Identification of physiological races

Using the monopostular technique and following the methodology described by ^{Huerta-Espino et al. (2020}) 150 isolates were obtained. For the identification of physiological races, a set of 20 differential lines was used, arranged in five sets of four differentials each. In addition to the 20 differentials, another 31 wheat genotypes were sown that allow corroborating the virulence and avirulence of the fungus in these genes, as well as the differentiation of breeds of bread wheat vs crystal wheat.

At 10 days after inoculation, the reaction of the genotypes was recorded using the 0-4 scale described by ^{Roelfs et al. (1992}). The nomenclature for the designation of races was based on the one proposed by ^{Long and Kolmer (1989}), modified by ^{Singh (1991}) and described for Mexico by ^{Huerta-Espino et al. (2020}).

Evaluation of genotypes in seedling and adult plant

At LANAREC, under greenhouse conditions (T_max 24 °C - T_min 18 °C), the resistance in seedlings and adult plants of eight control varieties and 27 advanced lines of bread wheat, from the genetic improvement program of INIFAP-CEVAMEX rainfed wheat. For the seedling evaluation, the 35 genotypes were sown in plastic trays, small holes were marked and 8 to 9 seeds were placed per genotype, later 11 days after sowing the seedlings were inoculated with a suspension of urediniospores of the strains of leaf rust called CBJ/QQ and MCT/SP, at a concentration of 1 x 10⁶ urediniospores ml^-1.

The urediniospores were suspended in mineral oil (Soltrol^® 170; Chevron Phillips Chemical Company) and sprayed with an atomizer connected to a compressor. The inoculated seedlings were placed in a bioclimatic chamber with temperatures of 24 °C for 24 h and sprayed at 100%. Later they were transferred to the greenhouse and after 10 days of inoculation their reaction to leaf rust was recorded using the scale proposed by ^{Roelfs et al. (1992}), where readings from 0 to 2 are considered resistant and from 3 to 4 susceptible.

On the other hand, for the evaluation for their resistance in adult plant to RH, the 35 genotypes were sown in plastic pots where 3 seeds were placed per pot. At 60 days after sowing, the plants were inoculated with a suspension of the CBJ/QQ and MCT/SP races, at a concentration of 1 x 10⁶ urediniospores ml^-1. The inoculated plants were placed in a bioclimatic chamber with temperatures of 24 °C for 24 h and sprayed at 100%.

Subsequently, they were transferred to the greenhouse and after 15 days of inoculation the severity of the disease was recorded, using the modified Cobb scale (^{Peterson et al. 1948}) and the host's response to infection was determined according to ^{Roelfs et al. (1992}). Where: R= resistant, tiny uredinia; MR= moderate resistance, small uredinia; MS= moderate susceptibility, small uredinia of moderate size; and S= susceptible, large uredinia.

Seven physiological races of leaf rust were identified, which occurred in the autumn-winter/2018-2019 cycle in the North of Sinaloa, in Table 1, their virulence/avirulence formula is presented and in this table it can be observed , that the most frequent breed was the CBJ/QQ with 29% frequency, this breed was identified for the first time in 1994 in the High Valleys of central Mexico as mentioned by ^{Huerta-Espino and Singh (1994}) and shares a certain similarity with the CBJ/QL and CBJ/QB breed reported by ^{Singh (1991)} identified in the state of Tlaxcala.

On the other hand, the most frequent breed was the MBT/SP with 23%, followed by the MCT/SP (18%), MGJ/SP and MBJ/SP with 10% and with 5% frequency the MBJ/QQ and MFJ/SP. The races with less spectrum of virulence were CBJ/QQ and MBJ/QQ; however, the latter was identified less frequently. On the other hand, the most virulent and frequent races were MCT/SP and MBT/SP. Of the seven identified breeds, a certain similarity was observed with other breeds reported mainly in the United States of America, such is the case of the MBT/SP breed, this breed shares similarity with the MBTSB breed identified since 2005 and reported by ^{Kolmer et al. (2011}); ^{Kolmer and Hughes (2018)}.

The MFJ/SP breed was identified similarity with the MFJSB breed reported by ^{Kolmer and Hughes (2018}). In general, most of the identified breeds presented a broad spectrum of virulence, which indicates that over time they have evolved in an accelerated manner and have overcome the resistance of most of the major genes available in Mexico.

The most viable strategy to control rusts is through genetic control, which is based on incorporating effective resistance genes into the germplasm of the various wheat breeding programs that exist in Mexico and the world. For which the search for germplasm with resistance to the most frequent races and with a greater spectrum of virulence is a constant work. Of the races identified in the present investigation, the most frequent was CBJ/QQ and one of the most virulent was MCT/SP, for these races genotypes with resistance in seedlings and adult plants to leaf rust were identified.

Table 2 indicates the level of resistance in seedling and adult plant under greenhouse conditions of eight varieties of bread wheat recommended for sowing under irrigation conditions in Northwest Mexico, of which Conatrigo F2015 was resistant in seedling and adult plant to both races, indicating that it possesses at least one gene for effective resistance to leaf rust.

On the other hand, the Villa Juarez F2009, Borlaug 100 and Bacorehuis F2015 varieties were moderately susceptible to the most virulent race (MCT/SP); however, with a behavior of moderate susceptibility to susceptible were the varieties Kronstad F2004, Roelfs F2007, Noreste F2018 and Hans F2019.

Of the 27 lines evaluated, only six showed a resistance reaction, moderate resistance and moderate susceptibility. Lines 10 (CHEWINK #1/MUTUS//MUTUS*2/HARIL#1) and 11 (MUTUS*2//ND643/2*WBLL1/3/BORL14) were resistant in seedling and adult plant to the two races evaluated as which indicates that they possess at least one effective gene for resistance to leaf rust.

Conclusions

In the Valle del Carrizo and Valle del Fuerte during the autumn-winter cycle, 2018-2019, seven physiological races of leaf rust were identified, of which CBJ/QQ was the most frequent. CONATRIGO F2015 showed resistance in seedlings and adult plants to the CBJ/QQ and MCT/SP races, which indicates that it has a greater gene for resistance to leaf rust. Most of the races identified had a broad spectrum of virulence. Races detected with low frequency may not become dominant in Sinaloa. It is very important to identify lines with genetic resistance to the main physiological races of rusts that occur in the North of Sinaloa, which will allow the generation of varieties that reduce the economic impact caused by leaf rust. Genetic resistance is one of the most viable sustainable management alternatives for the control of wheat rusts in Mexico.