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Ecology and Epidemiology

Infection of Milk Thistle (Silybum marianum) Leaves by Septoria silybi. D. Moscow, Department of Plant Pathology, University of California, Berkeley 94720; S. E. Lindow, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 79:1085-1090. Accepted for publication 23 May 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1085.

Septoria silybi infected milk thistle (Silybum marianum) frequently when daylight was provided during high humidity inoculation periods but rarely when light was excluded, because of the pathogen’s requirement for open stomata to penetrate thistle leaves. When stomata were artificially closed with abscissic acid, S. silybi failed to penetrate even during daylight. Conversely, the application of fusicoccin opened stomata on thistle leaves kept in the dark and permitted penetration. Hyphae of S. silybi encountered stomata as frequently on milk thistle leaves held in the dark as on those exposed to daylight. Hydrotropism, the directed growth of germ tubes and branches toward stomata, was not exhibited by S. silybi. Superficial hyphae reached or crossed stomatal pores with similar frequency, irrespective of whether humidity gradients might have existed at such sites or not. S. silybi infected milk thistle after periodic (8 hr/day for 6 days) or prolonged (continuously for 2.5 days) postinoculation drying. The amount of disease occurring on plants exposed to periodic drying was comparable to those incubated continuously at high humidity. Severity of disease resulting from inoculation with a given number of spores of S. silybi increased proportionally with leaf age. Whereas S. silybi infected young and old leaves of milk thistle with similar efficiency, the rate of lesion expansion was 2.5 times higher on old leaves than on young leaves.