Principles of Plant Genetics and Breeding

522 CHAPTER 31
James R. Smith
Figure 1 Soybean plants infected with M. phaseolina that have
wilted and died prematurely. Note the dead leaves and petioles
attached to the plants. (Courtesy of A. Mengistu.)
Figure 2 Microsclerotia on the lower interior (vascular,
cortical, and pith tissues) of a split soybean stem that was
inoculated with M. phaseolina at the time of planting and
harvested at physiological maturity.
Industry highlights
Estimating inheritance factors and developing cultivars for
tolerance to charcoal rot in soybean
USDA-ARS Crop Genetics and Production Research Unit, Stoneville, MS 38776, USA
The following is a summary of work in progress for the
purpose of understanding the inheritance of soybean
(Glycine max (L.) Merr.) tolerance to charcoal rot and
for developing improved varieties with tolerance to it.
The summary demonstrates the kind of thinking and
planning that needs to take place to successfully
achieve these objectives.
Charcoal rot is a disease of soybean caused by the
fungus Macrophomina phaseolina (Tassi) Goidanich
and occurs throughout the world, where it can cause
severe yield losses to soybean. Charcoal rot symptoms
appear during midsummer under conditions of high
temperature (28–35°C) and low soil moisture availability.
Diseased plants may wilt and prematurely die,
with dead leaflets and petioles remaining attached to
the plant (Figure 1). The most diagnostic symptom
of charcoal rot is the black speckled (charcoal-like)
appearance of microsclerotia on the exterior and interior
(vascular, cortical, and pith tissues) of the lower
stem of affected plants (Figure 2). Disease severity can
be affected by heat, drought, and the presence of other
pathogens. Attempts to control charcoal rot in soybean
have included crop rotation, application of fungicides,
biological controls, and adjustments to plant population,
planting date, and irrigation protocols. Because
these strategies have largely failed, researchers have
become increasingly interested in the potential of varietal
tolerance as a means of reducing yield losses.
However, little progress has been made in developing
charcoal rot-tolerant varieties, and the inheritance of
varietal tolerance to charcoal rot is unknown.
In order to understand the inheritance of tolerance
to charcoal rot and utilize it in breeding programs, it is
necessary to generate segregating families (populations)
of plants. Therefore, highly tolerant and highly
susceptible soybean lines were chosen as parents,
controlled pollinations between them were successfully
made, and the appropriate segregating populations
were generated.
Various options are available with segregating
populations for estimating the importance of genetic
factors in tolerance to charcoal rot. One way is by
estimating heritability. Heritability can be expressed
as the ratio of genetic variation over the total variation
(genetic and non-genetic) influencing the trait. Traits
with high heritability can be easily selected by plant
breeders, whereas those with low heritability are not