WHAT IS TOMATO BLIGHT?



Tomato blight is a disease caused by a fungus-like organism which spreads rapidly in the foliage and fruit of tomatoes - typically in wet weather, causing them to collapse and decay.

In particular, it is a serious disease outdoor tomatoes, but not as common on tomatoes grown in greenhouses.

Blight is specific to tomatoes and potatoes, and some ornamental relatives of these two crops are also susceptible. Cases have been recorded on some ornamental Solanum species as well as bedding Petunias.

Symptoms of Tomato Blight

The initial symptom of blight on tomatoes is a rapidly spreading, watery rot of leaves which soon collapse, shrivel and turn brown. During humid conditions, a fine white fungal growth may be seen around the edge of the lesions on the underside of the leaves. Brown lesions may also develop on the stems. If allowed to spread unchecked, the disease will begin to attack the fruit. This is recognised by brown patches appearing on green fruit. If infected, the more mature fruits will decay rapidly.

How to Control Tomato Blight

Perhaps the biggest problem with blight on tomatoes is with its cousin the humble potato. With the majority of potato varieties being highly susceptible to this virulent fungus, the late summer air is full of pathonogenic spores just waiting for a suitable host plant to infect. It's Unfortunate that the closely related tomato plant more than readily fits the bill.

This can be a particular problem with tomatoes, especially when grown outside in the more temperate regions of the country. With the late cropping of most true outdoor varieties – and even later cropping if glasshouse varieties are grown outside – the ripening fruit will often coincide with the seasonal incidence of ‘Late Blight’. If the late summer season is particularly hot and humid, your tomato crops will probably stand little hope of survival and your years worth of work can end up as another pile being burned at the local incinerator.

Because infection is so dependent on specific combinations of temperature and rainfall,  periods of high risk  can be predicted accurately. To find out when your tomatoes are at their greatest risk it is worth contacting your local horticultural advisory service. You will be able to access these warnings (visit the Potato Review website), but because this information is more for the commercial grower you must rely on a more restricted range of protectant fungicides containing copper (Bordeaux Mixture or Fruit and Vegetable Disease Control), as the more effective systemic products will not be approved for amateur use.

As a safety net, when wet weather is forecast from June onwards, begin applications of protectant sprays as a matter of course.
 
Blight Resistant Tomato Varieties

Recently there have been some new introductions that have performed extremely well against Late Blight. Given time - and improved availability - these hardier varieties will hopefully give tomato growers around the world a well deserved break. The three best performing varieties are listed below.

TOMATO ‘LEGEND’

This particular variety was bred in the USA by Dr. Jim Baggett at Oregon State University. In recent tests ‘Legend’ had shown impressive blight tolerance, and in particular during trials in a ‘garden’ situation. It produces large, glossy red fruits with an expected crop of up to 6lbs per plant. The fruit have a slightly flatter shape compared to the norm and come almost completely seedless. Fortunately for most gardeners, best results are produced when the plants were grown outdoors but they are also perfectly fine for growing under glass. They have an excellent flavour and should be sown 6-8 weeks before expected lasts frosts - in the United Kingdom this will be any time from March onwards.

TOMATO ‘FERLINE’ F1 Hybrid

Not only has this new variety shown excellent tolerance to ‘Late Blight’, it has also proven itself to be highly resistant against both fusarium wilt and verticillium wilt. As with the new ‘Legend’ cultivar, ‘Ferline’ has also tested extremely well in garden trials. It produces heavy crops – up to 5lbs per plant – of flavoursome, deep red fruits. Although it does well sown outdoors ‘Ferline’ is also suitable for growing under glass.

TOMATO ‘FANTASIO’ F1 Hybrid

This is a deliciously flavoured variety that has also trialled well in the garden situation against ‘Late Blight’ infection. In fact it has also shown good resistance to Tobacco Mosaic Virus, Verticilium wilt, Fusarium Wilts, and nematodes too. Tomato ‘Fantastico will bear you a good crop of round fruits, with each plant producing up to around 6lbs of tomatoes.

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