Tony Dickerson answers your questions
Q Why is the green on my outdoor tomato leaves turning yellow? Margaret Hemsworth, Ilminster, Somerset
A Tomatoes may display a range of leaf symptoms during the course of the growing season. Generally, well-fed and well-watered plants will grow out of the problems but some of the symptoms can be quite alarming. If leaves turn white and papery it’s a sign of temperature injury. This is common early in the season, where tomatoes have been put outside too early or before they’ve been properly hardened off. In early summer, days can be warm but nights surprisingly chilly and temperatures below 12C (54F) will often result in symptoms, including marked leaf curling. Later in summer, such damage can be due to very high temperatures and high light intensity such as we experienced in southern England in June. Tomatoes in windy situations are more likely to show symptoms, so a protective windbreak against the prevailing wind is useful. Usually, affected tomatoes will grow out of these leaf problems as the season progresses and the damaged leaves can be removed.
If the loss of colour is more a yellowing between the leaf veins (interveinal chlorosis), especially on the lower leaves, it’s likely to be magnesium deficiency. Magnesium is mobile within the plant and can be moved from the older, lower leaves to use in the younger, newer ones. If just a few of the lower leaves are affected it can be ignored.
A liquid tomato fertiliser will usually provide adequate magnesium for the plant, but if leaf yellowing is more extensive the cause could be insufficient feeding or too much. Overfeeding with tomato fertiliser, which is high in potassium, can cause magnesium deficiency as plants will take up potassium in preference to magnesium. Typically, aim to feed every seven to 14 days once the second truss has set.
The quickest way to rectify magnesium deficiency is to apply Epsom salts as a foliar feed. Dilute the salts at 20g per litre (⅓oz per pint) of water, plus a few drops of liquid detergent. Apply two or three times at fortnightly intervals, spraying in dull weather to avoid leaf scorch.