Studies on some diseases of sainfoin ( Onobrychis sativa) : II. The life history of Ramularia onobrychidis Allescher

Abstract

Summary Ramularia onobrychidis causes a leaf spot of sainfoin in Glamorgan, Wales. In field material, chains of conidia are produced on conidiophores of sub-stomatal sporodochia from late spring to early autumn. Conidia vary from phragmospores through didymospores to amerospores; moist conditions promote the formation of shorter conidia with fewer septa, and very dry conditions some longer, somewhat narrower conidia with up to four septa. Sclerotia develop in winter in the substomatal cavities and are arranged usually in a ring around an old spot. They germinate very readily in the field under moist conditions without any resting period and retain their viability for at least nine months indoors, when kept dry. Sclerotia consist essentially of a more or less globose ‘body’ of homogeneous plectenchy-matic cells with brown and slightly thickened walls, and a ‘neck’ of regularly columnar cells. It is from the apices of these neck cells that conidiophores arise. Sclerotia store oil, which disappears during the production of a single crop of conidia. Single spore isolations of the different spore forms from summer spots and from germinating sclerotia have given similar cultures. The disease was reproduced with difficulty when healthy sainfoin leaflets were inoculated with spore suspensions and R. onobrychidis was re-isolated from the spots. No perithecial stage in the life history has been discovered and it is probable that the fungus overwinters, in South Wales, by means of its sclerotia. A (?)pycnidial form, found near spots caused by Ramularia may represent the slimy conidial apparatus. The fate of the original species of Ramularia is discussed and a history of the genus is given.