White rot

Sclerotinium cepivorum

Leaves of plants infected with the white rot pathogen show yellowing, leaf dieback, and wilting. Leaf decay begins at the base, with older leaves being the first to collapse. A semi-watery decay of the bulb scales results. Roots also rot, and the plant can be easily pulled from the ground. Associated with the rot is a fluffy white growth, the fungal mycelium, which develops around the base of the bulb. As the disease progresses, the mycelium becomes more compacted, less conspicuous, with numerous small spherical black bodies (sclerotia) forming on this mycelial mat. These sclerotia, the resting bodies of the pathogen, are approximately the size of a pin head or poppy seed. Plants can become infected at any stage of growth, but in California, symptoms usually appear from mid-season to harvest.

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