Bud rot, caused primarily by the fungus Botrytis cinerea, is a highly destructive plant disease that targets the flowering parts of a plant. This necrotrophic pathogen, also known as gray mold, is widespread and can infect over a thousand plant species, leading to significant crop losses globally. It is the most common and devastating fungal issue for flowering plants because it attacks the most valuable part of the harvest. Immediate action is necessary upon detection, as the infection spreads rapidly, capable of destroying a plant in under a week under favorable conditions.
Accurate Identification of Bud Rot
A Botrytis infection often begins subtly, making early detection difficult before the mold progresses significantly. The fungus typically enters the plant through wounds or natural openings and establishes itself deep within the dense structure of the flower. Initial symptoms may include small, water-soaked lesions on leaves or stems, with the first visible sign on the flower being the yellowing of a small leaf bract emerging from the bud.
If left unchecked, the infected area of the flower will feel softer than the surrounding healthy tissue and will begin to display a gray or brown discoloration. To confirm a systemic infection, a gentle pull test on the yellowing leaf bract can be performed. If the leaf pulls out easily, revealing a dark, soft, and decaying base inside the bud, it confirms the presence of internal rot.
As the disease matures, the affected areas become covered with a dense, fuzzy layer of gray or brownish mold. This mold is the pathogen sporulating and releasing airborne spores, which are the primary mechanism of spread to nearby healthy plants. The infected bud tissue will become mushy and necrotic, often emitting a musty, unpleasant odor.
Immediate Action for Containment and Removal
Once bud rot is confirmed, the infected area must be isolated and removed immediately to prevent widespread spore dispersal. The infected plant or branch should be carefully moved away from healthy plants before any cutting begins. All tools used for removal, such as scissors or pruning shears, must be sterilized between cuts by wiping them with a disinfectant like 70% isopropyl alcohol.
The physical removal process requires cutting well past the visible margin of the soft or discolored tissue, ensuring that no signs of internal browning remain in the remaining stem. Handle the infected material gently, ideally encasing the affected bud in a small plastic bag before cutting it off, to minimize the release of spores into the air. All infected material must be sealed and removed from the growing area entirely.
Following the removal of the infected plant material, the surrounding growing environment requires sanitation. Spores can settle on walls, floors, and equipment, leading to re-infection if not addressed. Wiping down all nearby surfaces and tools with a cleaning solution is necessary to reduce the fungal load.
Environmental Management to Halt Spread
The most effective strategy for preventing the spread and recurrence of bud rot involves strict control over the environmental conditions that favor fungal growth. The Botrytis cinerea fungus requires a film of moisture for its spores to germinate and infect the plant tissue. Therefore, managing relative humidity (RH) is the most important factor.
During the flowering stage, when buds are dense and prone to trapping moisture, the relative humidity should be maintained below 50%, with 40–50% being ideal. This low humidity level actively discourages the germination of fungal spores. Dehumidification is especially important during the lights-off period when temperatures drop and RH naturally spikes, often creating conditions for rot development overnight.
Airflow is equally important for preventing the formation of stagnant, moist microclimates. Oscillating fans should be used to ensure continuous air movement, even pointing directly at dense buds to prevent moisture from condensing. Plant management techniques, such as pruning excess interior foliage or defoliation, help to improve air penetration and light exposure.
Temperature management also plays a role, as the fungus thrives particularly well in cool, damp conditions, with optimal growth occurring between 59°F and 77°F (15°C to 25°C). Maintaining stable temperatures and avoiding large fluctuations, especially cool, wet nights, reduces plant stress and the likelihood of condensation. Additionally, irrigation practices should minimize water accumulation on the plant surface, ensuring the canopy is dry before the lights turn off.
Post-Treatment Safety and Disposal
The disposal of all contaminated material is a safety measure to prevent the pathogen from returning to the growing environment. Infected buds, leaves, and stems must be sealed securely in plastic bags before being discarded with household waste. Never attempt to compost plant material infected with Botrytis, as most home composting systems do not reach the sustained high temperatures required to kill the fungal spores, allowing the disease to persist and spread.
Consumption or processing of material infected with mold poses a significant health risk due to the potential presence of mycotoxins. These are toxic secondary metabolites produced by various fungi, including those that cause bud rot, which can cause illness if ingested. Even if a portion of the plant appears healthy, if it was near a heavily infected area, it may be contaminated with these unseen compounds.
For the remaining healthy harvest, careful post-harvest handling is necessary to prevent rot from developing during the drying and curing process. Ensuring that the harvested material is dried sufficiently reduces the water activity below 0.7, a level that prevents most fungal growth. Proper airflow and humidity control must continue throughout the drying and curing stages to safeguard the final product against late-stage mold development.