic events.

Results and conclusions
Years with low disease incidence appeared to be characterized by a low accumulation of temperature (degree-days base O°C) during the period following sowing (less than 500 ;degree-days for the period 0 to 60 ;days after sowing) and cold temperatures during winter (less than 400 ;degree-days 160-150 ;days after sowing). Years with high disease incidence were those with more than 550 ;degree-days accumulated during each of these two periods of time after sowing. Rainfall did not appear to be a limiting factor during these years in the region, except when more than 300 ;mm of rain were recorded in the 60-150 ;days after sowing period. This could have led temporarily to water-saturated soil, and might be responsible for slower disease progression, although temperatures were favourable (more than 600 ;degree-days accumulated during the same period). Results are discussed according to the biology of the pathogen and its requirements for temperature, humidity and soil aeration.

References
1. Colbach N, Lucas P, Meynard JM, 1997. Phytopathology 87, 26-32.