The genus Setosphaeria consists of an important group of species that includes pathogens of monocots, such as the maize pathogens Setosphaeria turcica (causal agent of Northern Leaf Blight, NLB) and Setosphaeria rostrata (causal agent of Rostratum Leaf Spot). Setosphaeria is sister to the related maize pathogen Cochliobolus heterostrophus, and to the wheat pathogens, Pyrenophora tritici-repentis and Mycosphaerella graminicola. Sequencing of S. turcica provides the opportunity to understand the interaction between a hemibiotrophic pathogen and its host. As with all hemibiotrophs and biotrophs, the expectation is that genes involved in the pathogen/plant battle are continually evolving. As new fungal effector molecules (secreted disease factors) are deployed, the host deploys new plant resistance genes.
Several races of S. turcica have been described based
on the symptoms they cause on a panel of maize lines carrying the
‘major resistance’ genes, Ht1, Ht2,
Ht3, and HtN (Table 1). By definition, maize
carrying e.g., Ht1 is susceptible to S. turcica
race 1. Genome sequence of race 1 strain NY001 will allow
comparison with race 23N strain 28A.
Northern Leaf Blight, caused by S. turcica, is a major disease of maize, both in the United States and internationally, and poses a significant production constraint. The deployment of host resistance remains the most effective strategy for S. turcica management. Efforts to isolate and utilize host resistance loci will be more efficient when informed by insights on pathogenic mechanisms.
There is an extensive collection of S. turcica field isolates available which may be used for analyses of natural variation. Sexual crosses can be made in the laboratory. Transformation is possible with S. turcica, allowing molecular manipulation of candidate fungal genes.
|Table 1: S. turcica and maize
|B73, Pa, B68 = maize genotypes carrying or not
carrying Ht1-3 or N genes.
Race designation = able to cause disease on corresponding line.
i.e., maize line with gene Ht1 is susceptible (S) to race 1, but resistant (R)
to races 0, 2, 3, or N or a combination thereof.