Lettuce Diseases and Pests: Keeping the Crown Dry

The rots start at the base

What costs you a lettuce crop is not leaf spots but the group of diseases where the base of the plant goes soft and water-soaked and rots. They usually appear once the head has started to form, and by the time you notice from outside the inside has already gone. You lose a whole plant days before you meant to cut it.

The common condition is a crown that stays wet. Prolonged rain, a low bed, crowded plants and lower leaves touching the soil together produce it. The corollary is that designing the bed so the crown dries removes most of the problem.

1. White mold and black specks White cottony growth at the base, later producing hard black bodies the size of mouse droppings. Those bodies persist in the soil and start next year's outbreak.
2. Soft rot with a bad smell The plant turns to soft mush with a strong smell. This one is bacterial and gets in through wounds and dead lower leaves.
3. Pale patches with mold underneath Vague yellow patches on the upper leaf surface with white mold on the underside, working up from the bottom leaves in cool, damp weather.

What to watch and when

The cast changes with the season. Warm weather brings insects and soft rot, prolonged rain brings the crown rots and gray mold, and cool damp weather brings downy mildew. Knowing that order gives your eye somewhere to go.

Build a bed that dries

This is design rather than treatment. Raise the bed so water drains, space the plants so air moves, and stop soil splashing onto the crown, and the conditions the disease needs are simply not there. It is one job, done before planting.

1. Raise the bed Six inches (15 cm) or more where the crop will run through wet weather. A low bed holds water, the roots suffer, and disease enters through the damage.
2. Cover the surface Black plastic over the bed stops soil splash and the soil-borne infection that comes with it. It also suppresses weeds so the crown stays open.
3. Do not crowd Twelve inches (30 cm) for heading lettuce. Closer and the outer leaves touch and never dry, and one infected plant passes it straight to its neighbor.
4. Plant shallow A crown buried in soil stays wet. Set the shoulder of the rootball level with the surface or slightly above.

Water gently at the base, never over the leaves. Evening watering leaves the foliage wet all night, so move it to the morning.

Settle the insects before it heads

Once the head closes you cannot reach anything inside it. Concentrate pest control on the seedling stage and the weeks straight after planting, and finish before heading begins. Getting the order right changes the nature of the damage entirely.

Insect netting is the most effective single measure. Put it over the bed at planting and weight the edges down with soil so there are no gaps. Putting it on later just shuts the pests in with the crop.

Do not bring it in, do not leave it behind

Disease either arrives from outside on the plants you buy, or comes out of the debris from the last crop. Cover those two points and the amount you get next year changes.

1. Look at plants before you buy Do not buy anything with a darkened base or yellowing lower leaves. It goes on to infect and can spread through the whole bed.
2. Pull affected plants immediately Lift anything soft at the base with the soil around it and take it off the plot. Left lying, the hard black bodies end up in your soil.
3. Do not dig the debris in Outer leaves and root stumps rotavated in are a platform for the organism to overwinter. Take them off the plot first, then cultivate.

Rotation and reusing beds

Grown repeatedly in the same ground, the soil-persistent diseases build up year on year. Do not follow lettuce with lettuce: leave two or three years and grow something from another family in between. Lettuce is in the daisy family, so count chrysanthemum greens as the same slot.

1. Record by plant family Note which family each bed carried. Recording by name means you miss that lettuce and chrysanthemum greens are relatives and grow them in sequence.
2. Put a grass family crop in between Sweet corn or a cereal between lettuce crops lowers the population of the soil organisms. A green manure does the same job.
3. Rest an affected bed Where crown rot appeared, do not put lettuce back next year. Three years is the target, with other families in between.

Wet autumns give more disease. If the forecast shows a long wet spell, cut everything that is ready rather than leaving it.

Sorting out what you are looking at

Once it has appeared there is not much you can do, but misreading it can cost you the whole bed. Establish first whether it is rotting from the base, damaged from the leaves, or eaten, and decide whether to pull or keep before anything else.

Do not leave what you pull in the path, or it spreads from there. Bag it and get it off the plot the same day.