2.5.11
INFECTION EFFICIENCY OF PUCCINIA STRIIFORMIS F. SP. TRITICI AND PUCCINIA RECONDITA F. SP. TRITICI ON WHEAT SEEDLINGS: TOWARDS CONSISTENCY BETWEEN CONTROLLED AND FIELD CONDITIONS

C DE VALLAVIEILLE-POPE1, L HUBER2, M LECONTE1 and R GOUJET2

1Laboratoire de Pathologie Végétale, INRA, BP01, 78850 Thiverval-Grignon, France; 2Station de Bioclimatologie, INRA, 78850 Thiverval-Grignon, France

Background and objectives
Comparative studies of monocyclic components of the infection cycle suggested the existence of epidemic strategies in fungi [1]. High infection efficiency was thought to compensate moderate sporulation for Puccinia recondita f. ;sp. tritici (brown rust) and low infection efficiency to compensate high sporulation per lesion due to systemic growth within the leaf for P. ;striiformis f. ;sp. tritici (yellow rust) [1, 2]. Those results were obtained in optimal controlled conditions of temperature and wetness duration for penetration and incubation periods. Our purpose was to test this compensation hypothesis in field conditions. Differences in maximal infection efficiency for both rusts between field and controlled conditions led us to investigate the optimal environment for infection.

Materials and methods
Seedlings were inoculated with either urediniospores of P. ;recondita f. ;sp. tritici or P. ;striiformis f.sp. tritici. To study penetration in the field under various conditions of temperature and wetness duration, inoculated seedlings were placed outside for 16 ;h, including at night. Afterwards, the seedlings were transferred to a climatic room under optimal conditions for incubation until lesions developed. Experiments were repeated 40 times. Infection was assessed by recording the number of pustules for P. ;recondita f. ;sp. tritici and the number of chlorotic spots for P. ;striiformis f. ;sp. tritici. Following this experiment based on temperature and wetness effect, another experiment was required to better characterize the optimal light environment for infection. Seedlings were exposed before inoculation to various regimes of light duration and intensity in both field and controlled conditions. After light exposure, inoculation and incubation were done under optimal controlled conditions.

Results and conclusions
Field experiments permitted us to identify conditions for which both fungi had the maximal infection efficiency. For plants exposed to a light period before inoculation, infection efficiency of P. ;recondita f. ;sp. tritici was as high as that of P. ;striiformis f. ;sp. tritici. In controlled conditions, the same conclusion was true for both rusts under high light intensity. In contrast, under a dark period before inoculation, infection efficiency of P. ;recondita f. ;sp. tritici was 12 times higher than that of P. ;striiformis f. ;sp. tritici. Both rusts had similar infection efficiency under optimal field conditions, although they showed contrasting responses to the radiation level, which explains the discrepancy between field results and previously accepted measurements in controlled conditions [2]. The optimal conditions for yellow rust infection, i.e. succession of light before penetration and dew period during the night, are those commonly found in natural conditions. By their high and similar infection capacities, both rusts in a temperate fluctuating climate are well adapted to short periods of favourable climatic conditions. Our results also showed that caution should be taken, when developing forecasting models, to avoid the incorporation of monocyclic parameters evaluated only in controlled conditions.

References
1. Sache I, De Vallavieille-Pope C, 1995. Canadian Journal of Botany 73, 1186-1195.
2. De Vallavieille-Pope C, Huber L, Leconte M, Goyeau H, 1995. Phytopathology 85, 409-415.