The results of recent experiments at Penn State University's Mushroom Research Center support those of previous studies that dry bubbles form when Verticillium infestation occurs in the early stages of sporophore development. When inoculum is introduced to the developing primordium, or even prior to pin formation (Figure 1), the maturation process is interrupted with either no further development or the appearance of shapeless tissue mounds known as dry bubbles within 12 – 22 days (Figure 2). If the developing sporophore is infested (Figure 3), necrotic lesions and stipe blowout occur as development continues. Both symptoms are visible within 4 – 9 days of Verticillium spore introduction (Figure 4). If infestation occurs later in sporophore development, dark brown spotting of the pileus is visible within 4 days (Figure 5). If Verticillium spore exposure occurs just prior to harvest time, the mushrooms could be symptomless at the time they are picked. However, the symptoms continue to develop and often render the product unmarketable by the time it reaches the retailer.
Figure 1. Infestation Prior to Pin Development, or of the Developing Primordium.
Prior to Pin Development Developing Primordium
Figure 2. Development of the Dry Bubble Symptom after Early Infestation.
12 Days after Infestation 14 Days after Infestation 22 Days after Infestation
Figure 3. Infestation of the Developing Sporophore.
Infestation 7 Days after Pin Initiation
Figure 4. Development of Necrotic Lesions and Stipe Blowout within 4 - 9 Days.
4 Days after Infestation 9 Days after Infestation
Figure 5. Infestation 12 Days after Pin Initiation and Cap Spotting within 4 Days.
Infestation 12 Days after Pin Initiation 4 Days after Infestation