Manganese and Diseases

The manganese status of a plant can affect, and be affected by, disease infection, often leading to the misdiagnosis of disease infection as manganese deficiency or toxicity (134). The manganese concentration in diseased tissues has been observed to decrease as the disease progresses (135). This occurrence may be due to the pruning of the root system in the case of root pathogens, leading to a reduction in the absorptive surface with a resultant decrease in the plant concentration (136,137). Additionally, microbially induced changes in manganese status, such as that caused by the grey-speck disease (manganese deficiency) of oats have been reported to be due to the oxidizing bacteria in the rhizosphere causing the manganese to become unavailable (138,139). Manganese concentration at the site of infection also has been reported to increase, in direct contrast to the overall manganese plant concentration, which has decreased (140).

The most notable interaction between disease and manganese is that of the wheat disease takeall caused by the pathogen Gaeumannomyces graminis var. tritici, commonly referred to as Ggt. The importance of manganese in the defence against infection by Ggt was demonstrated by Graham (23). Manganese is the unifying factor in the susceptibility of varieties to Ggt under several soil conditions, including changing pH and nitrogen forms as shown in a table by Graham and Webb (141). The role of manganese fertilizer in the amelioration of Ggt has been reported in numerous papers (137,142,143). The effect of manganese fertilizer on infection by Ggt has been shown to impact before the onset of foliar symptoms (137,142).

Several mechanisms have been proposed for the interaction between manganese and disease resistance. These include lignification, with maximal levels reached at the same concentration of manganese as maximal biomass production (144); the concentration of soluble phenols, where manganese deficiency leads to a decrease in the their concentration (144); inhibition of aminopeptidase, which supplies essential amino acids for fungal growth, under manganese-deficient conditions (145); inhibition of pectin methylesterase, which is a fungal enzyme for degrading host cell walls, under manganese-deficient conditions (146); inhibition of photosynthesis leading to a decrease in root exudates and thus becoming more susceptible to invasion by root pathogens (142), though this mechanism has been shown not to be important in controlling Ggt by the lack of effect of foliar-applied manganese (137,147). A plant capable of mobilizing high concentrations of Mn²⁺ that are toxic to pathogens but not to plants in the rhizosphere may directly inhibit pathogenic attack (141).

TABLE 12.1

List of Critical Concentrations of Manganese in Various Agricultural Crops

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