Late blight at nine weeks from harvest on a farm four months from organic certification

The pathogen's clock. At 18–22 °C with free water on the leaf, P. infestans completes a generation in roughly 4–5 days: infection, lesion, sporulation, dispersal. Twenty sporulating plants are not twenty plants - each lesion can release thousands of sporangia, and rain splash plus wind moves them across the canopy.

The multiplier. If each infected plant generates a handful of new infections per generation, the count moves from 20 to roughly 100 in 5 days, then to several hundred, then to thousands. On a 0.6 ha crop at a typical 30,000–35,000 plants/ha, the field holds roughly 20,000 plants; 20 is 0.1%, but one week of unprotected spread can move it past the point where the epidemic is controllable.

The weather is the amplifier. The forecast of 25 mm in 36 hours plus five more wet days provides exactly the leaf wetness and humidity the pathogen needs, and rain physically splashes sporangia. The canopy is also closing, which raises humidity inside it.

The economics sharpen the urgency. Expected revenue is 30 t × ($0.60 + $0.85) = $43,500 if organic, or $18,000 conventional. Against $6,200 already spent, the crop is worth protecting - but only if the control decision is made in the next 24–48 hours. Late blight is one of the few diseases where a delay of two days genuinely changes the outcome, because a protectant applied after the epidemic has started does not stop lesions that are already incubating.

Order matters: remove the inoculum before protecting the canopy.

Hours 0–8, before the rain

1. Rogue the 20 plants and a 1 m buffer around each patch - cut, do not pull, to avoid shaking sporangia loose; bag the material in heavy plastic, remove it from the field, and destroy it (burn or deep bury). Do not compost on site and do not leave bags at the field edge.
2. Do not walk through the crop when it is wet. Work the infected patches last, or better, change boots and outer clothing afterwards. Tools and gloves should be disinfected.
3. Check the whole field, not just the two patches - walk every row, because there will be lesions the scout has not yet seen.

Hours 8–24, before the rain arrives 4. Apply copper at the remaining 4 kg/ha metallic equivalent budget (2 kg used, 6 kg allowed), with a sticker, at the label rate, covering undersides of leaves. Copper is protectant: it must be on the leaf before the spores land, which is why it goes on before the rain, not after. 5. Add the approved biofungicide on the same pass or in alternation, and record both products, rates and dates for the certifier. 6. Open up the canopy where practical - remove lower leaves that touch the soil and improve airflow; this lowers leaf wetness duration, which is the variable the pathogen depends on.

Hours 24–48 7. Re-scout daily for the next five days, especially after rain. Set a decision threshold before you start: if new lesions appear on more than about 1% of plants despite copper, the organic strategy is failing. 8. Do not irrigate overhead, and keep workers and machinery out of the field when wet. 9. Contact the certifier and the buyer now, before any decision, and ask in writing what a conventional application would mean for the transition status - the answer determines the economics in the next task.

The two revenue scenarios

• Stay organic: 30 t × $1.45/kg = $43,500.
• Apply a conventional fungicide and lose certification: 30 t × $0.60/kg = $18,000, assuming the crop is saved.

Break-even loss under organic control. Organic revenue = 30 × (1 − L) × $1.45, where L is the fraction of the crop lost. Setting this equal to $18,000: (1 − L) = 18,000 ÷ 43,500 = 0.414, so L ≈ 59%. In other words, staying organic remains the better financial outcome unless you expect to lose more than about 59% of the crop. That is a high bar, and it is the central calculation.

What that means in practice

• Organic control has a real chance. Copper plus biofungicide plus roguing, applied before rain with a protectant in place, often holds an epidemic at this stage - but not always, and the weather is against us.
• Conventional control would probably save far more than 41% of the crop, but it converts a $43,500 outcome into $18,000 and, critically, may reset the transition period. Prohibited inputs in the record can mean the land restarts the three-year conversion; the certification has option value far beyond this one crop.

My decision: stay organic for this event, apply the remaining copper budget plus biofungicide immediately, rogue aggressively, and re-assess at 48–72 hours against the 1% new-lesion threshold. If the epidemic takes off despite this, the fallback is not a conventional spray on the whole field but a written discussion with the certifier about options - including sacrificing the crop rather than the certification.

What would change my answer

1. A certifier ruling that a conventional application on this field would not jeopardise the certificate (unlikely, but decisive).
2. Evidence that the crop cannot be saved organically - for example, lesions on more than about 5% of plants within 48 hours.
3. A much higher conventional price, which raises the $18,000 counterfactual and lowers the break-even loss.
4. Any indication that the buyer's premium is not secure for future seasons anyway. Apply the same logic to the pre-harvest intervals: mandipropamid at 3 days and mancozeb at 7 days are both compatible with a harvest nine weeks away, so residue is not the constraint here - certification is.