With regards to watercress, it’s worth noting that they don't suffer too much from pests or disease but should you be unlucky enough for your plants to become infected by a fungal infection, it is likely only to occur at the propagation stage rather than on mature plants in their aquatic environment - with the exception of crook root. Therefore, it is particularly important to apply good high standards of hygiene during the propagation stage in order to minimize the need for subsequent chemical use. Be aware that watercress diseases tend to be most prevalent during dull, cool weather when the crop is unable to grow away.
DAMPING OFF (Pythium species)
Cause: This is very common fungal infection which is implicated in the disease commonly known as 'damping off'. When propagating seed in damp soil, the soft underground plant tissues can be attacked particularly if conditions are extremely and if plant growth is slow. Any aerial parts of the plant should remain unaffected unless they have contact with the soil.
Symptoms: You will typically lose part – or in extreme cases – all of the root system and your plants (rather obviously) will begin to lack vigor. Some Pythium species only attack the root tips, which can turn from white to brown and ending with their eventual collapse.
Treatment: Washing and disinfecting seed trays will help to minimize disease risk. Compost should always be kept from sources that may have been exposed to the fungus, e.g. by contaminated dust, or water , and your watering supply must also be kept free from sources of infection.
Cause: This destructive fungal disease 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. The organism will also multiply within the root, producing large numbers of viable spores. At certain stages during its life cycle, only dormant spores are produced which are highly resistant to poor environmental conditions. When ffavorableconditions return, the dormant spores become active, and become released into the environment where it can infect more host plants. Crook root is especially most damaging in winter when watercress is growing more slowly.
Symptoms: Once the fungus reaches the new growth, the plant will begin to produce swollen and malformed, leaves and stems. As the disease moves into the secondary shoots, these will become stunted resulting in malformed plants.
Control: 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 reducing the rate of infection by washing away the viable spores.
Cause: This is perhaps the most recognized pest of watercress due to the immense, characteristic damage that these beetles can cause.
Symptoms: These beetles can cause significant damage by leaving copious amounts of small holes in the leaves. This normally serious infestation is usually experienced at two distinct times of the year, usually in April and July.
Control: Beetles can be removed by submerging your crop of watercress for about 2 hours. This causes the beetles to float off, at which point they can be skimmed off the surface of the water. You can also consider “trap crops” such as radish which may help lure the flea beetles away from your watercress. With the appropriate equipment – and if you don’t have too many plants - Flea beetles can be vacuumed off the foliage. However this must be repeated frequently to avoid another invasion of your plants.
MUSTARD BEETLE (Phaedon cochleariae)
Causes: Like flea beetles, adult mustard beetles can lead to an almost continuous infestation of watercress throughout the summer.
Symptoms: Eggs are not laid on Watercress so damage is limited to the leaf and by the feeding adult beetles.
Control: Again, these beetles can be removed by flooding the cropping beds for about 2 hours to allow them to naturally float off before being skimmed off the surface of the water.
Cause: Most aphid infestations are caused by the black bean aphid or the Peach blossom aphid (Myzus persicae).
Symptoms: Aphids generally feed on watercress over the summer, but severe infestations may cause serious losses. They damage and weaken the plant by sucking the sap out of pressurized parenchyma cells found just below the leaf's surface. Clusters of these small insects are readily identifiable, normally at the plants tips or on the underside of their leaves. In severe cases, the infected parts can begin to curl down due to the quantity of sap being removed from that area. Aphids will also excrete honeydew which causes leaves to first become sticky and shiny. They eventually turn black because of a sooty-mould fungus growth.
Control: There is no chemical control that is recommended against use on watercress, however using trap crops such as lettuce or bedding nasturtiums may help to entice aphids to these alternatives rather than to your edible crop.