Charcoal stalk rot is a common disease of maize in hot, dry environments and is caused by the fungi Macrophomina phaseolina. Drought and high temperature encourage disease severity. Charcoal stalk rot is characterized by the stalk turning grey-black. The name of the disease is derived from its symptoms.

Pathogen

• Macrophomina phaseolina (syn. Macrophomina phaseoli, Botryodiplodia phaseoli and Sclerotium bataticola).

Symptoms

Symptoms of charcoal stalk rot are generally first observed once the crop has tasselled. Typically, the upper leaves dry out and characteristic sclerotia can be observed on the rind and vascular tissues. Sclerotia are minute, black, and abundant. The pith or internal sections of the stem show a black discoloration. Vascular bundles within the stem often shred. Symptoms on the stalk are usually more prevalent in the lower stalk internodes. Lodging is often caused by the decomposition of stalk tissue due to infection.

Confirmation

Charcoal stalk rot can be distinguished from other stalk rots by the black discoloration of the stalk pith and abundant production of minute sclerotia on the rind and vascular tissue. Sclerotia typically measure 0.05 to 0.22mm in diameter. Isolates of M. phaseolina that infect maize do not form conidia.

Incidence and factors favoring severity

Charcoal stalk rot is most prevalent in regions and growing seasons that are hot and dry. The disease is strongly associated with drought conditions. Soil temperatures of 30-42°C increase disease severity. M. phaseolina has many hosts including soybean and sorghum; therefore, rotation with these crops can increase inoculum levels and enable the pathogen to perennate in fields. High crop densities and poor soil nutrient status also favor disease severity.

Host range

M. phaseolina infects an extremely wide range of hosts, including sorghum, soybean, cucurbits and various weed species. Over 500 hosts have been documented for M. phaseolina.

Geographic distribution

Life cycle

M. phaseolina overwinters as sclerotia in the soil and can remain viable for several years. In dry and hot conditions fungi infect the roots of maize plants and colonize the lower stalk, eventually giving rise to characteristic symptoms (abundant, minute, black sclerotia and charring and shredding of the pith tissue). Isolates that are pathogenic to maize are not known to produce conidia. Charcoal rot is a soil borne disease.

Damage

• Mechanism of damage: Infection of the stalk can lead to premature senescing of the crop, lodging, and interference with translocation of water and nutrients, which all result in yield loss.
• When damage is important: The disease is particularly prevalent in drought years and in arid regions where maize is regularly cultivated in rotation with other host crops. The disease is heat and stress (drought) driven and is therefore rare in cooler climes and irrigated fields. Increased losses may be experienced where maize is machine harvested due to lodging.
• Economic importance: Charcoal stalk rot is recorded as a serious disease of maize in several arid maize growing regions where extensive yield loss has been documented when the crop is infected early.

Management principles

Host resistance

• Cultivation of resistant hybrids offers the most practical and cost-effective means of disease management.
• Varieties that are resistant to Gibberella and Diplodia stalk rot also tend to be resistant to Charcoal stalk rot.
• Plant varieties with a good stalk strength.

Fungicide

• Foliar application of fungicides is not effective, as once symptoms are apparent, damage has already occurred.
• Fungicide-treated seed may offer protection during seedling growth.

Cultural control

• Reduce water stress by reducing crop density or irrigating the crop during the reproductive growth stages.
• Ensure crop is not under nutrient stress. Application of potassium has been shown to delay senescence of pith cells and thicken their walls.
• Harvest when the crop is mature (30% moisture content) as delayed harvesting can result in extensive crop loss due to lodging.
• Due to wide host range, crop rotation may not provide adequate control.
• It has been reported that survival of Macrophomina phaseoli in soils rich in Penicillium and Trichoderma species is reduced due to parasitism of sclerotia by these species.

References

CAB International. 1985. Macrophomina phaseolina. Distribution maps of Plant Diseases. Edition 1 (August), Map 566.  Wallingford, UK: CAB International.

CIMMYT. 2004. Maize Diseases: A guide for Field Identification. 4th Edition. Mexico, D.F.: CIMMYT.

Durojaiye, J.A., R. Ikotun and J.M. Fajemisin. 1987. The survival of propagules of Macrophomina phaseoli and Diplodia maydis in Nigerian soils. Journal of Basic Microbiology 27: 67-73.

Shaner, G.E. and D.H. Scott. 1998. Stalk Rots of Corn. Purdue University Cooperative Extension Service. Extension Sheet BP-59. http://www.ces.purdue.edu/extmedia/BP/BP-59.pm65.pdf (28 August 2007).

White, D.G. 1999. Charcoal Rot. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 41.

Contributor: Biswanath Das