Beetroot Problems: Leaf Miner, Leaf Spot and Poor Roots

Problems arrive in a predictable order

Beetroot is a robust crop and most of what goes wrong with it is not a pest at all. The failures divide by stage: seedlings are eaten or damped off, half grown plants get leaf miner and fungal spotting, and mature plants disappoint in the root rather than the leaf.

The useful habit is to check whether the damage is on the leaf or in the root, because the two rarely share a cause. Leaf damage is usually cosmetic and the crop grows through it. A root that has not swelled is almost never a pest problem and almost always a growing condition you set weeks earlier.

Leaf miner: unsightly, rarely serious

The commonest complaint about beetroot is pale winding trails or translucent blistered patches inside the leaf. That is beet leaf miner, the larva of a small fly feeding between the upper and lower leaf surfaces. Because it is inside the leaf, sprays applied to the surface do very little, which is why the standard advice is physical.

It looks dramatic and it matters less than it appears. A plant losing a quarter of its leaf area to mines still makes a perfectly good root. It only becomes a real problem on small seedlings, where losing two leaves out of four genuinely sets the plant back, and on plants you are growing for the leaves.

1. Squash the larvae in the leaf Hold a mined leaf up to the light, find the dark larva at the end of the tunnel, and pinch it between finger and thumb. On a small bed this is quicker and more effective than anything else.
2. Remove badly mined leaves Take off leaves that are more than half tunneled and put them in the bin rather than the compost pile. Do not strip the plant: leave at least two thirds of the foliage.
3. Cover young plants with fine mesh Insect mesh over the row from sowing until the plants are well grown stops the adult fly laying. It must go on before the flies arrive, not after you see the first tunnels.
4. Clear crop debris in autumn The pupae overwinter in the soil under the old crop. Clearing the bed and rotating away from beetroot and chard next season breaks the cycle.

The same fly attacks chard and spinach beet, so treat those as one crop with beetroot for rotation and mesh purposes.

Leaf spot and rust

Small round spots, tan or gray in the center with a purple or reddish margin, spreading from the lower leaves upwards, are fungal leaf spot. It appears in warm humid weather and is worst where the plants are crowded and the foliage stays wet. Rust shows instead as small raised orange pustules on the underside of the leaf.

Neither usually costs you the crop, because beetroot keeps producing new leaves from the crown faster than the disease works upwards. The response is to remove the worst leaves, improve airflow, and stop watering over the tops. Spraying is rarely justified in a garden.

1. Take the lower leaves off Remove spotted leaves at the base and take them off the plot. Left lying, they release spores onto the plants above them for weeks.
2. Water at the base and in the morning Overhead watering keeps the leaves wet for hours and splashes spores upward. Water the soil, early enough that any wet foliage dries before evening.
3. Respect the spacing Crowded rows never dry out. The four inch spacing you set at thinning is disease control as much as it is root size.
4. Rotate away from the family Beetroot, chard, spinach beet and sugar beet all carry the same leaf diseases. Leave three years before beetroot returns to the same bed.

The root that never swelled

A row of healthy vigorous leaves over roots the size of a marble is the most common beetroot disappointment, and it is a growing problem rather than a pest. There are four realistic causes and the spacing plus what you fed the bed usually identifies which.

The first is not thinning: because each seed is a cluster, an unthinned row is three or four times as crowded as it looks and no plant gets room to swell. The second is nitrogen. The third is drought, which stops swelling outright. The fourth is simply time, where a late sowing runs out of warmth.

Bolting and what causes it

A beetroot that sends up a tall flower stem has decided it has been through a winter. The trigger is cold: young plants held below about 50F (10C) for a couple of weeks, particularly in the first month after emergence, are vernalised and run to seed as the days lengthen. Drought stress makes it more likely on top of that.

Once the stem is up the root goes fibrous quickly, so there is nothing to save by waiting. Lift the plant, use the root at once if it is still tender, and keep the leaves, which stay perfectly good.

1. Do not sow too early into cold soil Wait until the soil is reliably above 50F (10C), or start under cover and plant out into warmer ground. This single decision prevents most bolting.
2. Use bolt resistant seed for early sowings Seed sold as bolt resistant is bred for exactly this situation. Reserve it for the first sowing of the year and use ordinary seed later.
3. Protect early plants from cold snaps A cloche or row cover over an early row through a cold spell keeps the young plants above the vernalising temperature.
4. Keep the water steady Drought stress in a young plant increases the chance of bolting on top of any cold it has had. Even moisture through the first six weeks matters.

Late summer sowings very rarely bolt. If bolting has spoiled your spring crop, moving the main sowing later is a more reliable fix than any seed choice.

Telling similar symptoms apart

Beetroot leaves discolor for several unrelated reasons and the leaf alone will mislead you. Where the discoloration starts, whether it is inside or on the surface of the leaf, and whether the root is affected, will separate them quickly.

Record the date you first see any problem each year. Leaf miner and leaf spot arrive at remarkably consistent times in the same garden, and knowing your own dates lets you put the mesh on or thin the row a week before rather than a week after.