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Vegetable Pest and Disease

By ACS Distance Education on June 9, 2016 in Gardening & Self Sufficiency | comments


Warmer weather is the best time of year to grow vegetables. The only down side to this however, is that it’s also the best time to grow pests and diseases! Variable weather , such as high wind, cold or flooding; can make things even worse.

Gardeners can take three steps to avoid problems with their spring vegetables:

1.    Know what the likely problems are, watch out for them.
2.    Keep the plants healthy so they can fight the problems. This means you need to plant them in the right place, feed, water, mulch and protect from extreme weather.
3.    Treat problems -take precautions to avoid exposure and try to stop diseases/pests from spreading - even if that involves early drastic action such as removing entire plants.

Once you identify that one of your vegetables has a pest/disease, do a bit of detective work and look through the following chart to see what problems are most common to that plant. You will then find discussions on many of the common pests/diseases further along in the article where you may identify a solution.   If not discussed in detail here, you may at least identify what the possible problem is and look on the Internet for further ideas on tackling the problem.



Aphids (sometimes called greenfly) are common pests of many vegetables, including bean, cabbage, capsicum, carrot, celery, corn, eggplant, lettuce, parsley, pea, potato, radish and tomato. Aphids sit on soft plant tissue and suck sap out of the plant, causing wilting and leaf curl. Aphids can also transfer viruses and other diseases from plant to plant. A strong jet of water (from a hose) may drown small infestations. Aphids can also be controlled by spraying garlic spray, “Dipel” spray or Pyrethrum. Nasturtium, spearmint, stinging nettle, artemisia and garlic are companion plants that are often used to repel aphids. “Dipel” spray is therefore considered safe as Dipel is a very specific type of bacteria, which infects and kills moths, butterflies and aphids without affecting bees, humans or anything else.

Carrot Fly
Carrot fly often attacks parsnip, parsley, celery and related plants. The larvae burrow in roots, kill young seedlings, and also damage larger roots. Heavy infestations can cause wilting or discolouration of foliage. Remove infected plants and burn.

White fly
These can affect beans, potatoes, tomatoes and some other vegetables.  They often hide underneath the leaves. Relatively safe chemical sprays such as pyrethrum can be effective, but only if they make contact with the insect. This might not happen if they are in the shadow of a leaf. How thoroughly you spray can be just as important as what you spray.

Weevils are a special type of beetle with a characteristic long “snout”. Fleshy-rooted vegetables (e.g. carrots, beetroot) are especially susceptible to weevils. Weevils may be controlled using sprays to kill females before they lay their eggs.

Nematodes are small, normally microscopic worms which burrow into roots or leaves. They can cause swellings, dead patches, dwarfing, yellowing of foliage and dieback. To control nematodes, keep tools clean, use resistant plant varieties, or plant marigolds in the soil (as these deter nematodes). Digging in organic matter can also help deter nematodes. Most chemicals available for home use are ineffective.

Heliothis caterpillar
This is also known as a “corn eelworm” or “tomato moth”. It will burrow into and spoil a tomato fruit, or into an ear of corn, eating out the grains. Similar caterpillars can infect all sorts of vegetables. The good news is that they belong to a family of insects that includes moths and butterflies; and you can safely spray with “Dipel” to control them.

Cabbage white butterfly caterpillars
Cabbages can be badly damaged by these caterpillars. Like all moth and butterfly larvae, these can be safely controlled by spraying “Dipel”.

Green vegetable bugs
These can be a serious problem on legumes, in particular beans. They will be controlled to a degree by wasps and other natural predators, but if numbers get too large, they can seriously damage a crop, leaving no option but to spray chemicals or discard the crop.

Mites are tiny (sometimes microscopic) spiders that feed by sucking nutrients from their host. Plants will thus often loose colour and develop a mottled appearance. If you use a magnifying glass, you can often discover tiny spiders crawling about. Remove and burn any damage plant parts before the mites spread. If the crop is too badly damaged, destroy the plant.

Snails and slugs
These will emerge and attack any tender leafy vegetables (e.g. lettuce, spinach), whenever conditions are moist. Gardeners have a dilemma in that they need moist conditions to get these vegetables to grow well.  Fortunately, there are many natural ways to control snails and slugs. Poultry will eat them. They will be deterred from crawling over a sheet of copper, a layer of wool fibre or diatomaceous earth. Organic gardeners often surround individual plants with such things to deter snails. VP8 IMAGE Wool scattered around vegetables to deter slugs or snails.

Larger animals
When there is lots of damage to a fruit or plant as seen with this tomato; the damage is probably caused by a larger animal (e.g. mice, rats, possums, rabbits, birds, bats).  The simplest way to control large animals is to exclude them from the plant with some type of netting. VP12  Net over vegetables to protect them from pests. If that doesn’t work, you might use a trap. Poisons should be a last resort; and if you do use a poison; put it under a cover to prevent bigger animals such as your pets, getting to it.  VP13  IMAGE  For example, use a box over the top of rat poison which has holes large enough to allow mice and rats to enter; but small enough to stop dogs, cats or native wildlife getting to the poison. Be sure the box is pegged down so animals can’t expose the poison though.


Leaf Spot
Leaf spot diseases, causing dead or discoloured spots on leaves, are some of the most common diseases. A large range of different fungi and bacteria cause different leaf spots on different types of plants. Most leaf spots flourish after wet weather. They rarely require attention, except at times when there is a serious outbreak. Affected leaves should always be removed and the plant sprayed though to prevent further infection.

Wilt (drooping foliage) is caused by bacteria or fungi interfering with the supply of water to the plant. Because the symptoms do not occur necessarily where the infection is, it is a lot more difficult to treat a wilt disease than many other diseases. On tomatoes, rapid wilting caused by the Fusarium occurs on hot days. The lower leaves become yellow and wilt first, before wilting spreads upwards.

Rust, a fungal infection, causes rusty brown, orange or yellowish spots or stripes, normally on leaves and stems but sometimes on flowers or fruit. It affects a wide range of vegetables, particularly beans and asparagus. Destroy infected plant material. Chemicals which may be used include sulphur sprays.

Viruses cause many serious diseases, generally weaken plants they infect, making the plant more susceptible to other problems, and frequently stunt growth. Sprays won’t kill viruses, but spraying insecticides to kill insects that carry the virus can help.

Mildews are common fungal disease on cucurbits, particularly in humid, moist and warm conditions. Affected leaves should be removed, and badly affected plants destroyed. You can spray a fungicide if the problem is caught early, but so long as moist conditions persist, it is likely to return. The best way to avoid mildew is to space your plants well apart to allow lots of air movement; and to grow the right variety at the right time of year for your locality.

This is a fungal infection, causing rot in tomato fruits. Sprays can be used for most fungal diseases but the better way to control them is to avoid them in the first place. Fungal diseases are usually associated with too much moisture. If the plants are well ventilated, grown in well drained soil, and water is usually only applied at the base of the plant, the problems will be fewer if any (i.e. don’t wet the fruit and foliage).

Blossom end rot
This rot is common on developing tomatoes when calcium levels are inadequate and the supply of water to the plant is inconsistent. It isn’t caused by a fungus or pest, so sprays do not help. It is more common in larger tomatoes than smaller cherry tomatoes. Controlling it is more about ensuring water levels in the soil don’t vary from dry to wet, but rather stay relatively even.


Foliage often changes colour in response to a nutrient deficiency. Sometimes the change is subtle such as a slight yellowing or reddish tinge, and sometimes it can be more obvious. Nutrient deficiencies often affect one part of the plant more than others. Some deficiencies affect older leaves, others young growth tips; while yet others may affect the leaf veins or some other part. Sometimes a deficiency is the result of the soil being too acid or alkaline, or too wet or dry; so check those things. More often than not, feeding the plant with a balanced all round fertilizer will help. Ensuring the right soil moisture content is also important in spring due to variable weather conditions.


Too much nutrient can be just as bad as too little. Strong fertilizers that dissolve readily in water applied heavily can burn the roots of plants, and may cause a dying back of tender parts above ground too. Nutrient burn may look similar to sunburn or wind burn, but if you have just fed plants and there is no extreme sun or wind, the problem is probably over feeding.


These are some of the most common problems seen in spring vegetables.  However, there are numerous other difficulties that can occur. If these don’t fit your problem, try looking at other sources such as reliable Internet sites for solutions.

If you really want to be able to manage pests and diseases better; try studying our short course on Plant Health. -click here