Pests and diseases are not so frequent on herbs. However, there are still some pests and diseases, which can cause substantial damage to herbs in one way or another. Healthy and strong plants are usually more resistant to pest and disease attack. As herbs are usually grown in small quantities, it is rather easy to get rid off them after spotting them.
The most common pests that cause damage to almost every type of plants are aphids, i.e. greenfly and black fly. These are very efficient and their reproduction rate is usually high. They cluster near the growing tips with soft lush growth. These are common on herbs grown in the greenhouse and house. If you spot any damage, the best way is to cut out the affected shoots and infested tips of plants. Another way is to take water with small amount of detergent mixed in it and spray it on the plant.
Lovage (Levisticum officinale) is a plant that can be spoiled by celery leaf miner, a grub that tunnels within the leaves leaving a thin ribbon of dead tissue. Remove the affected leaflets from the plant. Carrot fly usually attacks the Parsley – the grubs which hatch out lives in the roots, eating tunnels out of them. As this reduces the plants' efficiency for obtaining water and food from the soil, the plants begin to wilt and eventually died away. The leaves begin to flag, becoming yellow in appearance and eventually the growth stops. It has been seen that carrot fly attack is more severe in some years than others. This can be controlled by using a soil insecticide.
The rusty orange, powdery spots on the undersides of the leaves are clear signs of mint rust. Almost all the varieties of mint are affected by this disease. Mint rust is a severe plant disease and its damage has no cure. The only way to keep your plants safe from this disease is by lifting some of the creeping underground stems in autumn and washes off the soil and put them in hot water, i.e. 44°C (110°F) for about ten minutes. You should be careful that the temperature remains constant; otherwise the rhizomes will get damage. Before replanting them rinse them in cold water.
In mild areas a different sort of rust affects the chives. If you see any severe damage, there will be no cure. You need to destroy the plants and grow new seeds.
Remember, using any chemicals, such as fungicides or insecticides on edible plants can be dangerous. It is important to read the instructions given on the pack carefully before applying them on plants. Make sure that the particular chemical is formulated for edible crops. Always go for the chemicals that have short intervals between spraying and picking. The intervals vary according to the chemicals, some require only a day and others require a fortnight. Some chemicals are not useful for certain plants; look for the list of plants given on most of the chemicals, which cannot be treated with particular chemical.