What is wrong with my watercress plant?

Are the seedlings dying?

Does the plant produce swollen and malformed, leaves and stems?

Do the leaves have large, ragged holes in them?

Are the leaves covered with small bumps?

Are the leaves curled and yellow?

If the seedlings are dying the probable cause is a damping-off disease. Damping-off is caused by various soil fungi. Damaged seedlings can't be healed so, the best course of action is to remove them. To prevent damping-off let the soil surface dry between waterings, thin seedlings to avoid overcrowding, and do not add nitrogen fertilizers until plants have developed leaves.

Crook rot is a destructive fungal disease that invades the plants by means of spores that penetrate the root cells. This fungus then goes on to systematically invade the leaves and stems of young shoots. Crook root is especially most damaging in winter when watercress is growing more slowly. There are no chemicals that can be used to control this pathogen, however, you can try purging your watercress with large volumes of water as this can reduce the rate of infection by washing away the viable spores.

Large, ragged holes in the leaves are caused by slugs and snails. Slugs and snails feed on the plant leaves usually at night. To control slugs and snails use diatomaceous earth around your plants. You can also handpick them during the night or use shallow dishes filled with beer as a trap.

Leaves covered by small bumps are caused by scales. These tiny, shelled insects often feed on the undersides of the leaves, causing an unhealthy appearance with yellowish blotches on the upper leaf surfaces. Scrape off minor infestations with your fingernail. Prune out badly infested growth, or use a soft brush and soapy water to gently scrub the scales off the stems. Control scales by spraying insecticidal oil during the growing season.

Distorted, curled leaves are caused by aphids. These tiny insects cluster under leaves and on growing tips where they feed on plant sap. Leaves, stems, and buds get distorted, and later leaves and flowers drop from the plant. You can control them by washing them off the plant with water spray. Insecticidal soap should be used with severe infestations.

If there are tiny holes in leaves they are caused by Flea beetles. These tiny, black, brown, or bronze beetles hop when disturbed. Larvae are small and white, feed mostly on the undersides of leaves. Prevent problems by covering young plants with row cover. Control severe infestations by spraying or dusting plants with pyrethrin.