Tomato troubles - Part 3

Tomatoes are the most popular and productive home garden crop in WA. Unfortunately, this popularity extends to pests and diseases, which find them delectable too. Do not give up hope because there are many ways of protecting your crop and not all methods involve chemical sprays. 

Selective breeding offers much hope for the development of resistant varieties.  It is an interesting fact that many of these breeding improvements capture natural characteristics from wild species and unite them with the large flavoursome fruits of modern types to come a few centimetres closer to the perfect tomato.
 
One of the most disease resistant varieties is the small-fruited cherry tomato. These varieties are very close to wild types and are resistant to most tomato problems. 

Many diseases build up in the soil, for this reason replanting in the same spot is not recommended.  It is a good practice to try rotating the locations so that you use any one area only once in four years. 

Insects such as whitefly and thrips can bring disease spores along for the ride which means they can transfer various maladies from affected vegetable shrubs and even weed elsewhere in the garden.  By eliminating weed in the adjacent area you will destroy a major launching site for these insects and the diseases they bring with them.

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Physical Disorders
Blossom end rot shows up as a leathery dark brown to black scar or rot on the base of the fruit at the opposite end to where it attaches to the plant.   The disorder is due to a calcium deficiency, which is often initiated and then made worse by uneven watering.  Mulching the soil, regular deep watering and a sprinkling of Blood and Bone fertiliser should help to overcome this problem.

Sunscald usually affects immature green fruits and can be seen as a yellow or white patch on the tomato.   As the fruit matures this spot can develop into a blister that ends up as a papery white or grey, sunken scald mark.  Poor foliage cover causes sunscald. Growing tomatoes in a cage that supports the foliage and creates a shady cover will help.

Fruit cracks are usually caused by summer rain or any condition that causes extremely rapid growth.   Fruit that is exposed to the sun is more affected than those well covered by foliage.

Insect pests
The tomato fruit caterpillar is the most common pest of this crop. This voracious pest grows to about 35mm long and varies in colour from green to brown.  They chew leaves and burrow into the fruit.  Because they are masters of camouflage it's likely that the first signs will be the damage they wreak.  Control can be achieved by spraying with Success or Dipel, both are biological in their nature and safe for other organisms. 

Thrips
are tiny black or brown insects found congregating on flowers.  Apart from carrying diseases thrips eat flowers and cause distorted fruits.  You can control this pest by spraying with insecticidal soap based products or Neem oil. 

Bronze surface mites
are microscopic
sap sucking insects that cause a silvering or bronzy appearance to stems and the underside of foliage. The result of severe attack is the premature death of the plant.   Regular dusting with sulphur or spraying with wettable sulphur is the control. 

White flies are small white sap sucking insects that appear to swarm around the underside of the foliage. Soap based sprays can be used to control the adult insects.  If you find considerable quantities of the nymphs on the underside of the leaves (they look like transparent oval scales), you may need to employ disturb and disruptive approaches such as squirting the plants with a jet of water regularly during the day.

Nematodes, also known as root knot eelworms, are a major problem of tomatoes in sandy soil.   They cause galls and swellings to develop in the roots, which eventually lead to the collapse of the plant when stressed by heat or dryness.  These pests disappear when your soil is rich with organic matter.  I recommend adding a 5cm layer of Soil Improver Plus to the surface as a feeding mulch. This can be applied two to three times during the growing season. 


Diseases
Early blight or target spot is one of the most common tomato diseases.  It appears as brown circular spots with a concentric ring pattern on the foliage.  The spots appear on the older, lower leaves first and cause them to turn yellow and drop.  Control this problem by dusting with sulphur.  

Verticillium and fusarium wilt are soil borne fungal diseases that block the sap conducting tissues of plants.  This causes yellowing of leaves and sudden collapse of plants in hot weather.  If you were to split open the stem at the base of an affected plant, you would find black discoloration just below the surface.  The best way to protect your plants from these two diseases is to practice crop rotation.  This means avoiding planting solanaceous crops such as tomato, capsicum, chilli, or eggplant in the same location for 2 years. 

Bronze spotted wilt disease is quite common in Perth gardens.  It is caused by a virus that gets into the plant, usually as a result of an attack by thrips. Bronze or black spots occur on the foliage particularly towards the top of the plant.  Wilting usually follows and the plant can die.  Once affected there is no cure. However, the fruit is still edible. It is possible to keep the plant alive by shading and lavishing extra care on it in the form of extra water and feeding.  If you have other tomato plants in the garden it is a good idea to destroy the affected one to reduce the risk of spread.  Vigilance in controlling thrips is also needed.  This can be achieved, partially, by weed removal and also by watching for thrips activity associated with garden flowers such as roses. 

Tobacco mosaic virus can produce the same loss of vigour as an attack of bronze wilt disease.  Tomatoes are related to tobacco and most cigarettes and tobacco products carry the mosaic virus. Smokers should wash their hands with soapy water before handling tomato plants to avoid transferring the disease. Leaves of affected plants develop a mosaic of yellow markings a sort of variegation.  Once infected, there is no control and the sick plant is best removed as the disease can be readily transmitted to healthy bushes nearby.  

One aid to controlling tomato foliage diseases is to apply irrigation by way of drippers or even underground weeping hose so that foliage is never left wet.  Overhead sprinklers are often associated with diseases. 

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