Libération des ascopores d'Epichloe typhina, agent de la quenouille du dactyle. Conséquences pour l'épidémiologie et la lutte

The choke disease of cocksfoot is an important threat in seed production in northern and eastern parts of France, where the main acreages of seed fields are located. The fungus inhibits seed formation. The average losses can reach 10 to 20% and increase with the age of the stands. Studies carried out in 1987 with a vacuum spore trap on wild choked plants, isolated in small parcels, showed that the ascospores were discharged in important quantities mainly from the end of June to mid-July, whatever the climatic conditions. The ascospores were essentially ejected during the night, because their discharge requires a high relative air humidity. Choked stems cut from plants and placed on a dry soil stoped sporulating quickly. When these chokes were well supplied with water, they continued to sporulate normally during a few days, provided that the air humidity was high. Complementary studies carried out in a climatic chamber, at various temperatures (15,20,25°C), in the light or in darkness, at a relative humidity of the air form 70 to 100%, on choked stems separated from plants, led to the same results as those obtained on chokes left on the plants. Possible control measures are only prophylactic, field trials with fungicides having proved to be unsuccessful, and should take into consideration the quoted conditions required for sporulation. One should harvest the crop when the weather is dry and sunny (anticyclonic conditions are the best ones). Under those conditions, chokes, cut by the combine-harvester, fall on the ground, dry quickly and are not able to sporulate any more. Moreover, during anticyclonic conditions, the wind is low or even non-existing during the night, so that the ascospore dissemination is presumably limited. One should leave non-cut areas as briefly as possible in the field, mainly on the edges, where numerous sporulating chokes may survive. The older fields should be harvested first, in order to inactivate their inocula. Ascospores beeing carried by the wind, these recommendations could only be effective if all the growd from plants, led to the same results as those obtained on chokes left on the plants. Possible control measures are only prophylactic, field trials with fungicides having proved to be unsuccessful, and should take into consideration the quoted conditions required for sporulation. One should harvest the crop when the weather is dry and sunny (anticyclonic conditions are the best ones). Under those conditions, chokes, cut by the combine-harvester, fall on the ground, dry quickly and are not able to sporulate any more. Moreover, during anticyclonic conditions, the wind is low or even non-existing during the night, so that the ascospore dissemination is presumably limited. One should leave non-cut areas as briefly as possible in the field, mainly on the edges, where numerous sporulating chokes may survive. The older fields should be harvested first, in order to inactivate their inocula. Ascospores beeing carried by the wind, these recommendations could only be effective if all the growers agreed to apply them in a small region.