Cinetique de la production d'ascospores de Sclerotinia trifoliorum Eriks en chambre de culture et en conditions climatiques naturelles. Incidences pratiques et epidemiologiques

Sclerotia of Sclerotinia trifoliorum produced in the laboratory were placed under favourable conditions for capogenesis at varying temperatures in the light or in the dark. Ascospores discharged by apothecia were counted on a daily basis throughout the life cycle of the apothecia, ie 18-20 d at 15 °C. The quantity of spores collected depended on the diameter and age of the apothecia. The greatest discharge was generally when apothecia were 7-12 d old (fig 1). The mean number of ascospores collected from apothecia over the entire life cycle was 1.4 x 106 from apothecia 4-5 mm in diameter and 4.7 x 106 for apothecia 7 mm in diameter. Temperature directly affected sporulation. Spore discharge was abundant at 25 °C, but lasted only a few hours and the apothecia died within a few days. At 5-10 °C discharge was weak, but lasted over 1 month. At 20 and in particular at 15 °C, discharge was optimal, with sporulation continuing for a mean of 3 weeks (fig 2). Light was more conducive to sporulation than dark. Apothecia that had become detached from their stipes maintained normal sporulation and responded to temperature in the same way as apothecia attached to sclerotia. However, they survived for a somewhat shorter period (fig 3). Sclerotia produced in the laboratory then subjected to natural climatic conditions in the soil from the beginning of September began to produce apothecia in mid-October . Ascospores that were discharged were collected with a vacuum spore trap (fig 4). In 1987 and 1988, ascospore discharge was studied from the end of October to the beginning of December, ie the period during which the apothecia remained alive. During that time constantly wet soil resulted in normal functioning of the apothecia. Spore discharge was maximal when the weather was mild (10-15 °C) and very humid (ie, fog, drizzle or rain). Discharge was slight under anticyclonic conditions when the nights were cold and afternoons sunny. Slight frost did not inhibit further sporulation once conditions became favourable again (fig 5). Ascospore discharge was almost non-existent at night, especially when temperatures were low. Discharge usually began about 10.00 h, was maximum between 15.00 and 16.00 h and ceased at about 19.00 h (table I and fig 6). This periodicity was related to temperature. The present study has resulted in an enhanced yield of ascospores from apothecia produced in the laboratory for artificial inoculation of red clover in screening for resistance. It has also better defined some important epidemiological factors affecting crown rot of forage legumes.

La production en ascospores d’apothécies de Sclerotinia trifoliorum est évaluée quotidiennement au laboratoire sous diverses conditions. Elle dépend de l’âge et du diamètre des apothécies. Elle est maximale pour des apothécies de 7 à 12 j. La sporulation moyenne, durant leur vie entière, d’apothécies de 4 à 5 mm de diamètre est de 1,4 x 106 ascospores. La température optimale est de 15 °C, les apothécies demeurant alors sporulantes pendant 18 à 20 j. À 5 °C et 10 °C, la sporulation est plus faible, mais les apothécies demeurent fonctionnelles plus d’un mois. À 25 °C, elles meurent en quelques jours. La sporulation est meilleure à la lumière qu’à l’obscurité. Des apothécies séparées de leurs sclérotes et mises en conditions d’HR satisfaisantes conservent un fonctionnement normal. Soumises au climat naturel, les apothécies projettent leurs ascospores de fin octobre à début décembre, comme l’indiquent des piégeages réalisés avec un capteur par aspiration. La sporulation est la plus forte par temps doux et très humide. Elle est faible en conditions anticycloniques. La projection des ascospores est presque nulle la nuit. Elle débute vers 10 h, est maximale vers 15-16 h et cesse vers 19 h. Outre leur intérêt pour une meilleure production d’ascospores utilisables en contaminations artificielles, ces observations précisent les facteurs agissant sur le développement de la sclérotiniose des légumineuses fourragères.