J_ Phyropathology 123, 233—243 (1988j © 1988 Paul Parey Scientific Publi.sht'rs, Berlin and Hamburg ISSN 0931-1785 Faculty of Agricultural Production, Maejo Institute of Agricultural Technology, Chiang Mai, Thailand Institut for Sporeplanter, Copenhagen, Denmark Danish Covernment Institute of Seed Pathology for Developing Countries, Hellerup, Denmark The Seed-borne Inoculum of Peronospora manshurica, Causal Agent of Soybean Downy Mildew U-TAJ ROONGRLANGSREE, L. W . OLSON and L, LANGE Author.s' addresses: U-TM KotiNCRUANGSRKF, faculty of Agricultural Production, Maejo Institute of Agncultural Technology., Chiang Mai (Thailand). L.. W. OLSON", lnititut tor Sporeplanter, 0 . farimagsgadc 2D, 1353 K0benha\n K (Denmark). L. LANGI:, Danish Ga\ernment ln&titute of Seed Pathokigv for Developing Countries, Ry\'angs Aile 78, DK-2900 Hellerup (Denmark). Send correspondence and reprint requtrsts to Dr. LE^NE; LANGL. NOVO BioKonirol, No\-o Industri A-'S, Novo Alle, 28S3 Bagsv3:rd (Denmark). Viith 14 figures Received July 27, 1987; accepted Soz'cmher 23_. 1987 Abstract The seed-borne inoculum ot P. manshunca is described using various microscopic techniques and staining reactions. The inoculum, in addition to the oospores. consists of a thm walled m"\'celium under the hourglass cell layer of the .seed coat and a thick walled resting m\ celmm m the oospore crust on the seed surface. By a modified Feulgen stammi; technique it was possible to demonstrate that the thin walled and the tliick wailed resting m)'celium as w ell as thin and thick •^'^•alled oogonia contam nuclei which are Feulgen positive suggesting that portions of the m) celium of P. manshunca may survive even on the surface oi dry, mature seeds. Zusammenfassung Samenburtiges Inokulum von Peronospora manshurica^ Verursacher des FaJschen Mehltaus be! Sojabohnen Das samenburtige Inokulum von Peronospora manshunca wird an Hand von \erschiedenen mikroskopischen Methoden und Fiirburigsreaktionen beschrieben. Das Inokulum, zuziiglich der Oosporen, besteht aus diinnwandigem Myzel, das sich unterhalb der Uhrglaszellenschjcht der Samenhiille betindet, und ein dickwandiges, ruhendes Myzel in der Oosporenkruste an der Samen- obcrflache. Mit einer niodilizlerten P'eulgen-Farbungstechnik wurde es moglich zn zeigen, dais das dunn- und dickwandige^ ruhende Myzel wje aucb die dunn- und dickwandigen Oosporen Kerne enthalten, die mit Feulgen positiv reagieren. Diese Beobachtung iafit ^ermuten. da!^ Teile des P. manshu-rica-Myzeh auch an der trockenen Oberflache reifer Samen iiberleben konnen. U.S.Copvri^htCk.r.nccCeme.aKk-Su,em«n; 0 9 3 1 - 1 785./88/2303-0233S02.50./0