Capsicum annuum is one of the pepper species with the highest morphological diversity and surface planted on a global scale. In Mexico, they are planted under open field conditions and in greenhouses, in which the “jalapeño”, “bell pepper” y “poblano” types stand out, since the production volume amounts to 3,296,875 t, and the states with the highest production levels are Chihuahua, Sinaloa, Zacatecas and San Luis Potosí (^{SIAP, 2018}). Mexico is the main exporter of bell peppers, with 150,303 t, and the main importing countries are the United States, Canada and Japan (^{Sánchez-Gurrola et al., 2019}). However, the production of chili peppers, and not just bell peppers, are affected by the presence of the Oomycete Phytophthora capsici in the soil (^{Barchenger et al., 2018}).

The pathogen P. capsici causes the wilting or drying of peppers, produced by the rotting of the root, chlorosis and falling of leaves, affecting between 10 and 100% of all plants (^{Jiménez-Camargo et al., 2018}), which depends on the prevailing weather conditions and the timely application of fungicides that help inhibit the fungal reproductive cycle; notwithstanding, the constant use of chemical products has led to the resistance of the pathogen, as well as environmental deterioration (soil, water and air) and an increase in production costs (^{Fernández-Herrera et al., 2007}).

Among the strategies used to reduce the negative effect of P. capsici, is the use of resistant rootstock; however, its use is limited by the type of rootstock, edaphoclimatic conditions and the species or variety used as a graft (^{Sánchez-Chávez et al., 2015}). In this sense, the most practical and useful thing is the use of the pre-existing physical (waxes, cuticles, cell walls, size, shape and location of natural apertures, among others) and chemical defense mechanisms (ethylene, jasmonic acid, salicylic acid, and essential nutrients) (^{Huallanca and Cadenas, 2014}), as well as the induction of defense mechanisms that are activated before and after the penetration of the pathogen (^{Qi et al., 2012}), providing a better response by the plant to a later attack (^{Castro et al., 2012}).

Nowadays, products from natural or synthetic sources (biocontrollers) and resistance inducers (synthetic) are available in the market, and with their application on the plant, they act on it and stop or delay the entrance of the pathogen, limiting its activity on the infected tissue or organ, without having a direct effect or specific activity on the phytopathogens (^{Huallanca and Cadenas, 2014}). Therefore, the aim of this research was to evaluate the level of resistance of different commercial bell pepper hybrids to P. capsici with the use of fungicides, biocontrollers and resistance inductors.

The experiment was set up in a tunnel-like greenhouse, P-5, located in the Experimental Agricultural Field in the Universidad Autónoma Chapingo (UACh), State of Mexico, Mexico (19° 29’ 33’’ N, 98° 52’ 21” W), at an altitude and with an annual average temperature of 2,267 masl and 15.9 °C, respectively. The plant material used consisted of commercial bell pepper hybrids: Caoba rojo, Dicaprio amarillo, Olvera pepper, Sympathy, California Wonder and PS16364212. On the other hand, the commercial name, active ingredient and doses of fungicides, biocontrollers and resistance inductors are described in Table 1.

The plants were sown in expanded polystyrene trays with 128 pits, using peat moss and perlite (1:2) as a substrate. The essay was carried out under greenhouse conditions, using 384, 60-day old seedlings, which were grafted in black polyethylene bags, with a capacity of 10 L, filled with tezontle sand as a substrate. Nutrition was carried out by a drip irrigation system, the nutrient solution of which had a nutritional composition of (mg L^-1): N (250), P (60), Ca (250), K (250), Mg (60), S (205), Fe (3), Mn (1), B (0.5), Cu (0.1) and Zn (0.5), where the volume of applied irrigation was of one liter per plant per day, distributed in two irrigations (morning and afternoon) with a duration of 30 minutes.

An experimental design of plots divided with a factorial 6 x 8 factorial treatments was used, and for this we considered 1) six commercial hybrids (Caoba rojo, Dicaprio amarillo, Pimiento Olvera, Sympathy, California wonder and PS16364212), 2) eight treatments (Acibenzolar methyl (ASM) + inoculation, Bacillus subtilis (BS) + inoculation, distilled water (TEST) + inoculation, Metalaxil-M (METM) + inoculation, Trichoderma spp. (TRICH) + inoculation, Potassium phosphonate (FP) + inoculation, Fosetyl-Al (FAL) + inoculation and distilled water (TESTAB) without inoculating). The experimental unit was composed of a plant with eight repetitions per treatment. The products were applied according to the dosage indicated by the manufacturer (Table 1), which considered a density of 2.5 plants m^-2. Treatments were applied 10 and 20 days after transplant, respectively. With exception from Actigard 50 GS and Aliette, the remaining products were applied by injecting directly into the substrate (stem base area). Plants were inoculated with 50 mL of a solution of mobile zoospores at a concentration of 1x10⁶.

In this experiment, we used the 6143 P. capsici strain, which was provided by the Plant Health-Pathology area of the Colegio de Postgraduados-Campus Montecillo, which was replanted in a V8 juice-agar medium. Once the pathogen developed, its multiplication was carried out by planting 0.5x0.5 cm squares in Petri dishes with a V8 juice-agar culture medium, and letting it stand for 24h at room temperature (19 °C), followed by incubation at 28 °C for six days.

Out of the total of experimental units, 336 were inoculated with 50 mL of a solution of mobile zoospores, at a concentration of 1x10⁶ zoospores mL^-1. Samples were taken at random from the roots of 10 plants with typical symptoms of chili wilting, which were washed with running water and disinfested with a 0.5% sodium hypochlorite solution for five minutes, to later be placed in the V8 juice-agar culture medium (^{Singlenton et al., 1992}).

The samples that developed isolations consisting of cottonlike white mycelia, were morphologically characterized. After corroborating the presence of the oomycete in the analyzed samples, the method was repeated to obtain sporangia. Subsequently, 50 mL of a solution of zoospores were prepared, at a concentration of 1x10⁶ zoospores mL^-1, and directly applied on the roots of 10 Caoba rojo, hybrid plants, 60 days to transplant, in order to satisfy Koch’s postulates; two trasplanting plants were used as controls.

Visual evaluations were carried out 8, 16 and 24 days after inoculation (dai) in order to detect plants with symptoms of wilting or dryness. The evaluation of incidence was carried out by identifying plants with typical symptoms of the disease in comparison with the total number of plants, and for the percentage of damage presented for each plant, a visual scale was proposed (Figure 1).

Using the data obtained on the damage levels, we calculated the severity of the attack of the oomycite, and the Area Under the Disease Progress Curve (AUDPC) was determined, using the trapezoidal research method proposed by ^{Shaner and Finney (1977}). The data underwent an analysis of variance (ANOVA) and a Tukey-Kramer multiple range test of comparison of averages (p≤0.05), using the SAS^® 9.0 statistical package.

According to Koch’s test, all plants inoculated with the P. capsici zoospore solution presented a loss in turgency until their complete wilting, whereas the control, which only received distilled water, displayed no symptoms. Initial results displayed an incidence of 19.5% of P. capsici on the hybrids, increasing to 38.8 and 48.9%, respectively for the second and third evaluations (Table 2). In this sense, it has been reported that the attack of the oomycete begins with the segregation of proteins within the host, causing physiological alterations (^{Thines and Kamoun, 2010}). During the evaluation, hybrid PS16364212 presented an incidence of 14.9% in comparison with California wonder, with 44.2%, and that has a longer time of use, and therefore has a lower resistance to the attack of the pathogen (^{Sánchez-Gurrola et al., 2019}).

Plants inoculated with the B. subtilis treatment displayed a damage severity of 33.5%, regarding Metalaxil-M and Acibenzolar-S-Metil (0% in both cases). At the end of 16 dai, this value increased to 80.4%; by contrast, Metalaxil-M registered 0.4%. In the last evaluation (24 dai), an increase was observed in the severity for plants treated with B. subtilis (92.9%), which agrees with reports by ^{Huallanca and Cadena (2014}). In addition, a slight increase appeared in the severity after applying Metalaxil-M and Acibenzolar-S-Metil, with values of 0.8 and 7.0%, respectively (^{Cosme-Velázquez et al., 2015}).

Figure 1 Visual guide for incidence evaluation and damage level (%) caused by P. capsici in bell pepper plants. 

The analysis of variance displayed highly significant effects on the hybrid x treatment interaction (p<0.0001) (data not shown). In general terms, the six hybrids displayed a lower AUDPC when they were given Acibenzolar-S-Metil (resistance inducer) and Metalaxil-M (fungicide) with values of 6.8 and 0.8% respectively, in relation to the TEST. Among the treatments, the use of Metalaxil-M displayed an AUDPC (Table 3), a behavior related to the systemic action of this fungicide, i.e., it interferes with the incorporation of uridine during the RNA synthesis process (^{Barchenger et al., 2018}). Studies performed with Metalaxil-M have shown its efficiency in reducing the damage caused by the oomycete P. capsici (^{Fernández-Herrera et al., 2007}); however, it is important to search for other alternatives of control, since the effectiveness of this fungicide has been affected by some cases of resistance to the oomycete (^{Qi et al., 2012}).

The use of Acibenzolar-S-methyl presented no statistical differences with Metalaxyl-M and the TESTAB, since it displayed the lowest AUDPC among the treatments (Table 3). In this sense, ^{Malolepsza (2006}) reports that Acibenzolar-S-methyl acts in a similar way to salicylic acid, since it is composed of analogous molecules, where the former is highly related to the acquired systemic resistance, given its interaction with the activity of enzymes related to the manifestation of resistance in plants.

In regards to hybrids, the values of the AUDPC fluctuated between 15.7 and 100.8 (% day^-1), where the most outstanding was PS16364212 (Table 3). This behavior may be linked to the synthesis of secondary metabolites with different chemical natures, including proteins and amino-acids, which help the plant create a level of resistance against the attack of P. capsici (^{Barchenger et al., 2018}). In some circumstances, these defense mechanisms are activated by the compounds segregated by the pathogens in the plant when attacked (^{Sánchez-Chávez et al., 2015}).

The activator of defenses Potassium phosphonate and the fungicide Fosetyl-Al displayed a similar growth in AUDPC (Table 4). In this sense, ^{Huallanca and Cadenas (2014}) indicate that Potassium phosphonate is a product with a limited efficiency in the control of P. capsici. The use of Fosetyl-Al has been reported to reduce the level of severity of the oomycete attack, although it does not surpass the use of Metalaxyl-M (^{Fernandez-Herrera et al., 2007}), which, in this study, displayed an AUDPC of 0.5%. On the other hand, the use of biocontrollers (Trichoderma spp. and B. subtilis) created the greatest AUDPC, similar to what was observed in the TEST, therefore its effectiveness was minimal to suppress the oomycete, which agrees with reports by Huallanca and Cadena (2014).

With an AUDPC of 4.3%, the use of Acibenzolar-S-Methyl displayed a good control of the disease, although it did not improve on observations for Metalaxyl. In this regard, ^{Cosme-Velázquez et al. (2015}) point out that Acibenzolar-S-Methyl, is a reducer of systemic resistance, and therefore reduces the AUDPC significantly, but does not eliminate the infection. On the other hand, ^{Baysal (2005}) reports that the reduction of the damage caused by P. capsici is linked to the induction of the product with the creation and synthesis of enzymes, phenolic compounds and PR proteins, reported as responsible for the effectiveness of defense mechanisms in plants against the attack of pathogens.

In this research and the specific conditions of evaluation, the bell pepper hybrid PS16364212 displayed the best results for resistance to the attack of P. capsici. The use of Metalaxyl-M and Acibenzolar-S-methyl displayed an efficient control, as opposed to Potassium phosphonate, Fosetyl-Al and the biocontrollers (B. subtilis and Trichoderma spp.).