Spinach Downy Mildew and Pests: Reading Resistance Codes

When downy mildew appears

Downy mildew is the disease that costs spinach crops. It shows as pale yellow patches on the upper leaf with gray mold on the underside, and it moves fast in cool, wet conditions with poor air movement.

It arrives when temperatures sit at 59 to 68F (15 to 20C) with continued rain, or when dew lies late into the morning. A crop under a closed cloche, or one that was never thinned, is the one that gets it.

Once it spreads close to harvest there is nothing to be done. Keeping the spacing is the insurance that actually works.

How to read resistance codes

Seed packets often list resistance to numbered races of downy mildew. The numbers refer to specific races, and a variety resistant to the races present in your area holds up while one bred for a different set does not.

Which races are around differs by region and changes over time, so a code alone is not a guarantee. Ask locally what has been appearing, and treat resistance as one factor rather than a solution.

Prevention that does not rely on the variety

Whatever variety you grow, the same three things reduce the disease: thin on time so air moves, keep the bed well drained, and ventilate any cover on fine days. All of these are decided by how you manage the crop rather than by what you spray.

Do not overfeed with nitrogen either. Dark, soft leaves are infected more readily, so the very deep green that looks like success is a liability here.

Soil mites show up as a soil preparation failure

When the growing point stops and the new leaves come out crumpled, the usual cause is mites feeding in the crown, and they build up on unrotted organic matter. They increase in cool, damp conditions, so autumn through spring crops are where you see it.

The response is not a spray but a change to how you prepare the bed. Use only well-rotted compost, add it a month or more ahead, and do not follow spinach with spinach.

If you see growing points stopping, look back at when and what organic matter you added rather than at this crop.

What to watch for through the year

Aphids bring virus with them, so the reason to control them is less the feeding damage than what they carry.

Telling similar symptoms apart

Several spinach problems produce yellow leaves, and reaching for a spray before you have identified which is how people waste time. Check the leaf underside, then the soil, then your records, in that order.