French beans, also known as green beans or snap beans, are among the most profitable horticultural crops grown in Kenya. They are especially popular among smallholder and export-oriented farmers due to their high demand in local and international markets, particularly Europe.
French bean farming thrives in cool to warm climates, such as those found in parts of Kiambu, Murang’a, Nakuru, and Meru counties.
However, despite their potential for high returns, French beans are highly susceptible to diseases that can significantly reduce yield and market quality. Left unmanaged, these diseases can lead to severe economic losses, especially for farmers targeting export standards.
Understanding the most common diseases and their management practices is essential for any farmer seeking to succeed in French bean production in Kenya.
To improve crop resilience, using the best french bean fertilisers is highly recommended. These fertilisers provide essential nutrients that support plant health and enhance disease resistance.
For a full guide on growing, managing, and marketing this high-value crop, check out our article on French bean farming in Kenya.
This article highlights 10 key diseases every French bean farmer should know about—what causes them, how to identify them, and effective prevention strategies to ensure a healthy, market-ready crop.
Diseases Affecting French Beans
1. Anthracnose Disease in French Beans
Causal Agent:Colletotrichum lindemuthianum
What is Anthracnose?
Anthracnose is a fungal disease that affects French beans. This disease is especially common in cool and wet weather. It can affect the leaves, stems, pods, and even the seeds of the plant.
One of the main ways it spreads is through infected seeds. When these seeds are planted, they can introduce the disease into a healthy field, making it hard to control if not caught early.
Key Symptoms of Anthracnose
The first signs of anthracnose usually appear on the leaves as small, dark brown or black spots, sometimes with yellow edges. These spots can grow bigger and may join together, causing large dead patches. On stems and leaf stalks, the disease causes dark, sunken lines or streaks that can weaken the plant and make it wilt.
The pods show round, sunken spots with reddish-brown edges. If the weather is moist, you might see pinkish fungal growth in the centre of these spots. Infected seeds look shrivelled, discoloured, and may not grow well when planted.
Disease Cycle of Anthracnose
The fungus that causes anthracnose survives in infected plant remains and on seeds. When infected seeds are planted, the fungus spreads to the new plant. The disease can also move from one plant to another through rain splash, wind, dirty tools, or hands.
Wet conditions and cool temperatures (around 15°C to 22°C) help the disease grow and spread quickly. The fungus forms spores on infected areas, and these spores move to healthy parts of the plant, repeating the cycle.
Control and Management Measures
To manage anthracnose, farmers should start by planting clean, disease-free seeds. Treating seeds with a fungicide like Thiram or Captan can help protect them. It’s also important to rotate beans with other crops like maize to reduce the chance of infection.
After harvest, remove and burn infected plant remains to stop the disease from surviving in the field.
Avoid working in the field when plants are wet to prevent spreading the fungus. During wet seasons, applying fungicides such as Copper oxychloride or Mancozeb can help protect the crop. If the disease appears, stronger fungicides like Carbendazim may be needed. Using disease-resistant bean varieties, where available, also helps reduce the risk of anthracnose.
2. Angular Leaf Spot in French Beans
Causal Agent: Pseudocercospora griseola (formerly known as Isariopsis griseola).
What Is Angular Leaf Spot?
Angular Leaf Spot is a fungal disease that commonly affects French beans, especially in warm, humid environments. It targets the leaves and pods, causing blemishes that lower the crop’s aesthetic quality and market value.
If not controlled early, the disease can spread rapidly through the field, leading to premature defoliation and reduced yields.
Key Symptoms on Leaves and Pods
The most distinctive symptom of Angular Leaf Spot is the presence of small, angular-shaped spots on the leaves. These spots usually start off as water-soaked lesions and later turn brown or reddish-brown with a yellow halo. Because the fungus develops between the veins of the leaf, the lesions often appear sharply defined in a geometric, angular shape.
On the pods, the disease causes brown to black spots that may become sunken. Infected pods may crack open, especially under dry conditions, which can expose the seeds to secondary infections. Severe infection often leads to leaf drop and poor pod development.
Disease Cycle of Angular Leaf Spot
The fungus survives between growing seasons in infected crop debris and on seeds. During wet and humid conditions, the spores are spread through splashing water, wind, and contaminated tools or hands. Once the spores land on a susceptible plant, they germinate and infect the tissue, especially if leaves stay wet for long periods.
Warm temperatures (around 24°C to 28°C) combined with high humidity or rainfall create ideal conditions for infection and rapid disease spread. Dense plantings also exacerbate the problem by reducing airflow, which keeps the canopy wet for longer.
Control and Prevention measures
Controlling Angular Leaf Spot begins with good farm hygiene and cultural practices. Always plant certified, disease-free seeds and avoid planting beans in the same field consecutively—rotate with cereals or other unrelated crops to reduce pathogen buildup in the soil.
Proper field spacing can improve air circulation and reduce leaf wetness. Irrigate early in the day using drip systems to allow leaves to dry quickly. Remove and destroy all crop residues after harvest to eliminate fungal survival sources.
Chemically, fungicides such as mancozeb, chlorothalonil, or copper-based products can be applied preventively or at the first signs of infection. Always follow the manufacturer’s instructions and alternate fungicide classes to avoid resistance development.
3. Rust(Bean Rust) Disease in French Beans
Causal Agent: Uromyces appendiculatus.
What Is Rust?
Rust is a common fungal disease that affects French beans at any stage of growth. It is recognised for its reddish-brown pustules that appear on both sides of the leaves, stems, and pods. If left uncontrolled, rust can cause significant defoliation, reducing photosynthesis, weakening the plant, and ultimately lowering yields and bean quality.
Symptoms of Rust
The earliest sign of rust is the appearance of small, whitish spots on the underside of leaves. These spots soon develop into characteristic reddish-brown pustules that may later darken and burst, releasing powdery spores. As the infection progresses, leaves turn yellow, wilt, and eventually fall off. In severe cases, infected pods may develop spots and become deformed or unmarketable.
Rust symptoms tend to appear first on older leaves before spreading upward. Affected fields often show uneven growth, and plants become stunted as they lose foliage and vigour.
Disease Cycle of Rust
The fungus survives in crop debris and can also be seed-borne. Spores are easily spread by wind, water splash, human activity, and equipment. Rust outbreaks typically occur in areas with warm temperatures (18°C to 26°C) and high humidity, especially when there’s frequent rain or heavy dew. Crowded fields with poor air circulation are particularly susceptible to rapid disease development.
Control and Prevention Measures
Integrated control is key to managing rust effectively. Start by practising crop rotation and avoiding planting French beans near previously infected fields. Choose resistant varieties when available, and maintain proper plant spacing to improve airflow and reduce humidity within the crop canopy.
Remove and destroy infected plant residues after harvesting to prevent fungal carryover. Avoid working in the field when plants are wet to minimise the risk of spreading spores.
For chemical control, apply fungicides such as mancozeb, chlorothalonil, or triazole-based fungicides at the first sign of symptoms. Reapply at regular intervals during favourable conditions, following label recommendations carefully. Alternating fungicides with different modes of action will help prevent the development of resistant fungal strains.
4. Powdery Mildew in French Beans
Causal Agent: Erysiphe polygoni (also known as Erysiphe polygoni var. phaseoli).
What Is Powdery Mildew?
Powdery mildew is a fungal disease that affects French beans by forming a white, powdery growth on the surfaces of leaves, stems, and pods. It’s one of the few fungal diseases that thrives in dry conditions with high humidity, making it a serious threat during dry spells with cool nights.
While rarely fatal, it weakens plants, reduces photosynthetic activity, and lowers both yield and pod quality.
Key Symptoms of Powdery Mildew
The most distinctive symptom is the appearance of white to greyish powdery spots on the upper and lower surfaces of leaves. These spots may start small but can rapidly spread to cover large areas, eventually affecting stems and pods. Infected leaves may curl, dry up, and fall off prematurely.
As the disease progresses, the plant’s growth is stunted, and pod formation is poor. Pods that are infected often appear deformed and may have a dusty white coating, reducing their market value, especially in export markets that demand flawless produce.
Disease Cycle of Powdery Mildew
Unlike many fungal pathogens, powdery mildew does not require free water to infect plants. Instead, it thrives in conditions of high humidity and moderate temperatures (20°C–27°C), especially when days are warm and nights are cool. The fungus survives in crop debris or on alternate host plants.
Spores are easily dispersed by wind and can infect healthy plants quickly, especially under dry, dusty conditions with intermittent humidity.
Control and Prevention Measures
Start with preventive cultural practices such as using resistant varieties and ensuring proper spacing between plants for air circulation. Remove and destroy crop residues after harvest to eliminate overwintering sources of the fungus. Avoid excess nitrogen fertilisation, which encourages lush, susceptible foliage.
5. Root Rot Disease In French Beans
Causal Agents: Rhizoctonia solani, Fusarium spp., and Pythium spp.
What Is Root Rot?
Root rot is a collective term for several soil-borne fungal diseases that attack the roots of French bean plants, often resulting in stunted growth, yellowing leaves, and even plant death. The disease is particularly devastating in poorly drained soils and fields with a history of continuous bean cultivation. It primarily affects seedlings and young plants, but mature plants can also be infected under favourable conditions.
Key Symptoms of Root Rot
The most common symptoms include wilting of the plant during the hottest part of the day, yellowing of lower leaves, and stunted growth. Upon uprooting affected plants, the roots appear brown, mushy, or completely decayed. The outer root tissues may peel off easily, revealing a rotting core. In some cases, reddish-brown lesions form on the stem base (especially with Rhizoctonia solani), leading to seedling damping-off shortly after germination.
In severe infections, entire sections of a field may show plant dieback, significantly reducing stand density and eventual yield.
Disease Cycle of Rot Root
Root rot pathogens persist in the soil or infected crop residues for several years and are favoured by wet, compacted, or poorly drained soils. Warm soil temperatures (between 20°C–30°C) combined with high moisture levels create ideal conditions for these pathogens to infect the roots. Over-irrigation, poor soil structure, and the use of infected seed can increase the risk of an outbreak.
These fungi often infect roots through wounds or natural openings, and spread slowly but steadily through the soil, especially when fields are continuously planted with French beans or other legumes.
Control and Prevention Measures
Managing root rot requires a combination of good soil management and crop hygiene. Begin by planting in well-drained soils and avoid areas with a known history of root rot. Practice crop rotation with non-host crops such as maize or cereals for at least 2–3 seasons. Improving soil structure by incorporating organic matter (like compost) and practising minimum tillage helps reduce compaction and enhance drainage.
Use certified, disease-free seeds and treat them with appropriate fungicides before planting to protect against early root infections. Avoid overwatering, especially during the seedling stage. Raised beds can help improve drainage in susceptible areas.
6. Halo Blight Diseases in French Beans
Causal Agent: Pseudomonas syringae pv. phaseolicola
What Is Halo Blight?
Halo blight is a serious bacterial disease of French beans that affects leaves, stems, and pods. It spreads rapidly under cool, moist conditions and can cause widespread damage if not managed early. This disease is seed-borne and is especially destructive in areas with frequent rain or high humidity.
Symptoms of Halo Blight
The first symptoms appear as small, water-soaked spots on the underside of the leaves. These spots soon enlarge and become surrounded by a pale green or yellow “halo,” which gives the disease its name. As the infection worsens, the spots darken and coalesce, leading to large necrotic patches and eventual leaf drop.
On pods, lesions appear as greasy, sunken spots that may exude a sticky bacterial ooze. These blemishes reduce the market quality of the beans and can cause secondary infections. Infected seeds may appear shrivelled or discoloured and can act as a source of infection for the next crop.
Disease Cycle of Halo Blight
Halo blight is primarily spread through infected seeds. The bacteria survive in crop debris and can be transmitted by rain splash, irrigation water, wind, tools, and even human contact. Cool temperatures (15°C to 22°C) combined with high humidity or leaf wetness create ideal conditions for the disease to develop and spread rapidly.
Entry into the plant occurs through natural openings or wounds caused by handling, pests, or hail.
Control and Prevention Measures
Start by planting certified disease-free seeds, as seedborne infection is the most common way the bacteria are introduced into the field. Avoid working in the field when plants are wet to prevent mechanical spread. Implement a 2–3 year crop rotation with non-leguminous crops to break the disease cycle.
Ensure adequate spacing between plants to improve air circulation and reduce humidity within the canopy. Promptly remove and destroy infected plant material and debris after harvest.
7. Bacterial Brown Spot
Causal Agent: Pseudomonas syringae pv. syringae
What Is Bacterial Brown Spot?
Bacterial brown spot is a widespread disease affecting French beans and is often confused with halo blight due to its similar appearance. However, it tends to occur under slightly warmer conditions and can cause significant damage to leaves and pods, especially in high rainfall or irrigated areas. The disease reduces yield and makes pods unmarketable due to scarring.
Symptoms of Bacterial Brown Spot
Initial symptoms appear as small, water-soaked spots on leaves, which gradually enlarge and turn brown. These spots often have a yellow halo surrounding them, though this is usually less prominent than in halo blight. The lesions can merge to form large necrotic areas, causing leaf distortion and premature drop.
On pods, brown, slightly sunken spots develop, which may crack open, allowing secondary fungal infections. These lesions make the pods look unattractive, significantly reducing their market value. Infected seedlings may also fail to thrive or become stunted.
Disease Cycle of Bacterial Brown Spot
Bacterial brown spot is seed-borne and also survives in plant debris and contaminated tools. It spreads primarily through rain splash, irrigation, and physical contact. Warm, wet conditions (20°C–30°C) and high humidity favour the rapid multiplication and spread of the bacteria. Wounds or natural openings on the plant provide easy entry points.
Unlike fungal pathogens, the bacteria do not require spores for dispersal, making the disease more dependent on surface water and physical spread.
Control and Management Measures
The best way to control bacterial brown spot is through prevention. Use certified disease-free seed and avoid planting beans in areas previously affected by the disease. Practice a crop rotation system of at least two years with cereals or grasses to reduce bacterial buildup in the soil.
Avoid overhead irrigation if possible, or water early in the day so foliage can dry quickly. Maintain good field hygiene by disinfecting tools and avoiding unnecessary handling of wet plants.
8. Bean Common Mosaic Virus (BCMV) in French Beans
Causal Agent: Bean common mosaic virus (a Potyvirus)
What Is Bean Common Mosaic Virus?
Bean Common Mosaic Virus (BCMV) is one of the most damaging viral diseases of French beans, affecting both yield and pod quality. It is prevalent in areas where susceptible varieties are grown and where aphid populations are high. BCMV can be seed-borne or transmitted by aphids in a non-persistent manner, making it difficult to control once established.
Symptoms of BCMV
Infected plants display a characteristic light and dark green “mosaic” or mottled pattern on the leaves. Other symptoms include leaf curling, stunted growth, and bushy appearance due to shortened internodes. Some plants may also exhibit blistering or malformation of the leaves.
Pods on infected plants are often deformed, poorly filled, or do not form at all. If infection occurs early, it can lead to total crop loss. Symptoms vary depending on the strain of the virus and the age of the plant at infection.
Disease Cycle of BCMV
BCMV is primarily seed-borne, which allows it to spread between seasons. Aphids, particularly Aphis fabae and Myzus persicae, act as vectors, spreading the virus from infected to healthy plants as they feed. The virus does not persist in the aphid, meaning that it is quickly acquired and transmitted, but does not remain in the insect for long.
The disease is favoured by the presence of infected seed and aphid activity, especially in warm weather.
Control and Management, and Measures
The most effective method of controlling BCMV is using certified virus-free seed and growing resistant or tolerant French bean varieties. Regular monitoring and early roguing (removal) of symptomatic plants can help reduce the spread.
Controlling aphid populations through insecticides or reflective mulches can also minimise transmission. However, because the virus is transmitted very quickly by aphids, insecticide control must be proactive rather than reactive.
9. Cercospora Leaf Spot Disease in French Beans
Causal Agent: Cercospora Leaf Spot is caused by fungi in the genus Cercospora.
What is Cercospora Leaf Spot
This is a fungal disease that commonly affects French bean leaves and sometimes stems. It thrives in warm, humid conditions and causes spots that damage leaf tissue, reducing the plant’s ability to carry out photosynthesis and weakening overall growth.
Key Symptoms of Cercospora Leaf Spot
The disease causes small, round to irregular brown spots with a lighter centre and sometimes a yellow halo around them. These spots usually begin on older leaves and can grow larger, merging to create big dead areas. Severely infected leaves dry up and fall off prematurely, which lowers the plant’s vigour and pod production.
Disease Cycle in Cercospora Leaf Spot
The fungus survives in infected plant debris and on seeds. Spores spread to healthy plants through wind and rain, especially in humid environments. The fungus invades leaf tissues, producing new spores that continue spreading throughout the growing season, particularly during wet weather.
Control and Management Measures
Use clean, disease-free seeds and rotate crops with non-host plants to interrupt the disease cycle. Remove and destroy infected plant residues after harvest to reduce fungal sources. Fungicides such as Mancozeb or copper-based compounds can protect plants when applied early.
Proper spacing to improve air circulation and good field hygiene also helps reduce humidity and disease development.
10. Fusarium Wilt in French Beans
Causal Agent: Fusarium wilt is caused by the fungus Fusarium oxysporum f. sp. phaseoli. This is a soilborne pathogen that attacks the water-conducting tissues of French beans.
What is Fusarium Wilt
Fusarium wilt is a serious disease that affects the roots and internal tissues of French beans. It enters the plant through the roots and spreads upward, blocking the flow of water and nutrients. This causes the plant to wilt, turn yellow, and often die.
The disease is more severe in warm soils and can survive in the soil for many years, making it hard to eliminate once present.
Key Symptoms of Fusarium Wilt
Symptoms usually begin with yellowing of the lower leaves. The leaves may wilt during the day but seem to recover at night, especially in the early stages. As the disease progresses, wilting becomes permanent, and the whole plant may collapse.
A clear sign of Fusarium wilt is brown discolouration inside the stem when it is cut open, showing the fungus in the plant’s vascular system. Infected plants are often stunted and produce fewer or deformed pods.
Disease Cycle of Fusarium Wilt The fungus survives in the soil as resting spores, which can live for years even without a host. It infects plants through the roots, especially in warm, moist conditions.
Once inside, it travels through the xylem, producing toxins and blocking water movement. The disease spreads through infected soil, water, tools, and seeds. It becomes more common when beans are grown repeatedly in the same field without rotation.
Control and Management Measures Managing Fusarium wilt begins with using resistant bean varieties that can tolerate the disease. Crop rotation is also important; rotating with non-legume crops like maize for several years can help reduce the fungus in the soil.
Removing and destroying infected plants after harvest helps to stop the disease from spreading. Good soil drainage and avoiding overwatering can also limit infection.
Using clean seeds and disinfected tools reduces the chances of spreading the disease. In some areas, soil solarisation—covering the soil with clear plastic during hot months—can reduce fungus levels in the upper soil layers. Since chemical control is not effective once the plant is infected, prevention is the best approach.
