The oat (Avena sativa) crop has, in the past three decades, increased in importance in rainfed cultivations, particularly when the traditional crops such as maize, wheat, barley and beans are damaged or it is no longer convenient to plant them (^{Villaseñor et al., 2021}). In the year 2020, 94% of the surface planted with oat was used for the production of animal feed in its different forms, where 75% of the cultivations were carried out under rainfed conditions (SIAP, 2022). Oat is a cereal that adapts well to high, cold and rainy areas, but it also adapts to semi-arid environments and it is a good alternative when other crops do not prosper (^{Villaseñor et al., 2021}). This crop, like other cereals, is exposed to diseases caused by fungi, out of which rusts are the most destructive. Stem rust caused by Puccinia graminis f. sp. avenae is one of the most devastating for the crop and it has a broad weather range. In Mexico, it is the disease that most affects oat production, since it can reduce yields by up to 70%, as indicated by ^{Leyva et al. (2004}). An important reason for this is the large diversity of races of the populations of the pathogens and their continuous evolution, as mentioned by ^{Mariscal et al. (2011}), who report the presence of 24 physiological stem rust races in production areas in Oaxaca, Puebla, Hidalgo, Aguascalientes, Durango and Zacatecas. On the other hand, the existence of secondary hosts, such as wild oat (A. fatua), which is susceptible to this disease, perpetuates the presence of the inoculum throughout the year in all the production areas, including those not reported as oat-producing areas.

The most effective control for stem rust in oat is achieved with the use of resistant varieties (^{Villaseñor et al., 2021}). The fungicides applied on the foliage can be used to efficiently protect the crops (^{Leyva et al., 2018}), although they increase production costs, therefore genetic breeding via the generation of resistant varieties is an effective method to manage the disease. Given the importance of stem rust in oat, the aim of this investigation was to evaluate the resistance of oat lines and varieties to stem rust in seedlings and adult plants.

The seedling tests were carried out in the National Laboratory for Rust and other Cereal Diseases (Laboratorio Nacional de Royas y otras Enfermedades de Cereales - LANAREC), of the National Forestry, Agriculture and Husbandry Institute (Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias - INIFAP) Valle de México Experimental Field (CEVAMEX) located at 19° 29´ LN and 99° 53´ LW at an altitude of 2,250 masl (^{García, 1981}). Eight sets of the 18th National Oat Trial (Ensayo Nacional de Avena - 18th EUAVENA) were planted in order to inoculate each set with two races and two stem rust isolates in a completely randomized design with two repetitions. The 50 genotypes of the 18th EUAVENA, out of which 13 are control varieties and 37 are advanced lines, were planted in 20 x 30 x 6 cm plastic trays, which were added a mixture of sterile soil and peat moss in a 60:40 ratio; small orifices were then marked and eight to nine seeds were added per genotype. Twelve days after planting, the 50 genotypes were inoculated with the races TNQ and TFQ and isolates AMEX18.21.1.1 (from Moxolahuac, Puebla, Mexico, with a virulence for the Turquesa variety in the year 2018) and AMEX18.18.1.1 (gathered in 2018 in Santa Lucia, Texcoco, Estado de México of the Chihuahua variety, and considered to have been dominant in the field). The inoculation was carried out with a suspension of urediniospores at a concentration of 1 x 10⁶ urediniospores/mL, which were suspended in mineral oil (Sotrol^® 170) and sprayed with a nozzle connected to a compressor. The inoculated plants were moved to a bioclimatic chamber at 25 °C for 24 h and dew at 100%. Later, they were moved to the greenhouse, where the temperatures were 24 °C in the day and 15-20 °C in the night. The infection types (IT) were recorded 14 days after inoculation. To determine the TI, the scale proposed by ^{Roelfs et al. (1992}) for wheat stem rust was used, in which the genotypes with values of 3 and 4 were classified as susceptible whereas 0, ;, 1, 2 and X as resistant.

For the test in adult plants, the 50 genotypes of the 18thEUAVENA were planted in the Santa Lucia Experimental Field of the CEVAMEX in the summer of 2018 (July 9) under rainfed conditions in a completely randomized block experimental design with two repetitions. Planting was carried out by hand, at a density of 120 kg ha^-1. The occurrence of the pathogen was natural, and to ensure the presence of the P. graminis f. sp. avenae inoculant, the edge was planted with the Chihuahua variety, which helped disperse the inoculant. The severity of the disease was registered three times with 8-day intervals, using Cobb’s modified scale (^{Peterson et al.,1948}) which ranges from 0 to 100% of the infected stem, including the peduncle.

Response in seedling. The varieties Diamante, Menonita, Karma and Turquesa were resistant to the race TFQ, but susceptible to TNQ and to isolates AMEX18.21.1.1 and AMEX18.18.1.1, whereas the other varieties displayed susceptibility to the four isolates. Among the advanced lines, labelled with the numbers 19, 24, 26 and 49, the same behavior was observed. Other lines that were resistant to at least one of the isolates were differentiated and classified in up to six groups, based on the response to the inoculation with the four isolates (Table 1); the lines not included in the table were susceptible to the four isolations.

Response of adult plant on the field. The environmental conditions favored the natural presence of P. graminis f. sp. avenae, which allowed for the expression of resistance in the varieties and advanced lines of the 18th EUAVENA. The levels of infection caused by stem rust in the evaluated genotypes were very similar for both repetitions, although the average of the final severity is still reported. The response of the varieties to isolate AMEX18.18.1.1 was of susceptibility at seedling, but in adult plants, they were classified as susceptible, intermediate and only Teporaca was considered as resistant, as shown in Figure 1. The 37 lines, due to their response in adult plants, were grouped by sister lines and by disease severity. Thus, 17 groups were identified, which are shown in Table 2. In reference to these same lines, only those identified as 25 (AI/P-V/11-5340-42C-0R-1C-0R-1C-0R), 28 (AI-O-I/11-5324-0C-17C-0R-1C-0R-3C) and three sister lines with numbers 34, 39 and 48 (AVEINIFAP-2012-102) were resistant as seedlings to isolate AMEX18.18.1.1, and the rest were susceptible to this same isolate. On the field, all lines were resistant with levels of infection no greater than 10%, and only line 19, from group 14, reached 20% of infection, as illustrated in Table 2.

The response shown in the seedling and adult plant of the lines allows us to infer that resistance is conditioned by adult plant genes. However, with the data obtained to date, it is still not possible to determine whether the resistance is conditioned by genes of race specific or if it is conditioned by the accumulation of multiple genes of additive effects.

Figure 1 Percentages of infection observed on the field, caused by stem rust in 13 varieties of oat included in the 18th EUAVENA. 

The resistance to stem rust in oat varieties worldwide is not very common and there are no recent data or reports of any identification of sources of resistance. Furthermore, in some countries, all varieties are susceptible to existing races, such as in Australia (^{Park et al., 2015}). In Canada and the United States, it has been reported that there are no oat varieties that are resistant to the predominant races (^{Harder, 1994}; ^{Fetch et al., 2002}; Jin, 2021, personal communication).

The search for sources of resistance to stem rust in oat has led to conclude that variability is low or inexistent to races that currently prevail in Mexico and the different countries that produce this cereal. For example, a study in Canada suggested that the resistance to stem rust in Avena spp. is apparently not common, since out of 9978 accessions evaluated, only 47 lines (0.5%) displayed moderate to high levels of resistance to the race NA67 and only 71 (0.7%) displayed intermediate levels (^{Gold et al., 2005}). These results were very similar to those reported by ^{Martens and Dyck (1989}), who pointed out that the resistance to stem rust in oat was relatively rare. In previous studies that searched for resistance to stem rust in accessions of Avena spp., both wild and cultivated, from the Middle East, only 6 accessions out of the 1,538 evaluated (0.4%) displayed resistance to the race NA27 (^{Martens et al., 1980}). In Argentina, the only oat variety in use that was resistant to stem rust until 2004 was the genotype UFRGS-16 (^{Martinelli, 2004}). ^{Li et al. (2015}) point out that they evaluated 35 oat genotypes for their resistance at seedlings to races TKR, TJM and TKM of stem rust, out of which only 13 were resistant to the three races.

This leads us to conclude that in the genus Avena spp. there is little variability for resistance to stem rust and thus the importance of this study, in which all the advanced lines that were a part of the 18th EUAVENA were resistant in adult plants to the same isolate to which they were susceptible as seedlings. The lines identified as resistant in adult plants were also resistant in other testing environments (no data shown). There are no reports of resistance in adult oat plants against P. graminis f. sp. avenae, making this the first of its kind. The challenge from here onwards is to use and maintain these levels of resistance in future oat varieties

Among the varieties evaluated, only Teporaca is considered resistant to stem rust in adult plants, whereas the other varieties range from susceptible to intermediate. The advanced lines included in the study were resistant, as adult plants on the field, to the same isolate they were susceptible as seedlings. These genotypes must be considered as sources of resistance in the crosses directed to the genetic breeding of oat in Mexico.