Macrophomina phaseolina is a common soil fungus in warmer regions, causing charcoal rot. It has a very wide range of host crops, many of which are economically important food crops.
Life cycle and appearance of Charcoal rot
The fungus survives in the soil and on crop residues as microsclerotia. In corn stalks, survival of up to 1.5 years has been reported. The density of the microsclerotia in the soil is directly related to disease incidence.
Within 24 hours the fungus infects the roots of the host by direct penetration via germ tubes. During the next two days, necrotic spots occur on the roots and the fungus spreads into the cortex and vascular tissue. In the infected host tissues, new sclerotia are formed. Wilting, falling of leaves and collapse of the whole plant may occur when the vessels become blocked.
Dispersal is almost solely by microsclerotia as opposed to by spores. Microsclerotia can be dispersed in soil particles by farm equipment, people, packing materials etc. The fungus may be transferred on seed but this is not considered a main source of infection.
Optimum conditions for this fungus are temperatures above 32 °C and dry conditions. In field crops in temperate regions it is therefore not a problem.
In strawberry, this pathogen causes wilting of foliage, stunting of plants and drying and death of older leaves. The youngest central leaves usually remain unaffected and stay green. The first symptoms are mostly found after plants are well established and after plants begin bearing fruit or are subjected to stress. Severely infected plants collapse and die. When the crown is cut, internal symptoms can be seen, such as discolouration of vascular and cortex tissue (outer tissue layer) to orange-brown to dark brown. Cutting the roots may show the same discolouration of the internal tissues of the main roots.
In most crops, necrotic lesions on the roots are the first symptoms, followed by wilting, dropping of leaves and collapse of the entire plant.