Horse’s tooth disease is caused by the teleomorphic state of the ascomycete Claviceps gigantea (^{Fuentes et al., 1964}), with the Sphacelia sp. anamorphous stage which has a wide distribution under field conditions. The disease has only been reported in Mexico, initially in the Valley of Toluca, State of Mexico, and the Tarascan range in Pátzcuaro, Michoacán (^{Fuentes et al., 1964}), but it has spread to locations in Puebla and Hidalgo (^{De León-García de Alba et al., 2017}), in which high humidity conditions and low temperatures prevail. In these regions, in which hybrids and local varieties are established, all susceptible to the disease, losses of up to 100% of the grain production have been reported (^{Meléndez-Carbajal, 2015}).

Figure 1 Horse’s tooth sclerotia in maize ears (A) and germination of C. gigantea sclerotia, where perithecia are formed, with ascospores (B). 

The disease originates when the mature maize grains infected by the fungus develop into sclerotia (Figure 1A), dormant structures to hibernate. During harvest, these sclerotia fall to the ground and remain in dormancy during the winter, to reactivate their metabolism with rains in the next planting season. When they reactivate, the sclerotia germinate, forming stipes (stems) with heads on the apex, where perithecia form, containing asci with ascospores (Figure 1B). These ascospores are the infectious structures of the fungus that penetrate the ear after infecting fresh stigmas. In their asexual phase, the sclerotia develop micro and macroconidia, which can also penetrate the ear, infecting the fresh stigmas (^{Moreno-Moreno, 2016}).

As a part of the control of the disease, a breeding program was initiated to develop synthetic open-pollinated maize varieties with good agronomic characters and genetic resistance to the disease. Genetic resistance is considered the most efficient and cheapest way to control this and other crop diseases (^{Pandey and Gardner, 1992}).

Breeding program. The breeding program to develop germplasm with desirable agronomic characters and genetic resistance to the horse’s tooth disease began with the formation of a maize population with white endosperm and a wide genetic base. This base population of white endosperm was formed by recombinaning in isolated plots, for two cycles, a total of 45 different types of maize collected in the highlands with desirable agronomic characters, including commercial hybrids, and improved and native varieties. The recombination plots were established in the field owned by a farmer in the ejido of Santa Teresa Tiloxtoc, in Valle de Bravo, State of Mexico (19° 13’ N, 100° 07’ W, 1880 masl, mean relative humidity of 65%). After the initial recombination (C₀), a recurring selection program was initiated with S₁ lines (^{Pandey and Gardner, 1992}), by self-pollination of approximately 400 desirable plants in each selection cycle and producing S₁ seed in each cycle. Seeds obtained from S₁ plants were planted ear-to-row, in a nursery established in the Experimental Field of the Autonomous University of the State of Mexico, in El Cerrillo, Piedras Blancas, Toluca, State of Mexico (19º24´N, 99º41´W, 2660 masl), where the disease occurs naturally. Ears from the S₁ families selected for agronomic characteristics were inoculated with an aqueous suspension of 100 000 macrospores mL^-1 of the pathogen Sphacelia sp. obtained in a T2 medium (^{Pažoutová et al., 2004}).

Approximately 33% of the S₁ lines inoculated and selected for desirable agronomic characters and resistance to the disease were recombined to begin a new selection cycle from the base population. This population improvement process was continuous. At the same time, in each S₁ evaluation cycle, groups of 10 to 12 lines with a specific desirable attribute, such as high grain yield, synchrony in male and female flowering, plant and ear heights, and resistance to the disease, were selected. These groups of lines were crossed in diallel to produce the F₁ of new experimental synthetic varieties, which were advanced to F₂ to be included in agronomic trials with other varieties obtained and commercial hybrids as checks, to measure their agronomic characters and grain yield.

Figure 2 Basic seed production plot for the synthetic maize variety CP-Tania 5. Montecillo, Edo. Méx. 2019. 

In yield trials established with F₂ seeds from experimental varieties obtained from lines of the C₃-S₁ cycle, the experimental variety Blanca 13 was selected for its outstanding characteristics over other genotypes included in the same trials (Table 1). In 2018, this selected variety was submitted for approval to the SNICS as a new variety called CP-Tania 5. The variety was approved and it was delivered to the Colegio de Postgraduados with the title of Breeder No. 2559, registered in the National Catalog of Plant Varieties-CNVV (MAZ-2179-210220) for its sale and distribution. In 2019, Basic seed of this variety was produced in the Experimental Field of the Colegio de Postgraduados in Montecillos, State of Mexico (Figure 2).

The maize synthetic variety CP-Tania 5 is high yielding (8.6 t ha^-1), genetically resistant to horse’s tooth, low-cost and can be planted for several years. Incorporating this variety into production may lead to important benefits, such as the increase in the farmer’s income, due to the low cost of the seed and the increase in maize productivity, due to its high yield and resistance to the disease, avoiding the spread of the disease.