Tobacco Barn Rots
Barn rot is a common issue in flue-cured tobacco and can cause major losses during curing. Affected leaves blacken, break down quickly, and do not cure properly. The disease can be caused by either bacteria or fungi, both of which thrive under warm, humid conditions. Decay usually appears on infested leaves during the initial yellowing phase of curing, when humidity is high and temperatures are around 30–35 °C (86–95 °F). Both bacteria and fungi are inactivated once curing progresses to higher temperatures.
Bacterial Barn Rot (Figure 1)
- Caused mainly by Pectobacterium spp., the same pathogen responsible for soft rot in the greenhouse and hollow stalk in the field.
- Rot often begins at the midrib and spreads into the lamina, causing soft, watery rot of the leaf tissue.
- Bacterial pathogens can grow up to 37 °C (98.6 °F).
Fungal Barn Rot (Figures 2–3)
- Caused by fungi such as Rhizopus, Mucor, and Aspergillus species.
- Rhizopus and Mucor form fluffy white growth with black-tipped sporangia.
- Aspergillus may cause colorful, powdery molds.
- Other fungi, including Alternaria, Phoma, and Cercospora, may also contribute and can produce dark spores.
- Fungal pathogens can grow up to 45 °C (113 °F).
