Longevity of Conidia of Peronospora tabacina Adam.

Abstract

DURING the three years ending 1961, Peronospora tabacina Adam. has spread from north-western Europe east to the U.S.S.R. and Turkey and south to Algeria and Tunisia. The evidence¹ indicates that this extensive distribution has been brought about by dissemination of the thin-walled, almost hyaline conidia that are produced in very large numbers. I have found up to 750,000 conidia per cm.² of infected leaf surface, while Corbaz² records twice this number. The effectiveness of conidia in initiating infections in previously free areas depends, inter alia, on their ability to survive during transfer. Survival (viability) on glass slides to which water is afterwards added to obtain germination is favoured by low temperatures and low relative humidities³, but with this procedure germination is extremely variable. More recently, Cruickshank⁴ and Shepherd⁵ have shown that reliable data on germination of conidia can be obtained on agar.