Tomato pests and diseases can be common in your garden.
The pests that you need to look out for include aphids, cutworms, leaf miners, army worms, bollworms, red spider mites, root-knot nematodes and thrips.
The diseases come in different forms. There are bacterial diseases, problems caused by fungi, as well as viral diseases.
It is important to focus on the health of your tomato plants, so that they are able to withstand attacks by pests. Proper watering and judicious fertilising with organic fertilisers allow plants to outgrow and tolerate pest damage.
Cutworms are the greyish, hairless caterpillars produced by a moth. They are often troublesome in the seedbed and a threat to newly transplanted tomatoes. They come out at night to feed on the stem of the plant, cutting it off from the roots. If you only have a few plants, you can wrap paper collars round the base of the plant. You can remove them manually at night. Try sprinkling diatomaceous earth around the plants. There are also eco-friendly insecticides for these worms. Plant tansy, sage and thyme amongst your tomato plants, as they repel the cutworms.
Leafminers are highly destructive insects that feed on leaves, stems and fruits of tomato plants. They are actually larvae of a small fly, which has a distinctive yellow dot between its wings. These and other flies lay their eggs on the underside of the leaves. The eggs usually hatch within ten days and the resulting larvae begin to tunnel inside the leaves, crea
ting the characteristic wavy lines that you see on the leaves. Keep a close watch on your tomato plants. If you see the wavy lines either remove the leaf or squeeze it, so that you squash the larvae.
Red spider mites are pale-orange to red and feed on the underside of leaves. They cannot be seen easily with the naked eye. They suck the chlorophyll from leaf cells, which causes shiny, pale yellow marks on the top of the tomato leaf. Eventually the leaves turn brown and die or fall off. Spray naturally derived miticidal sprays like neem oil or pyrethrins onto adult mites, larvae, nymphs and eggs to kill on contact. There are a number available.
Root-knot nematodes are microscopic, parasitic roundworms that inhabit the soil. Because they affect the roots, they are difficult to detect. Above ground symptoms include stunting, yellowing and wilting of plants. Interesting research is being done on biological control of the root‐knot nematode. There are also varietals of tomato that are more resistant to nematodes. Once your tomato season is over, plant French marigolds in those beds. Allow them to grow for about three months and then dig them into the soil.
Whiteflies are tiny insects, measuring between 0.6 and 2 mm in length. Their bodies are covered with a fine white powder. The nymphs and the adults damage plants by extracting sap from the leaves. This results in severe yield reduction. Particularly problematic for tomatoes are the Bemisia tabaci species, as it is the vector of a tomato yellow leaf curl-like disease. The other challenge is that whiteflies have developed resistance to chemical insecticides. Research is being carried out on various biological control options.
Diseases of Tomatoes
Tomatoes are susceptible to a number of diseases. Some are bacterial, some fungal and some are caused by viruses.
Health and sanitation: It is helpful to focus on keeping your plants strong and healthy. So you have planted at the right time and in the right position. You have looked for disease resistant varieties. You have planted with sufficient spacing to allow free flow of air around each plant. You have given each plant support through some kind of trellis.
Watering: Where and when you water, as well as how much is also important. Water at the bottom of the plant. This keeps the leaves dry. Water in the morning, so that the roots get what they need and evaporation means that there is not a build-up of humidity. The plants need less watering than one would expect. Once the fruit has started to form, only water when the leaves look limp during the day and when the top 10cm of soil becomes dry.
Pests: Keep the plants as pest free as possible, so that they have not been damaged or weakened. Some pests also carry diseases.
Mulch: Surrounding the plants with a layer of mulch prevents spores that might be in the soil from splashing up onto the plants. It also helps to keep the soil moist and cooler.
Pruning: We have written about pruning. Thin out excessive foliage to encourage free airflow.
Disinfect: If you use tools on diseased plants, be sure that they have been disinfected. This prevents diseases from being carried to other plants.
Fertilise: Boost applications of fertiliser during the growing season. Tomatoes are heavy feeders, but if they are strong they will be able to withstand diseases better.
Different diseases are prevalent in various parts of the country. Diseases are spread in various ways. Excessive heat and humidity provide good conditions for some of them. Some of them are on infected seed or seedlings, infected water and the movement of infected soil. Others may be carried by pests, implements or workers.
Diseases brought about by bacteria include bacterial canker, black stem, bacterial speck and bacterial wilt.
Fungal diseases on tomatoes are early blight, powdery mildew, late blight and blossom end rot.
Viral diseases are potato virus, grey mould, tomato spotted wilt virus and tobacco mosaic virus.