The Genetic Basis of Disease Resistance in Maize - Translational Genomics for Crop Breeding: Biotic Stress - page 32

Because of maize's importance, its genetics
and biology have been the focus of considerable
research effort in the public and private sectors.
A number of advanced breeding and genomic
resources have been developed for understand-
ing the genetics of resistance in maize, includ-
ing populations derived from bi-parental crosses
(e.g., Coe et al. 2002; Szalma et al. 2007;
Belcher et al. 2012), several association map-
ping panels (Flint-Garcia et al. 2005; Yan et al.
2011), a nested association mapping population
(McMullen et al. 2009; Yu et al. 2008), and large
genomic and sequence datasets (Ganal et al.
2011; Gore et al. 2009). These resources have
been utilized, at least to some extent, to better
understand the genetic architecture of resistance
for multiple diseases (e.g. Kump et al. 2011;
Poland et al. 2011; Wisser et al. 2006; Wisser
et al. 2011).
With its advanced genetic and genomic
resources, maize can be used both as a model
system for understanding plant-pathogen inter-
actions and as a practical system in which these
basic biological findings can be applied in breed-
ing programs to address farmers' production
constraints. The challenge is to produce more
resistant varieties in the context of various scien-
tific and resource constraints and within the orga-
nization of the global maize breeding infrastruc-
ture. The purpose of this chapter is to summarize
the current understanding of the genetic basis of
disease resistance in maize and to note some of
the challenges and frontiers in its application.

1972). As it turned out, T-urf13 also conferred
specific hyper-susceptibility to a toxin produced
by C. heterostrophus race T (Wise et al. 1999).
The ensuing SLB epidemic of 1970 was one of
the most economically damaging plant disease
epidemics of all time: yield loss throughout the
United States for that season was estimated at 20-
30%, with some areas suffering 50-100% losses
(Ullstrup 1972). The amount of maize lost to
the disease was much larger than, for instance,
the amount of potato lost during the Irish late
blight epidemic of the 1840s. Because suscep-
tibility was under very simple genetic control,
simply switching to germplasm lacking T-urf13
was sufficient to rapidly control the disease in
the following seasons. It should be noted that the
cause of the epidemic was not an overall lack
of genetic diversity, but rather the ubiquity of
a single-disease susceptibility gene within elite
germplasm.
Today, global maize diseases that pose
threats to yield and human health include fungal
diseases that attack the leaves, stem, and ear
(Balint-Kurti and Johal 2009). Globally impor-
tant foliar diseases include southern leaf blight
(SLB) caused by C. heterostrophus , southern
rust caused by Puccinia polysora , common
rust caused by Puccinia sorghi , northern leaf
blight (NLB) caused by Setosphaeria turcica ,
and gray leaf spot (GLS) caused by Cercospora
zeae-maydis and Cercospora zeina. Diplodia
and Fusarium stalk and ear rots and Fusarium
and Aspergillus kernel and ear rots are also
important in many regions. Diseases of regional
importance include tar spot complex (caused
by Phyllachora maydis and Monographella
maydis ) in Latin America and maize streak virus
(MSV) in Africa (Shiferaw et al. 2011). In addi-
tion to established diseases, emerging diseases,
such as banded leaf and sheath blight (BLSB)
in Asia (Pingali 2001) caused by Rhizoctonia
solani , also pose potential future constraints to
maize production. Maize diseases have been
reviewed elsewhere more extensively (Balint-
Kurti and Johal 2009; Pratt and Gordon 2006;
White 1999).

Understanding the Intruders:
Diseases of Maize

Historically, maize has suffered major losses due
to disease, with perhaps the best-known epiphy-
totic being the southern leaf blight (SLB) epi-
demic caused by Cochliobolus heterostrophus
in 1970-71 in the United States. At the time of
the epidemic, the T-urf13 gene conferring cyto-
plasmic male sterility was widely used in maize
hybrid seed production. About 85% of the U.S.
maize crop carried this gene in 1970 (Ullstrup