Head smut, a disease caused by the Sporisorium reilianum f. sp. zeae basidiomycete, has quickly spread across several Mexican regions, including the states of Jalisco, Durango, Hidalgo, Puebla, Querétaro, Guanajuato, Michoacán, Oaxaca, Sonora, Tamaulipas and Aguascalientes, but the disease could spread even farther (^{Aquino-Martínez et al., 2011}). Recently, it has been detected in the Mezquital and Toluca valleys where hybrid maize seed that has shown to be susceptible to the disease is planted. The disease invades the plant systemically and its visible infection symptoms are damage to maize ears and/or tassels (Figure 1).

As part of head smut control management, a project was launched to develop open-pollinated maize varieties with good agronomic traits and genetic resistance to the disease. Genetic resistance is considered the most effective and economic method for controlling this and other crop diseases (^{Pandey and Gardner, 1992}).

The improvement program responsible for developing a synthetic variety containing desirable agronomic traits and genetic resistance to head smut started by developing a broad-based white endosperm maize population. The population was formed by recombining a total of 45 samples collected in the Mexican highlands in isolated plots during two cycles; the collection included commercial hybrids, as well as improved and native materials, with good agronomic performance. To avoid self-fertilization, the collected germplasm was recombined by planting each component in 2 rows 5 m long that were detasseled, and a balanced mixture of all entries was planted as male every 2 rows. The recombination plots were established in a farmer’s land in the ejido Santa Teresa Tiloxtoc, Valle de Bravo, State of Mexico (19° 13´ N, 100° 107´ W, 1740 masl). After two recombining cycles, the base population (C0) was formed, and from this, a S₁ recurrent selection program was started (^{Pandey and Gardner, 1992}) by self-pollinating approximately 400 desirable plants in each improvement cycle and obtaining S₁ seed in each cycle. Seed from the selected S₁ families was inoculated with 1% carboxymethyl cellulose aqueous suspension with 1.7x10⁷ teliospores mL^-1 from the pathogen (^{Quezada-Salinas et al., 2013}). The inoculated seed was planted, one ear per row without replications, in a greenhouse established in the ejido Cinta Larga, Mixquiahuala, Hidalgo (20° 11´ N, 99° 14´ W, 2100 masl), where the disease occurs under natural conditions.

Figure 1 Signs and symptoms of head smut in the reproductive parts of a maize plant. 

Approximately 33% of the S₁ lines that were inoculated and selected based on their desirable agronomic traits and disease resistance were recombined plant to plant, in order to start a new improvement cycle of the base population. Simultaneously, during each evaluation cycle of S₁ lines, groups of 10-12 lines with a desirable specific trait (uniform flowering date, plant and ear height, early maturity, among others) were selected, and then crossed using the diallel scheme with direct crosses to generate F₁ of new experiment synthetic varieties that were advanced to F₂ to be included in agronomic trials along with other varieties obtained from that or another cycle, and commercial hybrids, and thus measure their agronomic performance and grain yield. The population improvement is continuous.

In yield trials using F₂ seed of the experiment varieties obtained from lines of the C₃-S₁ cycle, Blanca 13 was selected because of its outstanding traits, such as better quality and aspect, as well as ear filling, compared to other materials included in the same trials (Table 1). In 2017, this variety was submitted to the National Service Seed Inspection and Certification (SNICS, for its acronym in Spanish) for approval as a new variety known as CP-Vero 1, and delivered to Colegio de Postgraduados, which holds the breeder’s title number 1831. Certificate seed of this variety is being produced in farmers’ plots with good results (Figure 2).

CP-Vero 1 is a high yielding, white grain synthetic maize variety, with a high level of resistance to head smut and low cost, that can be planted for several years. The impact of this project on society is farmers’ income increase, because of the low cost of the seed and increased maize productivity, as a result of the variety’s good yield and disease resistance, which prevents the disease from spreading. This synthetic variety is registered with SNICS and owned by Colegio de Postgraduados for distribution and commercialization.

The S₁ recurrent selection improvement program established as part of this project involves the improvement of an initial genetic broad-based population to obtain S₁ lines whose agronomic performance and resistance to head smut are evaluated using artificial inoculations of the pathogen, thus contributing to the development of new experiment varieties during each improvement cycle.

Figure 2 Certified seed production plot of CP-Vero 1 synthetic variety. Huejotzingo, Puebla, 2019.