<p>The production of the pepper seedling (<em>Capsicum annuum</em>) is affected by the fungal complex that causes the ‘damping-off’, in which some species of the oomycete <em>Pythium</em> spp., stand out. The objective of the present study was to identify the causal agent of the death of pepper plants and evaluate its pathogenicity in pepper seeds and seedlings. A fast and aggressive growing oomycete was isolate from pepper plants, morphologically identified as P. aphanidermatum based on its sexual and asexual reproduction structures and, by molecular techniques. This isolate had a high degree of<em> in vitro</em> pathogenicity in pre-emergence and post-emergence in chile, showing 100% mortality. In addition, it presented a high rate of mycelial growth in different culture media (V8-Agar, Corn Agar, Corn Potato Agar, Potato Dextrose Agar, Czapek &amp; Oat Agar), being in V8-Agar medium the only medium where it developed reproduction structures sexual and asexual. The isolation presented a mycelial growth rate of 58.3 ± 0.3 mm / day at 26 ± 2 °C in PDA medium. Due to its rapid growth and its high degree of pathogenicity <em>in vitro</em>, it is an unusual and aggressive isolate for pepper seedlings.</p>

Mexico is considered one of the main vegetable producing countries, however, its production is affected by several phytopathogens, whose control of these pathogens is based on the use of agrochemicals, which cause consequent environmental and health damages. In 2018, Mexico ranked 5th in the world in the consumption of fungicides and bactericides in agriculture. For this reason, new alternatives to control phytopathogens are important, with natural compounds with antifungal properties such as chitosan, a biopolymer of N-acetyl-D-glucosamine and D-glucosamine units linked by β-(1-4) bonds derived from the deacetylation of chitin with alkalysis (NaOH) or enzymatic hydrolysis. Chitosan is extracted from exoskeletons of arthropods. Research shows that it inhibits the growth of phytopathogens, due to its polycationic nature depending on molecular weight and deacetylation percentage. With other antifungal properties, they are inducer of phytoalexins, promotion of plant growth and biostimulant of beneficial microorganisms, due to its useful and economic potential agricultural application in technological innovation.

Se evaluó la efectividad de Pseudomonas donghuensis y de Bacillus spp., para el control de Fusarium oxysporum y Pythium aphanidermatum causantes del ahogamiento de plántulas de chile en invernadero. Se comprobó su patogenicidad para ocasionar la enfermedad y se realizaron pruebas de antagonismo en placas con PDA enriquecido. Mediante la confrontación in vitro de estos fitopatógenos con siete cepas Bacillus spp., se comprobó que este esporulado inhibió en mayor o menor grado (39.35- 56.24 %) el crecimiento micelial de F. oxysporum, pero no tuvo efecto alguno para P. aphanidermatum, ningún Bacillus spp., inhibió a este Oomycete, debido a su rápido crecimiento en la placa de cultivo (36 h de incubación). En contraste P. donghuensis inhibió en un 76 % el crecimiento de P. aphanidermatum, y no presentó efecto inhibitorio para F. oxysporum, por lo que la mezcla de ambos antagonistas pudiera tener potencial para el control de los problemas de ahogamiento (‘‘Damping off“) causados por estos fitopatógenos.

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