Macrophomina phaseolina
Written by Darcy
Partridge
A class project for PP728
Introduction
Macrophomina
phaseolina (Tassi) Goid. a
soilborne fungus causes charcoal rot. The
fungus can infect the root and lower stem of over 500 plant
species and is widely distributed in the United States (8). Charcoal rot is
an important disease during hot, dry weather or when unfavorable environmental
conditions stress the plant. M. phaseolina causes disease on soybean, peanut, and corn. In peanut, it causes seed and seedling rots, wilt, root and stem
rots, leaf spot, and rotting of developing pods and seed. Charcoal rot on
soybean leads to early maturation, chlorosis and incomplete pod filling. While in corn the fungus causes a stalk rot
during hot, dry conditions.
Host Range and
Distribution
M. phaseolina infects over 500 plant species and has a wide
geographic distribution. Major cultivated
hosts include: Arachis hypogaea (peanut), Beta
vulgarius, Brassica oleracea (Cabbage), Capsicum annuum (pepper), Cicer arietinum (chick pea), Citrus spp. Corchorus sp., Cucumis spp., Fargaria sp., Glycine Max (soybean), Gossypium sp., Helianthus annuus (sunflower), Ipomoea batatas (sweet potato), Medicago sativa (alfalfa), Phaseolus spp., Pinus spp., Prunus spp., Sesamum indicum (sesame), Solanum tuberosum (potato), Sorghum
bicolor (sorghum), Vigna unguiculata (bean), and Zea mays (corn).
Isolation
When microsclerotia are visible on host
tissue isolation on culture media is easily accomplished using a dissecting
scope. If microsclerotia are not
evident, isolation should be attempted from the areas of the plant likely
infected. Surface disinfestations of
the tissue with 0.525% NaOCl solution for 1-3 minutes will enhance successful
isolation. Standard solid culture media such as potato dextrose agar (PDA),
cornmeal agar (CMA), lima bean agar (LBA), or water agar (WA) can be used and
incubation at 28 to 35 C for 3-5 days will promote rapid growth of the fungus
and exclusion of other microorganisms.
The microsclerotia
density of M. phaseolina can be determined in soil using
selective media (3). A 5 g-subsample is
first washed in 10% bleach and then rinsed over a 325 μm sieve. The subsample is then added to 100 ml of
molten PDA amended with rifampicin (100 ml/L) and tergitol (1 ml/L) and
distributed evenly over 10 plates. Colonies of M.
phaseolina can be enumerated
with an unaided eye by morphology after incubation at 28 C in darkness for 3 to
5 days.
Identification
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M. phaseolina (Tassi) Goid. (syns. M. phaseolina (Maubl.)Ashby, Rhizoctonia bataticola (Taub.) Briton-Jones, Sclerotium bataticola Taub., and Botryodiplodia phaseoli (Maubl.) Thrium.), is a soilborne plant pathogen belonging to the phylum Deuteromycetes and class Coelomycetes. It is highly variable, with isolates differing in microsclerotial size and presence or absence of pycnidia. The pycnidial stage is not common on soybean, but is on peanut. Pycnida are initially immersed in host tissue, then erumpent at maturity. They are 100-200 μm in diameter; dark to grayish, becoming black with age; globose or flattened globose; membranous to subcarbonaceous with an inconspicuous or definite truncate ostiole. The pycnida bear simple, rod-shaped conidiophores, 10-15 μm long. Conidia (14-33 x 6-12 μm) are single celled, hyaline, and elliptic or oval. |
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| Microsclerotia of M. phaseolina are jet black in color and appear smooth and round to oblong or irregular.
Across isolates, microsclerotia vary on size and shape and on different
substrates. Microsclerotia
are formed from aggregates of hyphal cells joined by a melanin material
with 50 to 200 individual cells composing an individual microsclerotia.
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Colony of M. phaseolina
on PDA.
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Colonies in culture range in color from
white to brown or gray and darken with age. Hyphal branches generally form
at right angles to parent hyphae, but branching is also common at acute angles.
Aerial mycelium with completely or partially appressed growth may or may not
be produce in culture. Some isolates may form concentric growth rings.
Symptoms
Soybean- Charcoal rot is a disease of the soybean root and stem. Charcoal
rot symptoms vary depending on the time of the year that the plant is infected.
Usually charcoal rot is develops later in the season but it can cause a seedling
disease since the roots of the plant can be infected anytime during the season.
Infected seedlings show a reddish brown discoloration at the soil line
extending up the stem that may turn dark brown to black.
Foliage of infected seedlings can appear off-color or begin to dry
out and turn brown. A twin-stemmed
plant may develop if the fungus kills the growing point.
Under cool, wet conditions young plants that are infected may survive
but carry a latent infection that will express symptoms later in the season with
hot, dry weather. In older soybeans symptoms of charcoal rot include
early maturation,
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Charcoal
rot symptoms in soybean field. (Image courtesy of SIU
Soybean Research
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Charcoal rot symptoms
in soybean taproot.
(Image courtesy of SIU
Soybean Research
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Peanut- Charcoal rot in peanut is both seedborne and soilborne. The fungus is commonly found in peanut seed
and pods and can be redistributed by their movement. Water-soaked lesions
on the hypocotyl near the soil-line are characteristic of charcoal rot on
peanut. Once the hypocotyl is girdled the seedling will die. Plant parts at all stages are susceptible to infection. Stem and root lesions first appear water-soaked
but then turn a dull light brown. Infection
can spread into the taproot and stem. Once the stem is girdled the plant will wilt
and the fungus rapidly colonizes the remaining tissues. The dead tissues turn
black as microsclerotia are abundantly produced. Sclerotia can also be produced
on roots, pegs, and pods. Disease
restricted to roots can cause the shredding of the taproot and lead to yellowing
and wilting of the plant (5).
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Foliar
symptoms on peanut
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Taproot
symptoms on peanut.
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Symptoms
in peanut field.
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Images taken from Compendium
of peanut diseases, 2nd Edition. N. Kokalis-Burelle et al.
eds. APS Press. St. Paul MN
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Charcoal
rot in corn stalk.
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Charcoal rot and microsclerotia in
corn stalk.
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Images taken from Compendium of Corn Diseases
3rd Edition. D. G. White ed. APS Press. St. Paul, MN.
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Disease Cycle and
Epidemiology
M. phaseolina survives as microsclerotia in the soil and on infected plant debris.
The microsclerotia serve as the primary source of inoculum and have been found
to persist within the soil up to three years (4). The microsclerotia are black, spherical to oblong structures that
are produced in the host tissue and released in to the soil as the infected
plant decays. These multi-celled
structures allow the persistence of the fungus under adverse conditions such as
low soil nutrient levels and temperature above 30 C. Microsclerotial survival
is greatly reduced in wet soils surviving no more than 7 to 8 weeks and
mycelium no more than 7 days. Seeds may
also carry the fungus in the seed coat. Infected seed do not germinate or
produce seedlings that die soon after emergence.
Germination of the
microsclerotia occurs throughout the growing season when temperatures are
between 28 and 35 C. Microsclerotia
germinate on the root surface, germ tubes form appresoria that penetrate the
host epidermal cell walls by mechanical pressure and enzymatic digestion or
through natural openings (2). The
hyphae grow first intercellularly in the cortex and then intracellularly
through the xylem colonizing the vascular tissue. Once in the vascular tissue M. phaseolina spreads through the taproot and lower
stem of the plant producing microsclerotia that plug the vessels (8). The rate of infection increases with higher
soil temperatures and low soil moisture will further enhance disease
severity.
Hot,
dry weather promotes infection and development of charcoal rot (8). In soybean
charcoal rot is a greater problem after anthesis and often occurs when the
plant is under drought stress (4). M. phaseolina can grow and produce large amounts of microsclerotia under relatively
low water potentials allowing this disease to be recognized as favoring drought
(6). The mechanical plugging of the
xylem vessels by microsclerotia, toxin production, enzymatic action, and
mechanical pressure during penetration lead to disease development. The population of M. phaseolina in soil will increase when susceptible hosts are cropped in successive
years and can be redistributed by tillage practices (8).
Management
Cultural management methods must be implemented
to minimize charcoal rot damage since there are no fungicides available for
effective disease control.
Crop rotation out of a susceptible host
is effective in some crop production systems.
Rotation out of soybeans for three years may effectively reduce
microsclerotia numbers and is useful for managing charcoal rot (7). Corn is also a host for M. phaseolina but isolates appear to be specific to each crop (soybean and corn)
though all isolates can infect both crops.
Corn is not as good of a host to M. phaseolina as soybean so rotation with corn for
three years may help reduce populations but not eliminate the pathogen from the
soil. Rotation with a poor host such as
cotton may only require one or two years to reduce inoculum levels in soil. For
peanuts rotation with cotton or rice for two to three years may help reduce
soilborne inoculum.
Early Planting will aid in earlier canopy
closure that will help reduce soil temperatures and therefore reduce the
competitive ability of M. phaseolina.
Avoid high plant populations. High plant
populations can contribute to increase plant stress and competition for water
increasing charcoal rot potential.
Fertility. Adequate levels of available P
and K will reduce nutrient stress and encourage health plant growth.
Soil Moisture. Tillage practices which
reduce soil moisture stress may reduce disease potential. Maintaining good soil moisture with
irrigation from planting to pod fill may reduce disease potential.
References
1.
Bowen, C. R. and
Schapaugh Jr., W. T. 1989. Relationships among charcoal rot infection, yield,
and stability estimates in soybean blends. Crop Science 29:42-46.
2.
Bowers, G. R. and
Russin, J. S. 1999. Soybean disease management. In Soybean production in the mid-south. L. G. Heatherly
and H. F. Hodges. CRC Press.
3.
Cloud, G. L. and Rupe,
J. C. 1991. Comparison of three media for enumeration of sclerotia of Macrophomina phaseolina. Plant Disease
75:771-772.
4.
Dhinga, O. D. and
Sinclair, J. B. 1977. An annotated bibliography of Macrophomina phaseolina. 1905-1975.
Universidade Federal de Vicosa, Minas Gerais, Brazil.
5.
Mehan, V. K. and
McDonald, D. 1997. Charcoal Rot. In Compendium of peanut diseases, 2nd Edition.
N. Kokalis-Burelle et al. eds. APS Press. St. Paul MN.
6.
Olaya, G. and Abawi,
G. S. 1996. Effect of water potential on mycelial growth and on production and
germination of sclerotia of Macrophomina phaseolina. Plant Disease 80:1347-1350.
7.
White, D. G.1999.
Fungal stalk rots. Compendium of Corn Diseases 3rd Edition. D. G.
White ed. APS Press. St. Paul, MN.
8.
Wyllie, T. D. 1988.
Charcoal rot of soybean-current status. In Soybean diseases of the north central region. T. D.
Wyllie and D. H. Scott, eds. APS Press, St. Paul, MN.
9.
Wyllie, T. D. 1993.
Charcoal rot. In Compendium of soybean diseases, 3rd Edition. J. B. Sinclair and P. A Backman, eds. APS
Press. St. Paul, MN.
Links to other sites
Charcoal
Rot in Corn – University of Nebraska
Charcoal
Rot/Black root disease forestry images
Pictures
of Charcoal rot in Maize DB
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