December 4, 2025
Kenya’s flower industry, is one of the most vulnerable to fungal plant diseases. Pathogens such as powdery mildew, botrytis, downy mildew, and black spot thrive in the high-altitude conditions ideal for rose production. As growers intensify protection measures to meet export standards, one challenge has become increasingly urgent: fungicide resistance.
Fungicide resistance occurs when a fungal population evolves to survive chemical control. This phenomenon, often driven by repeated use of the same mode of action, threatens the effectiveness of vital crop protection tools. To maintain the productivity and profitability of Kenya’s rose farms, growers must embrace a comprehensive resistance management strategy that integrates cultural, chemical, and biological approaches.
- Cultural Practices: The Foundation of Resistance Management
Healthy roses are less susceptible to disease. Cultural practices that suppress fungal populations form the cornerstone of any resistance management program. These include:
Crop hygiene – Remove and destroy infected leaves, buds, and prunings to reduce inoculum sources.
Ventilation and humidity control – Maintain proper air circulation in greenhouses to limit powdery mildew and botrytis.
Avoid continuous cropping – Break disease cycles by rotating rose varieties and sanitizing structures between crop cycles.
Optimized nutrition – Balanced fertilization promotes vigorous plants that can better withstand infection pressure.
Reducing disease pressure at the cultural level not only decreases the need for fungicide sprays but also slows the rate at which resistance develops. - Spray Only When Necessary: Preventive fungicide use has its place, but unnecessary spraying accelerates resistance and wastes resources. Decisions should be based on scouting data, weather forecasts, and disease risk models. Record disease trends and apply only when conditions indicate likely infection or historical data suggests high disease pressure.
- Target Hotspots
Most fungal infections in roses start in localized “hot spots” often humid microclimates within the greenhouse. Early detection and spot treatments in these areas can prevent spread, conserving product and minimizing selection pressure on the fungal population. - Use Protectant Fungicides Early
Apply protectant fungicides ahead of predicted high-risk periods, especially during cool, humid weather that favors powdery mildew or botrytis. Once these pathogens become established, they are far harder to control. In advanced infections, even systemic fungicides may fail to restore clean foliage or blooms.
Protectant programs should therefore form the backbone of every spray schedule, complemented by eradicants only when necessary.
- Restrict Use of Curative and Eradicant Fungicides
Curative fungicides, should be reserved for critical growth stages or periods of high disease pressure, such as during flush cycles or peak humidity months. Overuse of curatives increases the risk of selecting resistant strains.
Disease management is most efficient when fungicides are preventive rather than reactive.
- Rotate and Mix Modes of Action
-Every fungicide belongs to a mode of action (MOA) group, identified by its FRAC code. Continuous use of one group allows fungi to adapt and survive. To delay resistance:
Rotate fungicides with different FRAC codes in successive applications.
-Use tank mixes or pre-mixed formulations combining multiple MOAs (e.g., a protectant plus a systemic). Confirm compatibility before mixing to prevent reduced efficacy or phytotoxicity.
By varying exposure, growers reduce the selection pressure for resistant strains within the fungal population.
- Follow Label Rates and Recommendations
Always apply the recommended rate and ensure good spray coverage. Sub-lethal doses may suppress symptoms temporarily but allow partially resistant pathogens to survive and multiply. Similarly, underdosing or poor calibration can lead to apparent “failures” that mimic resistance.
A good practice is to regularly calibrate spray equipment, check nozzle performance, and monitor water quality to ensure optimal fungicide performance.
- When Control Fails, Is It Resistance?
Poor disease control does not always mean resistance. Factors such as spray drift, inadequate coverage, wrong timing, unfavorable weather, or blocked nozzles can all lead to failure. True resistance can only be confirmed through laboratory testing, a process that is time consuming and costly. Before assuming resistance, review spray records, product sequences, and application conditions. - Integrated Approach for the Rose Industry
Kenya’s rose sector must champion integrated resistance management. Two guiding principles summarize this approach:
Avoid repeated use of fungicides from the same mode of action group.
Combine chemical, cultural, and biological measures to maintain long-term disease control.
Biological fungicides and microbial antagonists can complement conventional products by reducing fungal pressure and extending the lifespan of chemical groups.
The Cost of Losing Chemistry
Developing a new fungicide molecule costs over $250 million and can take 10–12 years. With regulatory restrictions tightening globally, the loss of an active ingredient due to resistance may have no replacement. Protecting existing fungicides is therefore a shared responsibility across Kenya’s flower value chain from grower to exporter.
Fungicides are valuable allies, not silver bullets. In roses, sustainable disease management depends on smart rotation, cultural diligence, and integrated planning. By using fungicides judiciously and proactively, Kenya’s rose growers can protect both their crops and the future of one of the country’s most important export industries