Molybdenum deficiency in tomato plants

These leaves show some mottled spotting along with some interveinal chlorosis. An early symptom for molybdenum deficiency is a general overall chlorosis, similar to the symptom for nitrogen deficiency but generally without the reddish coloration on the undersides of the leaves. This results from the requirement for molybdenum in the reduction of nitrate, which needs to be reduced prior to its assimilation by the plant. Thus, the initial symptoms of molybdenum deficiency are in fact those of nitrogen deficiency. However, molybdenum has also other metabolic functions within the plant, and hence there are deficiency symptoms even when reduced nitrogen is available. At high concentrations, molybdenum has a very distinctive toxicity symptom in that the leaves turn a very brilliant orange.

Role of molybdenum in tomato plants

Molybdenum is needed by plants for chemical changes associated with nitrogen nutrition. In non-legumes(such as cauliflowers, tomatoes, lettuce, sunflowers and maize), molybdenum enables the plant to use the nitrates taken up from the soil. Where the plant has insufficient molybdenum the nitrates accumulate in the leaves and the plant cannot use them to make proteins. The result is that the plant becomes stunted, with symptoms similar to those of nitrogen deficiency. At the same time, the edges of the leaves may become scorched by the accumulation of unused nitrates.

Symptoms

The main symptoms of molybdenum deficiency in non-legumes are stunting and failure of leaves to develop a healthy dark green colour. The leaves of affected plants show a pale green or yellowish green colour between the veins and along the edges. In advanced stages, the leaf tissue at the margins of the leaves dies. The older leaves are the more severely affected. In cauliflowers, the yellowing of the tissue on the outer leaves is followed by the death of the edges of the small heart leaves. When these develop, the absence of leaf tissue on their edges results in the formation of narrow, distorted leaves to which the name ‘whiptail’ has been applied. Affected leaves are usually slightly thickened and the leaf edges tend to curl upwards, especially in tomatoes

Control

In most soils, molybdenum present in an unavailable form will be released by applying lime or dolomite.The effect of liming on molybdenum availability is slow and it may take several months to correct the deficiency. The amounts of lime or dolomite needed may range from 2 to 8 tonnes per hectare, depending on initial pH of the soil and whether it is sandy or heavy textured. Unless lime is likely to be beneficial for other reasons, it is quicker and cheaper to apply a molybdenum compound to the soil or to the crop. Where one of the molybdenum compounds is used, the quantities recommended vary from 75 g to 1 kg/ha depending on the crop and the molybdenum material.

Molybdenum can be applied in the following ways:
• mixed with fertiliser; or
• in solution, to
— seedlings in the seedbed before transplanting;
— the leaves of plants in the field; or
— the soil at the base of plants in the field.