Blue Mold Rot
Janna Beckerman, Associate Professor of Plant Pathology, Department of Botany & Plant Pathology, Purdue University
Blue mold of bulbs is a fungal disease caused by Penicillium spp. (Fig. 1). Penicillium is a common soil mold best known for producing penicillin, the antibiotic many people use over the course of their lives to kill bacteria. Penicillium is often found growing on dead or dying plant debris. In its process of reproduction, it produces millions of blue-green spores (Figure 2). This fungus is closely related to the fungus commonly found on moldy green oranges in the fruit drawer of the refrigerator (you know, the ones you forgot you had in there!) and your Roquefort cheese.
Although some species of Penicillium (like P. roqueforti) are enjoyed in cheese, Penicillium mold occurs on stored bulbs, on the outer bulb scales that may have suffered previous damage (Figure 3). When injured, lily bulbs (which are in a dormant state) can be easily infected and invaded by the fungus, which grows and produces a fuzzy blue-green patch, possibly covering the bulb. This fungus thrives in cool, humid conditions, and can spread throughout the bulb, or between bulbs stored in close proximity to each other. In rare instances, it can kill the bulb (usually, in concert with other fungi or bacteria).
For most plants, the Penicillium fungus persists on the outer scales, and causes no problem upon planting. It simply spreads throughout the soil, and is held in check by all the other soil microbes. This disease isn’t limited to lilies, but any plant that is propagated by bulb, corm, rhizome, or tuber, including freesias, tulips, daffodils, gladiolas, and others.
Blue mold on a bulb is rarely a cause for concern: Plant the bulb, and let all the other microbes in the soil do their job keeping the Penicillium in check. In a few months, barring any other problems, your lilies with the blue mold will be indistinguishable from any other lilies.
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Figure 1. Blue mold on lily bulb. Photo by Janna Beckerman
Figure 2. Micrograph of Penicillium. From: http://top-10-list.org/2009/10/11/ten-types-importan-fungus/penicillium-roqueforti/
Figure 3. Close up of lily bulb with blue mold. Photo by Janna Beckerman.