Wheat (Triticum spp.) is the cereal with the greatest adaptation to different weather conditions in the world. Along with maize (Zea mays) and rice (Oryza sativa), it provides 75% of the carbohydrates required for human nutrition. However, it is estimated that around 30% of the world’s wheat production is affected by biotic factors such as pests and diseases that cause important losses in yields (^{Bockus et al., 2010}); some of their most important diseases are caused by fungi that cause rust, smut, leaf blight and root rotting (^{Zillinsky, 1984}).

Blights in wheat are diseases that spread across the globe and are particularly aggressive when relative humidity is high, the temperature is around 20 °C (^{Zillinsky, 1984}), and soil fertility is low (^{Duveiller and García-Altamirano, 2000}).

CIRNO C2008 is the main cultivar of durum wheat planted in the north of the country, due to its high yield, which has been over 8% higher than in the commercial cultivar Júpare C2001 (^{Figueroa et al., 2010}). However, it is starting to be replaced by the varieties Quetchehueca Oro C2013 (^{Fuentes-Dávila et al., 2014}) and Baroyeca Oro C2013 (^{Chávez Villalba et al., 2015}) due to its susceptibility to leaf rust and yellow rust.

^{Villa-Rodríguez et al. (2016)}, reported B. sorokiniana as the causal agent of spot blotch in wheat in the Yaqui Valley, Sonora. However, in other areas of Mexico, ^{Duveiller and Dubin (2002)}, reported as related to the same disease, fungi such as Alternaria spp. and Pyrenophora tritici-repentis.

The strategy to control diseases such as blight, including spot blotch, is based on an integrated approach, where genetic resistance is an important element. Resistance to these diseases by high yielding genotypes is not satisfactory (^{Saari, 1998}); leaf blight is said to be caused by at least one fungal species of the genus Alternaria, although what species are involved, or if it is only that genus, is unknown. Therefore, the following goals have been established: identifying the causal agent of the disease in samples from southern Sonora, and to determine the aggressiveness of the pathogen or pathogens in commercial cultivars and advanced wheat lines under greenhouse conditions.

In the 2014 and 2015 autumn-winter cycles, fifty samples were collected of the cv. CIRNO C2008 at heading stage with symptoms of leaf blight in agricultural and experimental fields in the south of the state of Sonora, Mexico. The symptoms displayed by the samples were small, dark maroon colored lesions with a light maroon center, along with chlorosis in advanced stages (^{Dickson, 1956}).

Using a stereoscopic microscope, the tissue was searched for the presence of conidia and transferred to Petri dishes with a potato-dextrose-agar (PDA) medium (Difco^®, France).

DNA extraction was performed using the DNeasy Plant Mini Kit (Qiagen^®, EE.UU.) using the method reported by the manufacturer. For this, the mycelium with 7 days of growth was carefully detached from a Petri dish, which was deposited in a sterile mortar and macerated with liquid nitrogen. For the PCR we used primers ITS5 and ITS4 (^{White et al., 1990}).

The four isolates of B. sorokiniana and the four A. alternata isolates were purified and increased, and the conidia were collected directly from the Petri dishes with mycelial growth by scraping with a spatula and adding distilled water. This conidia suspension was filtered through two layers of cheesecloth to eliminate fragments of the fungus and culture medium, adjusted to a concentration of 10⁶ conidia mL^-1 using a hematocytometer, also adding a drop of Tween 20^® (^{Cardona and González, 2008}).

For pathogenicity tests, 19 wheat cultivars were planted under a totally randomized experimental design, and the second leaf in a seedling stage was taken as a basis for the evaluation, within which there were 13 cultivars of bread wheat and 6 cultivars of durum wheat. For each cultivar, 5 seeds were planted in a sterilized substrate to evaluate 5 seedlings; 8 repetitions were planted for each classification (bread and durum) along with 3 repetitions for each one of the fungal isolates. The experiment was carried out twice under greenhouse conditions.

Inoculation was carried out by spraying, according to descriptions by ^{Singh et al. (1996)} and it was in equal volumes of 15 mL conidial suspension for every isolate directly to each germination tray, which were then transferred to a controlled environment incubation chamber with a relative humidity of 100%, with the aid of an ultrasonic HU-820A humidifier (Samsung Electronic^®, Korea), and an average temperature of between 19 and 21 °C for 48 h. They were later transported to the greenhouse, where they were kept at a temperature that ranged between 21 and 30 °C for 15 days until signs and symptoms appeared.

Evaluations began five days after inoculation, requiring the use of the severity scale proposed by ^{Perelló et al. (1998)}, where: 1 = free of infections to minimum spots; 2 = 1 to 5%; 3 = 5 to 12%; 4 = 12 to 20%; 5 = 20 to 35%; 6 = 35 to 45%; 7 = 45 to 60%; 8 = 60 to 80%; 9 = over 80% of the foliar area affected with necrosis and generalized chlorosis. The data were converted into percentages and underwent an ANOVA analysis of variance using the program SAS 9.1.3 (SAS Institute^®,U.S.A.). In order to stratify the results, the means were compared isolated using Fisher’s test (LSD) (P≤0.05).

In the foliar tissue, we isolated anamorphic fungi of the genera Bipolaris and Alternaria due to the shape of the characteristic conidia that each one has, and to the type of germination for the case of Bipolaris.

^{Zillinsky (1984)}, ^{Duveiller and García-Altamirano (2000)}, ^{Sharma and Duveiller (2004)}, and ^{Mehta (2014)} coincided in that the only causal agent of the disease known as spot blotch is Bipolaris sorokiniana, although this study found the presence of A. alternata, which can participate as a saprophyte or damage enhancer. ^{Singh et al. (1996)} considered only Alternaria triticina es the pathogenic fungus related to leaf blight. Meanwhile, ^{Mehta (2014)} claimed that A. alternata is related to B. sorokiniana to promote the black dot symptom in wheat grain, which causes the blackening of the embryo.

Out of the isolates obtained in this study, four were identified as B. sorokiniana and four as A. alternata, according to the analysis of the ITS sequences of the ribosomal DNA. The comparison in the BLASTn of the sequences obtained with accession numbers MF036007-MF036010, showed an identity of 99 to 100% with other ITS sequences of B. sorokiniana deposited in the database of the GenBank. Meanwhile, the ITS sequences with an accession numbers of MF036011-MF036014 presented 100% of identity with the sequences deposited in the GenBank, which confirmed the result of the morphological identification of the fungal isolates.

Symptoms were clearly observed in inoculated seedlings with the B. sorokiniana isolates, where the disease reached levels of severity of 100% in some genotypes 13 days after inoculation. With this, we standardized temperatures for inoculation and development of the infection process of B. sorokiniana and A. alternata in the greenhouse, which were between 19 and 21 °C with a humidity of 100% for 48 h.

In 2013, rainfall in southern Sonora ranged from low to scarce, and therefore the region has a characteristically dry weather, and the disease was considered secondary (^{INIFAP, 2016}). In 2014 and 2015, the relative humidity of the region was high (60 to 100%), and therefore the diseased was overwhelming (^{Villa-Rodríguez et al., 2016}). On the other hand, ^{Dubin and Bimb (1994)} pointed out that in regions in which wheat is grown under conditions of scarce rainfall, C. sativus (anamorph: B. sorokiniana) can not only cause rotting of the root but is also transmitted via the seeds, a disease known as black point in cereals. The fungus, therefore, displays a great ability to adapt to the conditions found in southern Sonora.

Significant differences were observed between the isolates evaluated. Isolate 3 of B. sorokiniana was the most aggressive than isolate 4. Meanwhile, isolates 1 and 2 were similar to each other (Table 1. This agrees with findings in studies by ^{Zhong and Steffenson (2001)}, ^{Chand et al. (2003)}, ^{Ghazvini and Tekauz (2007)}, in which they indicated the variability of B. sorokiniana isolates due to the lack of clear virulence patterns. This leads to suppose that the natural populations could be the result of genetic exchange through the parasexual exchange in the fungus population (^{Maraite et al., 1998}; ^{Duveiller and García-Altamirano, 2000}).

^{Oliveira et al. (2002)} and ^{Gyawali et al. (2012)} found high genetic variability between the B. sorokiniana isolates in studies using the random amplification of polymorphic DNA, or RAPD, which explains the difference between isolates 3 and 4, observing that isolate 3 caused higher percentages of severity, although there was a large difference between them, since they displayed an average severity of 75% by the isolate 3, and 40% by isolate 4. By contrast, ^{Mann et al. (2014)} reported a low variability between isolates that they evaluated and concluded that there have been no genetic changes in the population of the pathogen in the last 15 years. This could explain the behavior of isolates 1 and 2, which displayed less aggressiveness, with an average of 51 and 53%, respectively.

The four isolates of B. sorokiniana turned out to be pathogenic for the 19 wheat cultivars and they also showed statistically significant differences (P= 0.0001) in terms of aggressiveness. Likewise, under greenhouse conditions, it was possible to differentiate and select genotypes such as highly susceptible, susceptible, moderately susceptible and moderately resistant. However, plant inoculated with A. alternata did not display significant differences, and so it was not possible to characterize the severity of the disease in the different wheat cultivars evaluated.

The evaluation in this investigation was carried out taking as a pattern the characteristic symptoms of the disease, which are small, dark maroon lesions, 1 to 2 mm in length, without a chlorotic margin. In susceptible genotypes, these lesions extend quickly to form light and dark maroon, elongated, oval stains that reach up to several centimeters in length before coalescing, resulting in tissue death. Abundant conidia were produced in the old lesions, along with chlorosis that spreads from the edge of the lesion, which could be caused by the production of toxins, as stated by ^{Mercado-Vergnes et al. (2006)}. These symptoms are produced depending on the type of host, although they coalesce in the same way, regardless of the genotype (^{Mercado-Vergnes et al., 2006}).

All cultivars displayed significant differences to the susceptibility of B. sorokiniana, although isolate 3 was observed to be the most aggressive. The resistant cultivars to isolate 3 could be considered in a future experiment on selection for resistance. Meanwhile, isolates 1, 2 and 4 had significant differences in severity with isolate 3.

Based on the ^{Fetch and Steffenson scale (1999)}, cultivars are considered highly susceptible when their percentage is higher than 90%, establishing the cvs. Bárcenas S2002, Morocco, Bacanora T88, Luminaria F2012, advanced line C, Villa Juárez F2009, Maya S2007, Urbina S2007, PBW343, and Nana F2007 in this range (Table 1).

No bread wheat cultivars were observed to be resistant to the evaluated isolates of B. sorokiniana, although the cvs. Vicam S70 and Kronstad F2004 behaved as moderately resistant. The remaining bread wheat cultivars were moderately susceptible and susceptible. The results obtained are based on statements by ^{Da Luz and Bergstrom (1986)} and ^{Duveiller and Dubin (2002)}, who observed variations in the aggressiveness of the pathogen and the fact that they have no relation with the genotype evaluated.

Regarding durum wheat, the cvs. Local Red, Noio and advanced line JC were highly susceptible, while Río Bravo C2016 was considered moderately resistant. According to the ^{Fetch nd Steffenson scale (1999)}, cv. CIRNO C2008 behaved as moderately susceptible to B. sorokiniana.

Cultivars Bárcenas S2002 and Morocco reached 100% severity for Bipolaris sorokiniana. The cultivar Borlaug 100 F2014 is resistant to leaf rust and yellow rust, as well as a moderate resistance to Karnal bunt and black point, including B. sorokiniana and Alternaria spp. (^{Mehta, 2014}). According to ^{Valenzuela-Herrera et al. (2012)}, the cv. Villa Juárez F2009 presents the same resistance as Borlaug 100 F2014, and it is considered moderately susceptible to the black point in grains. However, in this investigation, both cultivars were susceptible, showing maximum infection levels of 94 and 86%, respectively, to the infection by B. sorokiniana.

^{Solís-Moya et al. (2014)} observed that the cvs. Luminaria F2012, Maya S2007 and Urbina S2007 are resistant to linear yellow rust and moderately resistant to leaf rust, although they do not mention resistance to blight, although in this study, these cultivars behave as susceptible to B. sorokiniana.

The cv. PBW343 is reported as moderately resistant to leaf rust and yellow rust, but susceptible to Ug99 stem rust and resistant to RTR stem rust in Mexico (Huerta-Espino and Singh, 2000), but in this investigation, it behaved as highly susceptible to B. sorokiniana.

The cv. Vicam S70 is considered resistant to spot blotch, but susceptible to yellow rust and leaf rust, which coincides with the results obtained in this experiment, since its reaction was of moderate resistance to B. sorokiniana. Local red is considered susceptible to wheat rusts and us used as a parent in wheat breeding programs (^{Bárcenas-Santana et al., 2016}); this is based on the results obtained and classifying Local Red as highly susceptible to B. sorokiniana. Likewise, the cultivar of durum wheat Noio used experimentally as susceptible to stem rust, but resistant to leaf rust and yellow rust; in this study it behaved as susceptible to B. sorokiniana.

Within the resistance to B. sorokiniana in durum wheat, it was observed that advanced line JC is highly susceptible and Conasist C2015 is moderately resistant.

On the other hand, for the case of isolates of A. alternata, a variance analysis was not performed, since most of these had a low percentage of severity according to the scale used, which ranged between 1 and 5%, and were therefore considered as not significant.

^{Villa-Rodríguez et al. (2016)} claimed that variable temperatures with increases of +1 °C in the average temperature and a relative humidity of 60 to 100% in 2014 and 2015 were probably the main driving force behind the epidemic of spot blotch in wheat in southern Sonora.

This study found that the main causal agent of the spot blotch in wheat in southern Sonora was Bipolaris sorokiniana. Also, Alternaria alternata is a secondary fungus related to the development of the disease caused by B. sorokiniana; likewise, quantitative differences were observed in the aggressiveness of the four isolates of Bipolaris sorokiniana evaluated in this study