Tomatoes

Identifying and Preventing Tomato Blight

Tomato blight is the rarest of the pests that affect both tomatoes and potatoes, but it is also the most destructive. It was a major factor in the potato famine in Ireland in the 1850s, when millions of people starved to death from the ravages of this deadly disease. In the case of tomatoes, the fungus-like organism can destroy a crop in a matter of days if conditions are favourable. Careful observation and pre-treatment are the only defences against tomato blight.

Symptoms of tomato late blight

Phytophthora infestans , the pathogen that causes tomato blight, needs tissue to survive. Sporangia from an infected plant are transmitted by air, sometimes over several kilometres, and once they land on a suitable host, germination is almost immediate. Late blight tomatoes need only a few hours to develop, all they want is a little free moisture on the leaves from rain, fog or morning dew.

Once infected, symptoms of mildew become visible within three to four days. Small lesions appear on stems, leaves or fruit. Weather permitting

is wet and the temperature is moderate – like most rainy days in summer – the pathogen will sporulate around these lesions and the tomato blight disease will be ready to spread to the rest of the garden and beyond.

The tiny lesions of tomato blight are difficult to detect and sometimes go unnoticed. Late blight symptoms become more obvious when the area around the lesions is waterlogged or bruised and turns greyish green or yellow. Each late blight lesion on tomatoes can produce up to 300 000 sporangia per day and each of these sporangia is capable of forming a new lesion. Once started, late blight disease of tomatoes can wipe out hectares within a few weeks. The foliage of the plant will be completely destroyed and the fruits will be ruined by the dark, greasy spots of necrotic flesh.

Prevention of tomato late blight

Sanitation is the first step in the fight against tomato blight. Clean up all debris and fallen fruit from the garden area. This is especially essential in warm areas where freezing is unlikely to last long and where tomato blight can overwinter in fallen fruit.

Currently, there are no varieties of tomatoes resistant to tomato blight. Therefore, plants should be inspected at least twice a week. Since late blight symptoms are more likely to occur in moist conditions, extra care should be taken during these periods.

For the home gardener, fungicides containing maneb, mancozeb, chlorotanolyl or fixed copper may help protect plants from tomato blight. Repeated applications are necessary throughout the growing season as the disease can attack at any time. For organic gardeners, there are fixed copper products approved for use; otherwise, all infected plants must be removed and destroyed immediately.

Tomato blight can be devastating for both the amateur gardener and the commercial grower, but by paying attention to weather conditions, garden hygiene and early detection, this crop destroyer can be controlled.

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