Abstract
Northern corn leaf blight (NCLB), the most devastating leaf pathogen in maize (Zea mays L.), is caused by the heterothallic ascomycete Setosphaeria turcica. The pathogen population shows an extremely high genetic diversity in tropical and subtropical regions. Varietal resistance is the most efficient technique to control NCLB. Host resistance can be qualitative based on race-specific Ht genes or quantitative controlled by many genes with small effects. Quantitative resistance is moderately to highly effective and should be more durable combatting all races of the pathogen. Quantitative resistance must, however, be analyzed in many environments (= location × year combinations) to select stable resistances. In the tropical and subtropical environments, quantitative resistance is the preferred option to manage NCLB epidemics. Resistance level can be increased in practical breeding programs by several recurrent selection cycles based on disease severity rating and/or by genomic selection. This review aims to address two important aspects of the NCLB pathosystem: the genetics of the fungus S. turcica and the modes of inheritance of the host plant maize, including successful breeding strategies regarding NCLB resistance. Both drivers of this pathosystem, pathogen, and host, must be taken into account to result in more durable resistance.
Highlights
Northern corn leaf blight (NCLB) Resistance in Maize airborne conidia produced on maize debris which are transported by wind, rain, and seed borne inoculum (De Rossi and Reis, 2014)
The most adequate resistance type used in a breeding program depends on the population structure and the evolutionary capacity of the pathogen
In environments with lower disease pressure and low diversity of S. turcica populations, like in the temperate regions, introgression of Ht genes by recurrent backcrossing might be favored, because it is easy to accomplish for the breeder
Summary
Setosphaeria turcica (Luttrell) Leonard and Suggs (syn. Helminthosporium turcicum, teleomorph Exserohilum turcicum [Pass.] Leonard and Suggs), subclass Loculoascomycetidae, order Pleosporales, is a heterothallic ascomycete overwintering on host plant debris as dormant mycelium or as chlamydospores in the soil (Leach et al, 1977). The toxin induces disease symptoms and is associated with fungal aggressiveness (Bashan and Levy, 1992), the quantitative ability of a fungus to cause infection in the host (Vanderplank, 1984; Becher et al, 2013) This qualitative interaction between the resistance (R) gene of the host, and the Avirulence (Avr) gene of the pathogen directly affects conidial germination and ramification, and increases lesion size when the phytotoxin concentration is >250 ppm (Bashan et al, 1996). NCLB may cause a reduction of feeding value and increases pre-disposition of maize to stalk rot (Hooker et al, 1965; Fajemisin and Hooker, 1974) To reduce these negative effects, fungicides, biological control, improved management practices, and resistant cultivars can be used. This often leads to low durability of resistances and, both drivers of this pathosystem must be analyzed to result in a sustainable management of resistance
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