Manganese mischief managed

This past season, manganese (Mn) toxicity was suspected in some cabbage plots at University of Guelph, Ridgetown Campus. Leaves and soil were sampled and submitted for analysis. The results were:

Leaf analysis:

• 671 and 830 ppm
• Adequate range: 20-40 ppm (OMAFRA Vegetable Production Recommendations 2008-2009)
• Excessive range: > 300 ppm (Micronutrients in Agriculture)
  

Soil analysis:

• pH: 4.9
• soil test Mn: 17 ppm
• Mn Index: 62
• The Mn Index evaluates the soil's pH affect on manganese availability. Generally a Mn Index > 30 is adequate.

Figure 1: Mn toxicity symptoms begin in older leaves

Figure 2: Leaf margins curl inwards

Symptoms

Symptoms often begin on lower, older leaves. Brown necrotic spots or chlorotic mottling develop near leaf tips and along margins (Figure1). These necrotic areas can provide opportunities for disease infection. Leaves curl inwards (Figure 2). The margins become ragged, giving a "crinkle leaf" appearance. Lower leaves eventually die.

Causes

Manganese toxicity may be a problem for sensitive plants growing in extremely acid soils (< pH 5. 0). It may also occur if excessive soil and/or foliar applications are used.

Diagnostic tools

Leaf and soil analysis can be used to confirm suspected Mn toxicity. Cabbage leaf Mn levels are usually considered adequate between 20-40 ppm. Adequate ranges of 25-200 ppm are also reported. Levels greater than 300 ppm are considered excessive. A soil pH less than 5.0 may indicate possible Mn toxicity, especially for sensitive plants. Besides cabbage, other sensitive plants include cauliflower, dry edible beans, potatoes, sugar beets and tomatoes.

Molybdenum deficiency

Cabbage, like some other cole crops, is susceptible to both a Mn toxicity and molybdenum (Mo) deficiency at a low soil pH. The leaves were not analysed for Mo. Plants did not show the typical whiptail symptom often associated with Mo deficiency in cole crops. But the marginal chlorosis and margin curling or cupping seen with excessive Mn can also be observed with a Mo deficiency. Unlike other micronutrient deficiency symptoms, older leaves can express Mo deficiency symptoms.

Management

Lime soils to the desired pH range, as determined by the soil test. This will prevent any excessive manganese levels and correct molybdenum deficiencies.