Potatoes are a critical staple and cash crop for many smallholder farmers in Kenya. Yet yields remain far below their potential: while Kenyan potato farms could produce 30–40 tonnes per hectare under ideal conditions, recent figures report average yields of only 8–10 t/ha in many regions. A major culprit is Phytophthora infestans, the pathogen that causes late blight. This disease can destroy fields entirely if left unchecked, and farmers often rely heavily (and sometimes exclusively) on synthetic fungicides in efforts to control it.

However, reliance on pesticides alone has led to a growing problem: pesticide (fungicide) resistance. This newsletter explores how resistance develops, why it is becoming an urgent problem in Kenya, and what farmers, extension agents, researchers and policy makers can do to mitigate it.

How Pesticide Resistance Develops: The Science Behind the Problem

Resistance in pathogens occurs when a chemical control (fungicide, pesticide) no longer kills or suppresses a large proportion of the target organisms. Key processes by which resistance develops include:

1. Genetic variation in pathogen populations. Phytophthora infestans populations are genetically diverse. Some individuals may carry mutations that confer tolerance or resistance to a fungicide’s mode of action.
2. Selection pressure: When a fungicide is used heavily (or repeatedly), sensitive individuals are killed, while those with resistant traits survive and reproduce. Over time, resistant genotypes come to dominate.
3. Overuse of a single mode of action: If farmers use the same chemical class repeatedly, the evolutionary pressure is very strong. Alternation or mixing of different active ingredients with different modes of action slows this selection.
4. Pathogen biology and environment: Late blight thrives in cool, moist, humid aerial conditions (heavy dew, rainfall) – conditions found in many highland potato-growing regions of Kenya. These conditions favor rapid pathogen reproduction, spread, and multiple cycles within a season – giving more opportunities for resistant individuals to increase. Farm practices: Use of farm-saved seed that may carry infection, monoculture (planting the same variety or crop repeatedly), poor field sanitation (leaving debris or infected material) all contribute to maintaining large pathogen inoculum and increasing exposure to fungicides, thus accelerating resistance.

Late Blight Resistance Issues in Kenya: What the Studies Show

In Kenya:

• Many potato farmers rely on just one fungicide class. In Nyandarua County, about 65.8% of farmers report using a single fungicide type
• Fungicide applications are frequent; in wet seasons, spraying may occur weekly or more. This intensifies selection pressure. Awareness of biological control and Integrated Disease Management practices is still low: only about one-third of farmers are aware of or use them
• Varieties commonly planted (e.g. Shangi) are moderately susceptible; resistant or more tolerant varieties (e.g. Asante, sherekea) are available but uptake remains limited.

Strategies to Mitigate Pesticide Resistance in Late Blight Management

To address pesticide resistance, a multi-pronged approach is needed. Below are strategies to mitigate pesticide resistance in potato production.

1. Varietal resistance / Genetic improvements
   • Use of potato varieties that carry resistance genes (or are bred for resistance) reduces dependence on fungicides. For example, trials with hybrid potatoes with double-stack resistance performed well—even without spraying—producing yields far above the average under no-spray conditions.
   • Kenya is developing GM potato varieties with multiple resistance genes introduced into popular local varieties (Shangi, Tigoni, Asante). Though regulatory review is still underway, these promise durable resistance and lower fungicide use
2. Diverse fungicide use & rotation
   • Rotate fungicides with different active ingredients and modes of action rather than relying on a single class, to reduce selection pressure. Studies show many farmers do not alternate, which hastens resistance development
   • Mix fungicides (within regulatory allowances) or use formulations with more than one active ingredient, where feasible and safe.
3. Integrated Disease Management (IDM) / Cultural practices
   • Good agricultural practices: field sanitation (removing infected debris), timely destruction of crop residues, avoid growing potatoes repeatedly in the same plot, intercropping or rotation with non-host crops. These reduce the base inoculum. Use certified disease-free seed material rather than farm-saved seed, to minimize spread of infected tubers
   • Adjust planting dates to avoid peak inoculum pressure if local climate and rainfall patterns allow.
4. Biological control and alternative approaches
   • Use of fungal antagonists (e.g., Trichoderma spp.) and plant extracts (e.g. neem, local botanicals) has shown promise in Kenyan field trials for suppressing late blight incidence and severity
   • Promote awareness and adoption of biocontrol agents, ensuring local availability and proving cost-effectiveness under local conditions.
5. Monitoring, early detection, and extension services
   • Pathogen monitoring: track which strains are present, and whether resistance to certain fungicides is emerging. This may involve local labs, extension staff, or research institutions.
   • Weather-based disease forecasting: since late blight outbreaks are strongly linked to temperature, humidity and rainfall, having early warning systems can enable timely fungicide applications (if needed) rather than reactive blanket spraying.
   • Farmer education & training: ensuring farmers understand correct dosage, spray intervals, safety, alternating fungicide classes, and cultural control practices.
6. Policy, regulation and seed systems
   • Support to release resistant varieties, whether bred conventionally or via biotechnology, and ensure they are affordable and adapted to farmer preferences (taste, maturity, storage qualities). Regulation of pesticide quality, ensuring that substandard or counterfeit fungicides are not contributing to resistance (e.g. by underdosing).
   • Incentives or extension support for adoption of integrated approaches, e.g. subsidies or credit for certified seed, or for new resistant varieties.

Pesticide resistance in late blight is not a distant threat—it is actively undermining potato yields in Kenya, increasing costs, threatening food security, and exposing farmers to health and environmental risks. But there is promise: through the adoption of resistant varieties, sound agricultural and cultural practices, diversification and rotation of fungicides, biological control options, robust monitoring, and supportive policy, Kenyan potato production can become more resilient.

The path forward will require collaboration: farmers, researchers, extension services, government bodies, and private sector must work in concert. It’s not enough to spray harder; we must spray smarter, combine tools, and build systems that anticipate resistance rather than reacting to it.

Figure 1: A picture showing potato plant infested by late blight