Crop Health
August 12, 20269 min readLate blight pressure is rising early this season
How Krawp helps growers spot disease before it crosses the row.
Dr. Amina Osei
Head of Plant Health, Krawp
Why this season is different
Late blight — caused by the oomycete Phytophthora infestans — is the same pathogen that triggered the Irish Potato Famine in the 1840s. Nearly two centuries later, it remains one of the most destructive crop diseases worldwide, capable of destroying an entire tomato or potato field within 7–10 days under favourable conditions.
Historically, late blight peaks in mid-to-late summer across most tropical and subtropical growing regions. But this year, conditions have shifted the timeline significantly. Warmer, wetter springs — driven by shifting rainfall patterns across East and West Africa — have created a perfect incubator for early spore release. Growers who typically begin scanning for symptoms in July are now seeing the first visible signs in late June, a full two to three weeks ahead of the historical average.
For smallholder farmers growing tomatoes, potatoes, and other solanaceous crops, this isn't just a calendar problem. Early blight pressure means the window for preventive action shrinks dramatically. By the time visible lesions appear on leaves, the pathogen may already be established in neighbouring rows. Spores can travel up to 30 metres on wind currents, meaning a single infected plant can become a field-wide outbreak within days.
Recognising the early symptoms
Late blight presents differently depending on the crop and the stage of infection. On tomato leaves, the first visible signs are small, water-soaked patches that rapidly expand into irregular brown lesions with pale green or yellow halos. In humid conditions, a white, fuzzy growth (the sporangiophores) appears on the underside of affected leaves — this is the pathogen actively producing spores.
On stems, late blight causes dark brown to black lesions that can girdle the plant, cutting off water and nutrient flow. On fruit, it produces firm, irregular brown patches that quickly develop into soft rot. The speed of progression is what makes late blight so dangerous: under warm, wet conditions, a single sporangium can germinate, infect a leaf, and produce new spores in as little as five days.
The challenge for farmers is that early symptoms — small water-soaked patches — can be mistaken for other problems: bacterial spot, sunscald, or even physical damage. Misidentifying the disease at this stage leads to delayed or inappropriate treatment, by which point the pathogen may have spread beyond easy reach.
What Krawp sees in the field
When a farmer opens Krawp and photographs an affected leaf, the app doesn't just match patterns against a database. It evaluates multiple visual features simultaneously: lesion shape and margin characteristics, colour gradients within the lesion, the ratio of affected to healthy tissue, and the specific pattern of spread across the leaf surface.
Beyond the image itself, Krawp cross-references local environmental conditions. Temperature, relative humidity, recent rainfall, and regional disease pressure data are all factored into the diagnosis. A lesion that might be ambiguous in isolation becomes much clearer when the model knows that humidity has been above 90% for the past 72 hours and that other farmers in the same district have reported similar symptoms.
In trials across three East African growing districts — covering 847 individual crop scans — Krawp identified late blight with 91% accuracy at the early necrotic stage. This is the stage where lesions are still small (typically under 10mm) and before most human scouts without laboratory support would flag the problem. The false positive rate was 4%, meaning farmers received a clear, actionable diagnosis in the vast majority of cases.
From diagnosis to action within 48 hours
Krawp doesn't stop at identification. Once a diagnosis is confirmed, the app provides context-specific guidance tailored to the crop, growth stage, and local conditions. This includes: which fungicide class is appropriate (chlorothalonil, mancozeb, or metalaxyl-based products, depending on resistance profiles in the region), how to calculate application rates based on plot size and equipment capacity, optimal spray timing based on weather forecasts, and when to schedule follow-up scouting to assess treatment effectiveness.
The 48-hour treatment window is critical. Research consistently shows that fungicide application within 48 hours of symptom appearance reduces late blight severity by 60–80% compared to application after 5 days. Beyond that window, the pathogen has typically produced enough spores to re-infect treated plants and spread to untreated areas, requiring multiple applications and significantly higher input costs.
For cooperatives managing hundreds of hectares, these individual diagnoses roll up into field-level dashboards that change how entire organisations respond to disease pressure. AgriUnion, one of Krawp's early partners, used this aggregated view to redirect spray crews to affected blocks within 24 hours of the first reported case — a response time that previously required a dedicated scouting team of six field officers.
The broader pattern: climate and crop protection
Climate data from the Regional Meteorological Centre suggests that early-season blight pressure will become more common across East Africa over the coming decade. Average temperatures during the primary growing season have increased by 1.2°C since 2000, and rainfall events have become more concentrated — creating longer dry spells punctuated by intense wet periods that favour oomycete sporulation.
For cooperatives and agricultural planners, this means the traditional calendar-based approach to crop protection — spray on fixed schedules regardless of actual disease pressure — becomes increasingly unreliable and wasteful. blanket fungicide applications cost farmers money, expose beneficial organisms to unnecessary chemicals, and accelerate resistance development in pathogen populations.
Precision crop protection — where treatment decisions are based on real-time field data rather than calendar schedules — is the path forward. Tools like Krawp, which ground-truth conditions at the plant level rather than relying on regional forecasts, will be essential for staying ahead of shifting disease patterns while reducing unnecessary chemical inputs.
What growers should do right now
If you're growing tomatoes, potatoes, or any solanaceous crop this season, here are concrete steps to manage the elevated late blight risk:
Immediate actions
- Start scouting earlier than usual — begin at 4 weeks after transplanting rather than waiting for the traditional mid-season check.
- Scout in the early morning when humidity is highest and white sporangial growth is most visible on leaf undersides.
- Photograph any suspicious symptoms immediately and use Krawp for a confirmed diagnosis before treating.
- If confirmed, apply the recommended fungicide within 48 hours and increase scouting frequency to every 2–3 days for the next two weeks.
- Remove and destroy severely infected plants immediately — do not leave them in the field as inoculum sources.
- Avoid overhead irrigation during high-humidity periods; drip irrigation reduces leaf wetness and slows sporangial germination.
- Keep records of affected areas to inform next season's variety selection and planting layout.
Farmers who received an early diagnosis were able to apply fungicide within 48 hours — compared to the typical 7–10 day lag — reducing severity by 60–80%.
Key Takeaway
Early detection is the single most impactful factor in managing late blight. The pathogen can destroy a field in 7–10 days, but a diagnosis within 48 hours of symptom appearance — combined with timely fungicide application — reduces severity by 60–80%. Use Krawp to identify symptoms at the earliest visible stage.
Frequently asked questions
How quickly can late blight destroy a tomato crop?
Under warm, wet conditions (20–25°C, >90% humidity), late blight can progress from initial infection to field-wide devastation in 7–10 days. The pathogen produces thousands of spores per lesion, and each sporangium can infect a new leaf within hours of landing in favourable conditions.
Can late blight be cured once established?
There is no cure for late blight once a plant is severely infected. The goal of treatment is to protect uninfected tissue and slow the spread to neighbouring plants. Fungicides are preventive, not curative — they must be applied before or immediately after infection to be effective.
Does Krawp work on potato crops as well as tomatoes?
Yes. Krawp's late blight detection models are trained on images from both tomato and potato crops, as well as other solanaceous species. The app identifies the pathogen based on lesion characteristics that are consistent across host species.
