FURTHER OBSERVATIONS ON THE MECHANISM OF GERMINATION OF THE CONIDIA OF VARIOUS SPECIES OF POWDERY MILDEW AT LOW HUMIDITY

Abstract

Collected data from the observations of the author and others indicate that nine species of powdery mildew fungi, obtained from 21 different hosts, have been examined with regard to germination of the conidia at various humidities. Of these species, four, viz., Erysiphe Polygoni DC., E. graminis DC., Uncinula Salicis (DC.) Wint., and Microsphaera Alni (DC) Wint., seem beyond doubt to produce conidia capable of germinating at low humidity. Five species do not tolerate low humidity, according to published information.Conidia of Erysiphe graminis, detached from the conidiophore but adhering in chains, do not germinate in as high proportion as separate conidia from the same sample; also, the longer the chain, the lower the percentage of germination. In chains it is usually the end conidia that germinate, not those in intercalary position. These observations are discussed in the light of the theory previously-put forward by the author to explain the germination of powdery mildew conidia at low humidity.The apparent osmotic pressures of the cell sap of conidia of two powdery mildews were determined by plasmolysis with potassium nitrate. For Erysiphe Polygoni, the figure obtained was 63 atm., and for E. graminis Hordei Marchal, 68 atm. These figures are higher than any previously recorded in the literature seen by the author. The significance of this high osmotic pressure is discussed.Study of the papillae that regularly appear on the end of conidia of E. graminis indicates that they are special structures with a special function and should not be considered as germ pores. It is suggested that the conidium wall is two-layered, at least in the region between adjacent conidia. Rupture of the outer part of the wall in a ring at the septum may serve to disjoin the conidia and form the papilla.