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Nutritional disorders of tomatoes

Iron deficiency

Symptoms are yellowing of the very youngest leaves at the top of the plant, with even the smallest veins remaining green; usually caused by over-watering and/or poor soil aeration. It can be treated by spraying with a proprietary iron foliar feed, but you also need to fix the underlying cause.

Magnesium deficiency

Symptoms are yellowing of the older leaves with the main veins staying green. This can be due to a number of causes:

  • Low soil magnesium levels (especially if potassium level is high)
  • Root problems caused by waterlogging, disease, over-application of soluble fertilizers
  • Stress due to heavy fruit load

Removing the underlying cause should alleviate symptoms. Use a liquid feed which contains magnesium as well as potassium. Have your soil analysed and apply magnesium fertilizer before the next crop (if required). If it’s really bad you can spray with 1 litre per 10 sq m of a 2% solution of Epsom’s salts, with a little washing-up liquid as a wetter. Be careful, as this can scorch the leaves if done in sunny conditions.

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Blotchy ripening & leaf curling

Blotchy ripening

Uneven ripening and fruit that tastes ‘flabby’ (low acidity) is usually due insufficient potassium. Feed every watering with a high potash tomato feed. Have your soil analysed and apply high potash fertilizer before the next crop (if required).

Leaf curling

Early in the season, there can be large differences in temperature between the warm, sunny days and cold nights. These wide diurnal fluctuations can cause the leaves to roll up or curl. This is due to large amounts of carbohydrates accumulating in the leaves because the nights are too cold for the plants to assimilate them. It is usually a temporary problem that cures itself as the nights get warmer.

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Diseases of tomatoes

Late (potato) blight

Late blight (Phytophthora infestans) affects both tomatoes and potatoes. The disease survives over winter on volunteer potatoes (plants growing from tubers left in the ground). Spores can also blow in from nearby infected crops. In warm, wet seasons the disease spreads rapidly – it’s less of a problem in fine weather. Commercial potato growers use ‘Smith periods’ to predict when their crops are at risk (two successive days with temperatures above 10°C and relative humidity above 90% for more than 11 hours each day). You can see when these are forecast here: Blightwatch. The only materials available to gardeners for the control of blight contain copper and are ‘protectants’ which means they have to be applied before the plants are infected, with coverage of all surfaces. Copper is toxic to aquatic organisms and it accumulates in the soil so we don’t use it in our garden.

If you are in a high-risk area, choose some blight-tolerant varieties; we have selected six of the best for our Blight-tolerant Outdoor Tomato Collection.

Other leaf diseases

Many modern F1 varieties e.g. Shirley F1 are resistant to the most common leaf diseases such as Leaf Mould (Cladosporium) and Tomato Mosaic Virus (TMV) (Gardener’s Delight is NOT resistant to TMV). Powdery mildew can also be a problem in some years.

Root diseases

If you have a small greenhouse and always grow your tomatoes in the same place, then there can be a build up of soil-borne diseases such as Fusarium (wilt; crown and root rot), Verticillium wilt and Corky root rot. You can get round this either by not growing in the soil or by using grafted plants.

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Pests, diseases and disorders of tomatoes

Pests, diseases and disorders of tomatoes
If you can’t find the answer to your problem here, tweet your veg gardening woes to me and I’ll advise/sympathise @AgonyPlant, or if you’re not on twitter, email agonyplant@delfland.co.uk

Pests, diseases and disorders

If you can’t find the answer to your problem here, tweet your veg gardening woes to me and I’ll advise/sympathise @AgonyPlant, or if you’re not on twitter, email agonyplant@delfland.co.uk.

There are 3 key areas:

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Blossom-end rot

Disorders of tomatoes – page discussing disorders – Blossom-end rot

Blossom-end rot

Blossom-end rot (BER) is due to a lack of calcium in the distal end of the fruit (the end where the flower dropped off) opposite the calyx (where it is joined to the plant). Sometimes there is an internal black rot with little or no external signs. It is seldom due to an actual deficiency of calcium in the growing medium or soil. Calcium is taken up passively and carried in the ‘transpiration stream’. It can only be absorbed by actively growing root tips. All this means that there can be several underlying causes:

  • erratic watering, especially in peat bags
  • sudden transition to low humidity after a period of dull weather
  • root damage
  • surge in vegetative growth due to high Nitrogen liquid feed or fertilizer, particularly if there is a high proportion of ammonium rather than nitrate-N
  • excessive fertilizer or feeding

Take off all the affected fruit (it tastes bitter). Try to maintain an even amount of moisture in the soil or growing medium. Feed with a high potash feed once the fruit start to swell, according to the instructions on the packet. If the plants are growing in a greenhouse, don’t keep the vents closed during humid weather, leave slightly open. Plum and beefsteak tomatoes are much more prone to this disorder than cherry varieties.

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Pests of tomatoes

Pests

Pests aren’t usually too much of a problem on outdoor crops, but in greenhouses commons pests include aphids, glasshouse whitefly and red spider mite (properly known as two-spotted spider mite). Commercial growers use biological control agents to manage these pests and these are available on our website: Natural pest control. Soft soap is useful for reducing the numbers of pests – use distilled, rain or soft tap water to dilute it.

There is a list of insecticidal soaps and oils here : Pesticides for Home Gardeners RHS Advisory Service February 2015.