Macrophomina phaseolina (charcoal rot of bean/tobacco)

SoyabeanRootsA reddish-brown discoloration occurs in the vascular tissues of the taproot. Black streaks are found in the woody portion of the crown.SeedlingsA reddish-brown discoloration of the hypocotyl is seen. Infection from the roots leads to dark brown to black discoloration at and above the soil line, followed by death of the seedlings, particularly in dry weather. Seedlings may survive under cool, wet conditions, carrying a latent infection.StemsOccasionally, superficial lesions extend from the soil line. Microsclerotia form in vascular elements. They also form in the pith, giving a greyish-black colour to subepidermal tissues. They are first visible as profuse small, black, randomly distributed specks at the nodes.MaizeRootsBrown, water-soaked lesions, which later turn black, are present on the roots. Sclerotia may be found on the roots.SeedlingsSeedlings rot when infected by M. phaseolina.StalksThe pathogen invades the lower internodes, causing premature ripening, shredding and breaking at the crown. Numerous black sclerotia may be found on vascular strands, giving the interior of the stalk a charred appearance. Sclerotia may be found just beneath the stalk surface.EarsKernels blacken following infection by the fungus.SorghumRootsWater-soaked lesions, which turn brown-black, are visible on the roots. Sclerotia form on the remnant walls of the vascular cylinder and cortex.SeedlingsCharcoal rot may be expressed as seedling blight, damping-off and dry rot.StalksAffected stalks are soft or spongy at the base, and tend to lodge in moderate winds. The pith of the stalk may be in various stages of disintegration, which extends toward the panicle across several nodes. Fibrovascular bundles are separated from one another and are profusely marked by the small, dark, charcoal-coloured sclerotia of the pathogen.SunflowerHeadsThe heads of infected plants are poorly filled.StalksPremature ripening and drying occurs. The base of the stalk is usually discoloured and the pith disintegrates. Vascular fibres have a shredded appearance and become covered with small, black sclerotia which seldom exceed the size of pepper grains. A destructive stalk rot may occur under high temperature and drought conditions; symptoms are usually not apparent until after flowering.GroundnutRootsLesions on the roots appear water-soaked at first, but infected tissues eventually have a dull, light-brown appearance. Later, affected areas become covered with sclerotia. Roots become rotten and blackened with shredding of the taproot.SeedlingsWater-soaked lesions appear on the hypocotyl near the soil surface. Girdling of the hypocotyl results in seedling death.Stems and branchesAttack is most common at or near the soil line in older plants. Where lesions girdle the stem, the plant wilts and the fungus rapidly colonizes the branches which turn brown and die. The dead tissues rot and turn black, as sclerotia of the fungus develop profusely. Infected pegs and pods also rot and become covered with sclerotia.Phaseolus vulgarisSeedlingsThe first symptom appears on the stem at the soil line as a small, irregularly shaped, blackish, sunken lesion. Lesions may occur before or soon after emergence.Upper plantInfection spreads upwards from the original canker. Several cankers may enlarge, coalesce and eventually girdle and kill the plant. Cankers possess a definite margin and commonly contain concentric rings. Wilting, chlorosis and death of leaves may be more pronounced on one side of the plant. Numerous small, black sclerotial bodies or pycnidia form on the ageing, ash-grey cankers. Large areas of the crop may be killed.Pods and seedsAsh-grey lesions develop on affected areas of pods and seeds.

Cultural Control and Sanitary Methods

Much of the effort on control of M. phaseolina has focused on the management of populations of microsclerotia. A range of factors that influence the disease have been combined into computer models that predict changes in these populations (Todd et al., 1987). Maize, sorghum and cotton are hosts for the pathogen, but they support lower populations of microsclerotia in soil than soyabean. One-year rotation with any of these crops has been suggested as a means of maintaining populations at an acceptable level for soyabean production, i.e. 15 microsclerotia per gram of soil (Todd et al., 1987). Herbicide stress has only a minor influence on microsclerotial populations, particularly on soyabean plants with root injuries. Effects also depend on the herbicide (Bowman, et al., 1986; Canaday et al., 1986). There is little direct evidence to relate fertility or plant nutrition to charcoal rot in soyabean (Todd et al., 1987). Higher nitrogen rates increased the disease in sunflower (Tosi and Zazzerini, 1990), maize (Pande et al., 1993), safflower (Singh et al., 1987) and chickpea (Taya et al., 1988).

Irrigation is beneficial as a means of reducing infection by M. phaseolina in several crops, including soyabean (Michail et al., 1979), sunflower (Blanco-Lopez and Jimenez-Diaz, 1983) and Phaseolus vulgaris (Diaz-Franco and Cortinas-Escobar, 1988).

Effective control can be achieved by growing soyabean cultivars of full-season maturity groups that escape the hottest and driest conditions in the period after flowering (Bowen and Schapaugh, 1989). Blends of pure lines of different maturity groups neither increased yield or decreased disease severity (Bowen and Schapaugh, 1989). In West Bengal, India, the incidence of M. phaseolina was favoured by planting in December, while early planting in November or late planting in January reduced disease incidence (Pande et al., 1993 ). M. phaseolina infection of inoculated jute plants was generally greater in mid-April and later sowings. Disease expression was related to the rainfall pattern, and air temperature was the most critical factor. Lesion length was affected by RH (Ji, 1984).

A range of organic amendments such as farmyard manure, Neem and mustardcake have been used to control Macrophomina diseases (Rathore, 2000). A 20-40% decrease in soil poulations of the fungus was achieved through soil amendment with pearl millet and weed-based composts and this was associated with a 28-50% increase in actinomycetes (Satish Lodha et al., 2000).

Soil solarization has been used with contrasting results. In one study in Egypt, solarization for a prolonged period of 6 months increased the population of Macrophomina (Botross et al., 2000). However, Hoda et al. (2000) also in Egypt, report that soil populations were decreased following solarization with clear polythene for 30 days, giving a decrease in root rot and wilt in cotton.

Soil populations of the pathogen were greater under no-tillage than either disc or mould-board tillage but the difference was not sufficient to not affect infection rates (Wrather et al., 1998).

Host-Plant Resistance

Resistance to charcoal rot, caused by M. phaseolina, has been most extensively investigated in sorghum. Non-senescence is a delayed leaf and plant death resistance mechanism in sorghum that circumvents the detrimental effects of reduced soil moisture combined with high temperatures during post-anthesis growth. This drought-tolerance mechanism is often equated with charcoal rot resistance. The inheritance of charcoal rot resistance was investigated directly by exposure of sorghum to M. phaseolina, and indirectly by determination of the inheritance of non-senescence. Experiments carried out under controlled and field conditions showed that non-senescence and charcoal rot resistance are not to be equated with each other. Therefore, non-senescence alone cannot account for, and should not be used as the sole breeding criterion for, resistance to charcoal rot in sorghum (Tenkouano et al., 1993). One study in India suggested that susceptibility was dominant over resistance to charcoal rot in sorghum. Inheritance of resistance was polygenic, and non-allelic interactions such as additive x dominance and dominance x dominance played an important role in the inheritance of resistance (Rao and Shinde, 1985). Another study indicated that resistance to M. phaseolina is recessive and controlled by three complementary genes (Venkatarao et al., 1983).

Monogenic inheritance of resistance was indicated in chickpea, with resistance dominant to susceptibility (Rao and Haware, 1987). Some highly resistant lines of sunflower have been identified in Pakistan (Hafeez and Ahmad, 1997). In sesame, mature plant reaction to M. phaseolina in four crosses between six local and introduced sesame cultivars indicated that susceptibility was dominant over tolerance and was controlled by one, two or three pairs of genes (Selim et al., 1976). In maize, the additive gene effect was found to be predominant over the non-additive effect (Singh and Kaiser, 1991). Searches for resistance in soyabean have not been successful (Schneider et al., 1974). This has been attributed in part to the variability of the pathogen (Todd et al., 1987).

Extensive genetic variation and the site-specific nature of M. phaseolina have made studies on genetics of charcoal rot resistance difficult. Therefore, genetics of resistance against M. phaseolina have not been clearly demonstrated and controversies are found in the findings of various workers. Resistance in sunflower genotype is a dominant character (Olaya et al., 1996; Michel, 2000). It has been reported that resistant genes against Macrophomina phaseolina do not exist or are unknown; however, it has also been reported that the presence of two dominant genes, MP 1 (which imparts resistance against Macrophomina phaseolina in peas) and MP 2 is essential in resistant cultivars.

Charcoal rot, caused by M. phaseolina, is economically important across a broad range of crops throughout the world, particularly in regions that experience hot, dry conditions during the growing time. Annual losses in soyabean were estimated at 5% throughout Missouri, USA, with some growers experiencing 30-50% loss (Todd et al., 1987). One report suggested that charcoal rot was responsible for greater losses in soyabean than any other disease from central Mississippi and Alabama to central Illinois and Indiana (Montes et al., 1975).In cotton, losses caused by soilborne fungi, particularly M. phaseolina and Fusarium oxysporum f. vasinfectum, were estimated at about US$481, 593 over 27, 000 ha of cotton in 1982 in Peru (Delgado and Agurto, 1984). Losses in seed weight per chickpea plant caused by M. phaseolina were 70.8% at full podding and 48.9% at pre-harvest stages (Quaiser-Ahmad et al., 1986). Yield losses in groundnut of 100, 94 and 63.03% occurred when disease appeared at the pre-emergence, pre-pod and pod-filling stages, respectively (Sharma and Bhowmik, 1986). The disease reduced grain yield in sorghum by 31-38% in two breeding lines (Choudhari and Tikhotkar, 1987). Yield losses of 36.8-79.2%, caused by M. phaseolina, were reported on sunflower in the western plains of Venezuela. The percentage of infected sunflower plants among the five hybrids studied varied from 10% in M-733 to 45% in GV-28074, and total yield losses varied from 0% in M-702 to 16.5% in GV-37027 (Pineda and Avila, 1993). Leaf and pod infection by M. phaseolina caused a 10.8% reduction in grain yield and 12.3% reduction in protein content of Vigna radiata seed (Kaushik et al., 1987).