DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
A opening bud of Rosa gallica versicolor (Rosa mundi), infected by the powdery mildew, Sphaerotheca pannosa. The picture shows an infected flower stalk and calyx (above centre) covered with the white mycelium of fungal hyphae. In the background, below, the fungus is visible the upper surface of leaves. The tiny dark particles on the stalk below the flower are glandular hairs of the rose within the mycelium. The infection has reached the lowest petal (above centre, right, patchy red). Powdery mildews (members of the Erisyphales) are widespread and can cause substantial commercial losses to crops such as cereals and soft fruit such as gooseberries. S. pannosa is disfiguring and weakens infected roses.It requires humid conditions for its spores to germinate, but paradoxically is more destructive in hot dry summers. Rosa mundi is a colour sport of R. gallica, first recorded in 1583.
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