Honeycrisp Scab Stopped on M26 Rootstock with a Bordeaux Mixture Spray Routine

For an M26 Honeycrisp, the first copper application belongs at green tip or bud-burst, usually in mid-April across temperate zones. Visible olive-brown blotches on the underside of a leaf in May mean Venturia inaequalis has already completed an infection period and is releasing ascospores from the litter under the tree. Copper works by contact as a protectant and has almost no curative value, so the schedule depends on rainfall timing and fresh leaf growth more than tank strength.

M26 is a semi-dwarfing rootstock, and its growth habit often leaves a low, dense canopy that stays wet long after a shower. Honeycrisp is rated highly susceptible to scab in most pomology references, and that combination gives the fungus humid tissue close to the ground.

Why mid-April carries so much weight

The ascospores that start the season come from last autumn’s fallen leaves. They mature and discharge during wet periods once temperatures sit above roughly 6C, with the heaviest releases from green tip through petal fall. An unprotected primary infection cycle gives secondary conidia a start, and those spores can keep the disease moving through the tree all summer.

The practical routine begins with copper at green tip, followed by a change in material or strength as flowers approach. Full-strength Bordeaux mixture near bud-burst can russet fruit skin if residues carry into the flowering period, and Honeycrisp skin marks easily. The first one or two sprays therefore sit in the dormant or green-tip window, before open tissue is present to scar.

After that, use lighter copper or another protectant through petal fall. Re-coat after every 25mm of rain, because rainfall removes the protectant film from the leaf surface and new leaf area has appeared since the last pass. Reinfection periods follow the Mills table logic: warmer, wet leaves need fewer hours of wetness for infection. At 18C, around nine hours of leaf wetness is enough, and a three-day-old residue that has been half washed off will give poor protection.

Mixing Bordeaux without scorching the tree

Bordeaux mixture is copper sulphate plus hydrated lime in water, and the ratio decides whether the spray protects or burns. A standard dormant-season mix runs roughly 100g copper sulphate and 100g hydrated lime in 10 litres of water, the classic 1:1:100 by older weight conventions. Dissolve the copper sulphate in most of the water in a plastic bucket; galvanised metal is a bad container because the solution eats zinc.

Slake the lime separately in the remaining water. Then pour the copper solution into the lime and use the mixture the same day. The lime neutralises free copper that would otherwise burn foliage, so a mix that has gone acidic can phytotoxify young leaves.

Old growers used simple checks for that acidity. A strip of newspaper dipped into a harsh mix will scorch. A knife blade left in the mixture for a minute should come out clean; if it appears plated with copper, add more lime.

Pre-formulated copper products sold under names like Bordeaux Mixture, or generic copper oxychloride sachets, save the slaking step. They tend to run weaker and need more frequent reapplication. Whatever the source, agitate the tank constantly. Copper settles fast, and the last few litres from an un-agitated sprayer can carry triple the concentration of the first.

The litter problem nobody sprays for

Raking and removing fallen leaves in autumn, or running a mulching mower over them, cuts the overwintering spore load that starts next spring’s epidemic. Less inoculum at green tip leaves the copper film with less pressure to hold back. This sanitation pass often does more per hour than a mid-summer spray, and it costs nothing except time with a rake.

Where the copper routine quietly fails

Coverage is the first weak point. Scab infects the underside of leaves and the calyx end of fruit, and a sprayer aimed only downward from above leaves both areas poorly coated. A hollow-cone nozzle and enough pressure to drive mist up into the canopy from below matter more than a heavy drench from the top.

On a trained M26 tree, that upward coverage is manageable. On a neglected, bushy tree, it becomes nearly impossible. Winter pruning that keeps the centre open is part of the disease programme, because air movement shortens wet periods and leaves space for the spray plume to reach the target tissue.

Soil accumulation is the second problem. Repeated annual copper applications build copper in the ground to levels that suppress earthworms and soil fungi over a span of years. Organic certification schemes cap annual copper metal per hectare for exactly this reason.

A home plot with one or two trees will take a long time to approach those orchard-scale caps, yet spraying the whole understorey every fortnight for a decade still has consequences. Keep the spray on the tree as much as possible. The film belongs on leaves, young shoots and fruit clusters, with minimal runoff onto the soil.

Resistance and tolerance creep form the third recurring failure. Copper itself rarely drives resistance, which is one reason it remains useful as a protectant material. Problems arise when a single synthetic fungicide is blanket-applied alongside it season after season, giving Venturia repeated selection pressure in the same direction.

Weather can also defeat a clean-looking schedule. A fortnight of warm rain through blossom can overwhelm any contact protectant, because new leaf area expands faster than the spray film can be renewed. In those seasons, autumn leaf clearance and a hard renewal prune carry extra weight.

What else gets done while the sprayer is out

A garden running copper on fruit usually has other jobs stacking up in the same damp weeks. CED Stone drainage gravel laid as a 50mm mulch ring around the base of the M26, kept clear of the trunk collar, sheds water from the root zone and reduces splash-up that carries soil-borne spores onto low fruit. The same 20mm clean gravel also suits a raised pond liner build, where it works as a blinding layer over compacted subsoil and protects the butyl from stones before the underlay goes down.

Slug pressure rises in the same wet spells that drive scab. Nemaslug nematode slug control, watered into soil above 5C, sends Phasmarhabditis hermaphrodita after slugs underground and gives around six weeks of control per application while staying separate from the copper chemistry. That timing matters before strawberry runners go into a self-watering planter, where the reservoir keeps fruit off wet soil and away from slugs that would otherwise hollow them overnight.

Propagation work often lands in the same April window. A heated propagator running at a steady 18 to 21C lifts germination rates on slow starters like chillies and aubergines far above a cold windowsill. Box hedging hit by Cylindrocladium responds to a Topbuxus box blight treatment regime of fungicide plus the firm’s leaf-feed tablets, although airflow through the plant can matter as much as the spray. None of these jobs treat the apple tree, yet they compete for the same wet fortnight when orchard sanitation is easy to postpone.

The one-year test

The measure of the routine is the August canopy and the fruit in storage, not a tidy set of leaves in May. A tree that holds its canopy through summer ripens fruit properly and enters winter with reserves; a half-defoliated tree limps. Run the schedule for a full season, clear the litter, then compare leaf retention after cropping with the corky scab that shows up in stored fruit by October.

The unresolved clue is the fruit in storage: which clean-looking apples carried quiet infections through summer?