5 Signs of Potato Blight on Maris Piper and the Bordeaux Mixture Response

Maris Piper accounts for roughly half of UK maincrop potato plantings, yet it sits at the bottom end of the AHDB blight resistance scale for foliage. Its score is 3 out of 9, which explains why it is often the first maincrop variety on an allotment to show damage after a Smith Period, or after its successor, the Hutton Criteria, reports high risk.

The Hutton threshold is specific: two consecutive days, each with at least six hours when relative humidity remains at or above 90 per cent and air temperature is above 10C. Blightwatch and the Fight Against Blight service publish daily maps against that trigger. A grower checking those maps gets lead time that a resistant variety such as Sarpo Mira is built to provide in the plant itself.

The first leaf mark usually sits on a wet edge

The earliest visible sign is commonly on leaf margins and tips. Look for a pale, water-soaked patch about 5 to 10mm across, with a greasy feel, usually on the lower and older leaves where the canopy stays damp longest. Under humid conditions it can turn brown-black within 24 hours, with a narrow yellow halo in the tissue around it.

Turn the leaf over in the early morning. The strongest confirmation is a fine white felt of sporangia along the underside edge of the lesion, before sun and air movement dry the dew. Alternaria early blight shows concentric brown target rings on leaf interiors and lacks that white margin.

One confirmed lesion on Maris Piper usually means the bed is already carrying inoculum. Sporangia move by wind and splash, and the incubation period runs three to five days. Treating a single visible leaf leaves the wider infection untouched, because tissue may already be colonised before symptoms break through.

When the stems and haulm go, the crop decision changes

The second sign is a brown-black lesion on the stem, often at a node or leaf axil. Sometimes it girdles the stem, after which the section above wilts and folds. Stem infection is especially damaging because it interrupts vascular flow to whole branches. Copper sprays coat exposed surfaces, while colonised stem tissue stays infected regardless.

The third sign can arrive quickly in humid weather. Whole-haulm collapse turns green foliage into a blackened, slimy mass over three to four days, with the sour smell of decay. At that stage, the decision is no longer about rescuing leaves.

Cut the haulm to ground level, bag it, and remove it entirely. If you cut foliage two to three weeks before the intended lift date, the skins can set and fewer spores wash down to tubers during the next rain. On Maris Piper, the trade is smaller tubers in exchange for tubers worth storing.

The fourth sign is in the tubers. Lift a suspect plant and check the skin for irregular, sunken, purplish-brown patches. Slice through one of those areas. Blight gives a rusty, granular brown rot that spreads unevenly inward from the skin, distinct from the soft wet ring of bacterial soft rot.

The fifth sign often appears after lifting. Secondary bacterial rot colonises blight-damaged tubers in storage and can turn a marginally infected sack into a liquefying mess within two weeks. Any tuber with brown marbling should go into household waste, and blighted haulm should stay out of compost heaps. Oospores and living mycelium overwinter in discarded material, and infected volunteers are the single largest source of early inoculum in the following season.

Bordeaux mixture works only while tissue is still clean

Bordeaux mixture is copper sulphate neutralised with hydrated lime. Copper ions disrupt fungal enzyme systems on the leaf surface and stop spore germination, while tissue that already contains the pathogen carries on breaking down. Applications belong before the Hutton Criteria are met, or during the first hours of a forecast risk period. Once lesions are visible, the spray has arrived too late for those infected leaves.

The classic garden ratio is 100g copper sulphate and 100g hydrated lime in 10 litres of water, often described as a 1 per cent solution. Dissolve the copper sulphate in warm water in a plastic container, and keep metal containers away from the mixture.

Make a separate lime slurry in another volume of water. Add the copper solution to the lime slurry in that order, stirring continuously. Check with a pH strip; the finished mix should be neutral to slightly alkaline, since an acidic mix scorches foliage.

Spray both leaf surfaces to the point of run-off. The underside matters because sporangia establish there. Reapply every 7 to 10 days through the risk window, and again after 15mm or more of rain because copper washes off.

How you water during that window matters just as much as what you spray. Overhead watering while blight risk is high spreads sporangia by splash, one of the main dispersal routes from an infected leaf to a clean one. Keeping water at soil level leaves the canopy dry and cuts that route out. A pressure-reduced 4mm micro-tube run laid along the base of each row, fed from a timer set for early morning, does this well. On a 3m potato row, four 2 litre-per-hour drippers spaced 750mm apart deliver 8 litres per hour, and a 20-minute cycle puts roughly 2.7 litres into the root zone with very little leaf contact.

Copper also accumulates in soil. It is toxic to earthworms and to the mycorrhizal fungi that a no-dig bed depends on. The EU restricts agricultural copper to 4kg per hectare per year averaged over rolling periods, which gives a useful sense of why repeated copper use should be limited in a domestic bed.

The better season plan reduces the number of sprays needed: early-maturing varieties lifted before August humidity peaks, 40cm spacing for airflow, and morning watering at the base so foliage dries by midday. If Maris Piper goes back in after a bad blight year, Bordeaux mixture is worth having ready. A switch to a Sarpo or Carolus type tackles the susceptibility at variety level.

The timing problem never really goes away

Copper only guards clean tissue, and the Hutton warning only tells you the weather has turned. Everything hangs on the gap between those two facts.

The hard part of growing Maris Piper is that its flavour and yield are exactly why people keep planting a variety scored 3 out of 9, which means the standing blight liability comes back every wet summer. Whether a grower can consistently hit that 24 to 48 hour window with a spray, or whether the season plan and a resistant substitute end up doing more of the work, is a question each allotment answers differently once the maps start turning red.