Onion Problems: Downy Mildew, Bolting and Split Bulbs

Risk arrives in three waves

Onion problems do not come evenly through the year. There are three peaks: the seed bed in autumn, the warming weeks of spring, and harvest and storage in summer. Deal with those three and you avoid most of the damage.

Midwinter, by contrast, is quiet. Leaves scorching in winter are the work of cold and dry wind, and no spray helps. The commonest mistake in that season is spending effort on control that is not needed.

Downy mildew starts in the seed bed

Downy mildew costs more onion crops than anything else, and by the time it is running across the plot in spring it is too late. What actually happens is that plants infected in the autumn seed bed carry it through winter and release spores in spring. The place to fight it is autumn, not spring.

1. Keep it out of the seed bed Sow thinly so air moves through, and pull anything with a vague yellow-white wash on the leaves and take it off the plot. A spring epidemic starts with what you overlook here.
2. Do not carry it in at planting Do not plant anything that looks even slightly wrong. Saving a few plants can cost you an entire row in spring.
3. Act before the rain It spreads suddenly in cool, wet weather, so the dry spell before is your only chance. Sort out spacing and drainage in advance.
4. Look in the morning Early lesions dry out and become invisible during the day but show while there is dew. Fixing your inspection to the morning means finding it sooner.

Pests are easier to handle by season than by species

At planting time in autumn the problem is root maggots, which are drawn to unrotted organic matter and whose larvae feed at the base of the plant. Not adding fresh manure at planting is the main prevention.

In spring thrips build up, leaving fine pale streaks along the leaves. They are worse in dry years, and heavy damage turns the whole leaf pale and stops bulbing. Leaf miners appear around the same time and leave winding pale trails inside the leaf.

With all of them, treating a large area after you see the damage rarely works. What changes the outcome is noticing early on a spring walk. Caught small, removing the affected leaves is sometimes enough.

Soil-borne disease and rotation

Grow onions in the same ground repeatedly and soil-borne rots build up. The pattern is a soft base, the tops falling over, and no roots left when you pull. Rotation is more realistic than any spray, and these want a two to three year gap.

1. Leave two or three years These organisms persist in soil. Not following onions with onions is the most reliable control there is.
2. Count the whole family Leeks, garlic and shallots share the same diseases. Putting them in between is not a rotation, it is a continuous onion crop.
3. Improve the drainage Outbreaks concentrate where water sits. Raising the bed and giving water a way out visibly reduces the number of affected plants.
4. Take affected plants off the plot Lift them, bag them and dispose of them elsewhere. Leaving them, or rotavating them in, spreads the organism through the bed.

Bolting and splitting are design faults, not diseases

Bolting is not a disease. It happens when a plant is too large going into winter: once it passes a certain size and then meets cold, it forms a flower bud and runs a hard stem up in spring. The causes are sowing too early, planting fat seedlings, or feeding too much in autumn, and by spring there is nothing you can do.

Splitting is the opposite kind of fault: the bulb divides into two or three, which happens when the plant grows unusually large, or when growth stops and then restarts. Fat seedlings and very uneven feeding both increase it. Split bulbs are perfectly edible but they do not store.

Both are reduced by managing the sowing date and the seedling size. Sow five days later after a bolting year, grade the seedlings harder after a splitting year, and after two or three seasons you will have found the right window for your garden.

A bolted plant will not fill and it takes light and nutrients from the ones around it. Pull it young and eat the stem and bud.

Build the prevention into the system

If you do spray, whether it works depends on whether this groundwork is in place. In a crowded, badly drained bed the damage repeats however much you apply. When the same disease appears every year, look at the conditions rather than the organism.

1. Keep the rotation Two or three years between onion family crops. Drawing a plan of the plot and rotating on paper is more reliable than memory.
2. Raise the bed At least four inches, with a way out for the water. Drainage is the best value control against disease in general.
3. Do not crowd the plants Below five inches (12 cm) the air stops moving and mildew travels along the row. Choose airflow over plant numbers.
4. Stop feeding at a fixed date Late feeding gives open necks and rot in store. Having a deadline at all is itself a control.