Greenhouse humidity is one part of Botrytis risk. To understand a problem, also consider the pathogen source and the plant tissue it can reach. University extension guidance describes different routes involving ornamental flowers, tomato stem wounds and infected plant debris. Those routes explain why lowering an air-humidity reading does not, by itself, show that a crop problem has been resolved.
How flowers can carry infection to leaves
In ornamental greenhouse crops, Botrytis can colonize dead, dying and wounded material, then attack healthy tissue. Infected floral parts may fall onto healthy leaves and provide a route for leaf blight. A spent flower can therefore matter as more than a sign of crop age: it can carry the pathogen to another plant surface. The Pacific Northwest Pest Management Handbooks describe this connection and recommend removing dead and dying material in closed containers to limit spore escape.
The handbook also distinguishes moisture conditions from dispersal: humid conditions favor sporulation, while changes in relative humidity can stimulate spore release. This is not a recommendation to maintain high humidity. It explains why changing the environment and managing colonized material address different parts of the problem.
Why tomato stem wounds deserve separate attention
For tomato, NC State Extension describes stem infections through leaf scars, cracks and pruning wounds during humid conditions. It also identifies fallen flowers as a common source of leaf infection. The fungus can survive in plant debris and organic matter, and greenhouse air currents can transport spores.
That makes the affected tissue an important part of the question. A stem mark beside a leaf scar calls for a different crop assessment from a damaged leaf beneath flowers. Humidity information helps describe the setting; it does not identify the cause of either mark. NC State's examples are tomato guidance, not a universal description of every greenhouse crop.
Look beyond the standing crop
For bedding plants, UConn Extension identifies infected plants and leaves on benches or floors as places where spores are produced. Flowers provide nutrients for the pathogen. Its guidance also draws attention to propagation under mist and to condensation during transport: the relevant environment may change as plants move through production.
When a humid greenhouse cools in the evening, condensation can form on plants. UConn therefore discusses leaf wetness, plant spacing, airflow and debris management together. Reading a room sensor alone leaves both the local surface conditions and the potential source material unexamined.
| Crop context | Mechanism described in extension guidance | Useful distinction |
| Ornamental flowers | Infected floral tissue falls onto healthy leaves | Contact pathway versus room humidity |
| Tomato stems | Leaf scars, cracks and pruning wounds provide infection routes | Tissue condition versus air condition |
| Bedding plants | Infected plants and fallen leaves can produce spores | Pathogen source versus moisture control |
The table summarizes the cited mechanisms; it does not diagnose symptoms. A qualified crop advisor can assess the affected tissue and decide whether diagnostic sampling is needed. For equipment context, see strawberry greenhouse dehumidification and tomato greenhouse dehumidification. Those discussions address climate and equipment inputs; disease interpretation still depends on the crop and its condition.
Related project questions
Separate the pathogen and exposure questions when investigating a crop problem. Continue with Botrytis warnings after cloudy weather and crop-specific downy mildew conditions. For the wider project context, return to agricultural humidity control.
