DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Damping-off is a disease of seedlings raised in overcrowded, overwatered and cool conditions. The causal agent is one of a variety of fungi; the most frequent being the Oomycete, Pythium ultimum, (Kingdom Chromista). The infection is initiated by motile zoospores in water films within the soil. The symptoms are the sudden collapse of seedling plants followed by their complete destruction. An attack starts locally but quickly spreads under the favoured conditions of overcrowding. The picture shows young plants of lettuce (Lactuca sativa) grown closely together in a small tray, sold for home use as salad leaves. Encourage by storage on a cool windowsill in November in the UK, damping-off has affected most of the foliage visible.The small off-white particles at the ends of thin hyphal outgrowths are zoosporangia. These germinate to zoospores when they contact water.
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