Horse’s tooth disease of maize, caused by the ascomycete Claviceps gigantea (^{Fuentes et al., 1964}), sexual phase of the anamorph Sphacelia sp., has disseminated from the regions in which it was originally reported, namely, the Toluca Valley in the State of Mexico and the Pátzcuaro mountains, in Michoacán, to different locations in the states of Puebla and Hidalgo, all of which display high relative humidity and low temperatures. In these locations, the disease causes losses in the production of the cereal where susceptible hybrids or local varieties of maize seeds are planted.

The disease begins when the newly matured maize kernels infected by the fungus develop sclerotia, resting structures that allow them to hibernate. During harvest, the sclerotia fall to the ground and remain dormant during the winter to reactivate their metabolism in the following rainy season. When they reactivate under favorable conditions of humidity and temperature, the sclerotia germinate forming stipes (stalks) with head-like structures in the apex in which perithecia are formed, containing asci with ascospores, the infective structures of the fungus which penetrate the ear and infect fresh silks. In its asexual phase, the sclerotia also produce micro and macroconidia, which may also penetrate the ear and infect the fresh silks (^{Osada, 1984}).

As a part of the integrated management of this disease, the project was started to generate open-pollinated maize synthetic varieties with good agronomic characters and genetic resistance to the disease. Genetic resistance is considered the most efficient and cheapest control for this and other crop diseases (^{Pandey and Gardner, 1992}).

The breeding program for the development of germplasm with desirable agronomic characters and genetic resistance to horse’s tooth began in 2009 with the development of a genetically broad-based maize population with yellow endosperm. This base maize population was formed by recombining a total of 55 different maize cultivars collected in the Mexican highlands with good agronomic characters, including commercial hybrids, and improved and native varieties in isolated plots for two cycles. The recombination plots were established in a farmer’s field in the ejido of Santa Teresa Tiloxtoc, in Valle de Bravo, Mexico (19º13´N, 100º07´W, 1870 masl). After two initial cycles of recombination (C0), an S₁ recurrent selection program began (^{Pandey and Gardner, 1992}), self-pollinating approximately 400 desirable plants in each breeding cycle and generating S₁ seed in each cycle. Seeds from the S₁ obtained were planted ear-to-row, in the field of the Universidad Autónoma del Estado de México (UAEM) in El Cerrillo, Piedras Blancas, Toluca, Mexico (19º24´N, 99º41´W, 2667 masl), where the disease is presents under natural conditions. Ears of S₁ families selected for their desirable agronomic characters were inoculated with an aqueous suspension of 1x10⁵ macrospores mL^-1 of the pathogen Sphacelia sp. (Figure 1).

Approximately 33% of the S₁ lines inoculated and selected by desirable agronomic characters and resistance to the disease were recombined to begin a new cycle of improvement of the base population. This population breeding process is continuous. At the same time, in each evaluation cycle of the S₁ lines, groups of 10-12 lines with specific desirable characters were selected, such as uniformity in days to flower, plant and ear height, earliness, etc. These groups of lines were crossed in a diallelic design to generate the F₁ of new experimental synthetic varieties, which were moved on to F₂ to be included in agronomic trials with other varieties obtained in that or other cycles, along with commercial hybrids, to measure their agronomic performance and grain yield.

In yield trials established with F₂ seed from experimental varieties obtained from lines of the C₃-S₁ cycle, experimental variety Yellow 2 was chosen for its outstanding characters over other entries included in the same trials (Table 1). In 2018, this variety was submitted to the National Seed Inspection and Certification Service (SNICS) for approval as the new CP-Hilda 2 synthetic variety, which was given to the Colegio de Postgraduados, and registered under the breeder´s title 2166. Certified seed from this variety is currently in production in farmers fields with good results (Figure 2).

Figure 1 Micro and macroconidia of Sphacelia sp., anamorph of Claviceps gigantea.  

The synthetic maize variety CP-Hilda 2 has yellow grain, it is high-yield, with a potential grain yield of 10 t ha^-1, it is genetically resistant to horse’s tooth, its seed is low-cost and it can be planted for several years without affecting its grain yield. The impact of this project on society is the increase of farmers income due to the low cost of the seed and the increase in maize productivity due to its good yield and resistance to the disease, which helps to prevent its spreading. The synthetic variety CP-Hilda 2 is registered in the SNICS and belongs to the Colegio de Posgraduados for its distribution and commercialization.

Figure 2. Production plot of certified seeds of the synthetic variety CP-Hilda 2 in Huejotzingo, Puebla, Mexico, in the year 2019.