The ‘tar spot’ disease in maize, caused by Phyllachora maydis, was initially identified in samples from Mexico (^{Maublanc, 1905}; ^{CIMMYT, 2004}). Currently, the name of “tar spot complex” has been proposed, due to the etiological association of the fungi Phyllachora maydis and Monographella maydis (Garrido-Ramírez, E. Com. Personal, 2020). The disease, in a simple etiology or as a complex, has spread throughout Mexico and in other countries, in maize-producing areas with warm weather, cultivated in subtropical regions as high as 2600 masl (^{Hock et al., 1989}); yet its main damage is found in maize grown in the coasts of Tabasco, Veracruz, Jalisco, Tamaulipas, Chiapas, Oaxaca, Michoacán and Guerrero, as well as in other non-coastal regions of the Mexican highlands, such as Morelos, the State of Mexico and Hidalgo, although its distribution can be more extensive (^{Hock, 1991}). On the other hand, Coniothyrium phyllachoraeis also related to the ‘asphalt spot complex,’ considered a hyperparasite of Phyllachora (Garrido-Ramírez, E. Com. Personal, 2020). The disease, in any of its etiological conditions, causes yield losses, depending on the susceptibility of the cultivar planted and the phenological stage of the maize plant when the infection occurs. The disease, under the condition of a complex, begins with the presence of P. maydis, with the production of a hard, raised, shiny, black spots on the maize leaves, with the appearance of asphalt, followed by the appearance of M. maydis around the asphalt spot (^{Quiroga-Madrigal et al., 2017}). The development of the foliar infection spreads from the lower leaves to the higher ones, and in advanced stages of development after flowering, the infection results in a severely blighted foliage (Figure 1A). A severe infection may affect the development and quality of the grain (Figure 1B).

As an alternative to minimize the effects of the tar spot on maize grain yield, attempts were made to generate synthetic, open-pollinated maize varieties with good agronomic characters and genetic resistance to the disease. Genetic resistance is considered the most efficient and inexpensive way to control this and other crop diseases (^{Pandey and Gardner, 1992}).

The breeding program to develop maize germplasm with desirable agronomic traits and genetic resistance to the tar spot complex began with the development of a maize population of white endosperm with a wide genetic base. This base population was formed in 2008, by recombining 37 different white grain maize genotypes with adequate agronomic traits, including commercial hybrids and improved and native varieties collected in Mexican highlands. These materials were recombined in isolated plots for two cycles to achieve genetic stability. The recombination plots were established in a farmer’s field located at the ejido of Santa Teresa Tiloxtoc, in Valle de Bravo, State of Mexico (19° 43’ N, 98° 53’ W, 1700 masl). After the second recombination cycle (C₀) an S₁ recurrent selection program was initiated (^{Pandey and Gardner, 1992}), self-pollinating approximately 400 desirable plants and generating S₁ lines in each breeding cycle. In the C₃ breeding cycle, seed of selected S₁ families were planted, ear to row, in a breedin nursery established in farmer´s field in Metztitlán, Hidalgo (20° 45’ N; 98° 39’ W; 900-2700 masl), where the tar spot disease appears severely under natural infectio. In thes conditions, the lines were evaluated and selected by resistance to the tar spot complex under severe natural field conditions.

Figure 1. Signs and symptoms of the ‘tar spot complex’; A) signs and symptoms on maize leaves and B) damage on maize ear (Photos: C. De León). 

In this nursery, seven groups of 10-12 S₁ lines were selected with synchronized days to male and female flowering (days to female flowering-days to male flowering should be equal to zero), plant and ear height (height index=product of the division between ear height and plant height), plant aspect, phenological development, time until harvest and resistance to foliar diseases measured with severity values of 1 (no damage) to 5 (severe damage) by severity of damage to the foliage caused by asphalt spot and diseases such as common rust, gray spot, Curvularia, and others. Each of these seven groups of lines were recombined in a diallel to generate the F₁ of new experimental synthetic varieties which were advanced to F₂ to be included in agronomical trials with other varieties obtained and commercial varieties to measure their agronomic performance, disease resistance and grain yield.

In two yield tests established in 2016 and 2017 in the ejido Cinta Larga (CL), in Mixquiahuala, Hidalgo (20° 13’ N, 99° 13’ W, 2100 masl) with F₂ seed of experimental varieties obtained from lines of the C₃-S₁ cycle, experimental variety CL16-TSR-2 was chosen for its outstanding traits over other materials included in the same trials (Table 1). In 2017, these varieties were also planted in a trial with randomized blocks with three replications in Metztitlán, Hidalgo, to evaluate their resistance to tar spot spot (Table 1). In late 2017, this variety was proposed to the National Seed Inspection and Certification Service (SNICS) for its registration and approval as the new variety CP-Elvia 3, which was given to the Colegio de Postgraduados, which was granted the title of Breeder Num. 2167 for its distribution and commercialization (Figure 2).

The synthetic maize variety CP-Elvia 3 (CL16-TSR-2) (Table 1) has a high grain yield (72% higher than the commercial control), it has a good floral synchrony, medium plant height, and its seed is affordable (approximately half the cost of a hybrid seed), it can be replanted for several years, it seed is easily disseminated between farmers and it is genetically resistant to the tar spot complex.

The impact of this project on society is the increase in the income of farmers, due to the low cost of the seed and the increase in grain yield due to its good behavior and resistance to the disease avoiding its dissemination in maize producing areas.

Figure 2 Seeds of the synthetic variety CP-Elvia 3.