Introduction

Root diseases are the main causes of economic losses in onions and related crops, throughout the world. White rot caused by Sclerotium cepivorum Berk is one of the most important root diseases. The development of the disease occurs in different regions around the world where environmental conditions are favorable for the pathogen (^{Velázquez-Valle and Medina-Aguilar, 2004}). In Mexico, this is the main cause of yield and quality losses in garlic ^{Delgadillo et al. (2004)}, which has led to total losses in the Bajio (^{Pérez-Moreno et al., 2009}), difficult to control due to sclerotia, reproductive structures of the fungus, which can remain viable for up to 20 years (^{Delgadillo et al., 2004}).

Sclerotinia sclerotiorum (Lib.) Bary is the causal agent of white mold or soft rot, which has caused losses of up to 70% in the cultivation of lettuce. Symptoms in this are manifested in the final phase of the crop cycle, showing wilting of the outer leaves of the plant, with the presence of cottony white mycelial growth towards the basal or central part of the stem, from which compact bodies are formed called sclerotia (^{Arias et al., 2007}).

A friendly alternative to the environment for disease control is the use of plant extracts ^{González et al. (2011)}. Plant secondary metabolites or bioactive compounds with fungicidal properties are an option in the management of pathogens, since they act against a limited number of species, are biodegradable to non-toxic products, have a low impact on human health and can be incorporated into programs of integrated management of pests and diseases (^{Salgado-Galciglia et al., 2008}).

Some genera of the Heliantheae tribe produce different types of alcamides, these compounds are hydrophobic amides or esters that contain unsaturated fatty acids as part of their chemical structure (^{García et al., 2004}).

Heliopsis longipes is a member of the Asteraceae family, it presents different compounds such as afinin, belonging to the group of alkamides (^{Molina et al., 1995}; ^{García et al., 2004}). Affinin is the alkamide found in greater proportion in the roots of this plant and mainly responsible for the biological effects such as insecticidal and bactericidal activity; this compound has biocidal action on some Gram-positive and Gram-negative bacteria, as well as on some fungi of the Ascomicetes class (^{Molina et al., 1995}; ^{García et al., 2004}; ^{Montes-Belmont and Prado-Ligero, 2006}; ^{Salgado-Galciglia et al., 2008}). The objective of this work was to evaluate the antifungal activity of the crude extract of H. longipes in strains of S. cepivorum and S. sclerotiorum and the viability of the sclerotia after the application of the extract.

Methodology

The research was carried out in the Department of Agricultural Parasitology of the Autonomous Agrarian University Antonio Narro in the Toxicology laboratory.

Results

The Sclerotium isolates showed white mycelium, white sclerotia that turned from dark brown to black. The pathogenicity test to differentiate the Sclerotium species was (negative) in the tomato fruits, which indicates that the strains belong to the species S. cepivorum according to the methodology of ^{Montes et al. (2003)}.

The results show a percentage of inhibition higher than 50% at the concentration of 15 000 ppm except for the strain from Zacatecas and the strains of S. sclerotiorum, being the strain PBGTO61 the one that showed the highest percentage of inhibition at the 25 000 ppm with a percentage of 77.01%. While strains of S. sclerotiorum (MBREP and MBLEC) showed the lowest percentages of inhibition of 39.58 and 25.09% in the concentration of 25 000 ppm, it should be mentioned that S. cepivorum strains showed a lower growth rate than strains of S. sclerotiorum (Table 1).

Ramírez et al. (2008) evaluated the effect of the crude extract of H. longipes on the development of mycelium based on dry weight in the species of S. cepivorum and S. rolfsii, obtaining a decrease in dry weight of 90% and 80% respectively at concentrations of 25 µg mL^-1. It is worth mentioning that the highest inhibition percentage obtained in this study was 77.01% at 25 000 ppm of the PBGTO61 strain. Additionally, the mean lethal dose of affinin and the crude extract for S. rolfsii varied between 15 and 20 µg mL^-1, while for S. cepivorum it ranged between 5 and 10 µg mL^-1. In Table 2, it shows the average effective dose for each of the strains used in this study and their fiducial limits.

Likewise, Ramírez et al. (2008) evaluated the ergosterol content in the isolates when exposed to affinin and the crude extract, with reductions in ergosterol in both species and in the two presentations (raw root extract and purified affinin), stating that the inhibition of ergosterol is not specific so it is parallel with the dry weight of the mycelium, which could be taken as an index of growth. While Pérez-Moreno et al. (2000) states that the use of chemical products such as tebucanzole for the control of S. rolfsii isolates which were collected in 2003, have not developed the resistance to the mode of action of this type of products; however, it may be possible that the use of this product is affecting the effect of other compounds such as afinin.

In Table 3, it can be observed that in the six strains there is a tendency to decrease the production of sclerotia as the concentrations of the extract increase in the culture medium, showing significant difference in all strains in the production of sclerotia, PBZAC being the only strain to show no difference in the production of sclerotia.

These results coincide with those reported by ^{Ochoa et al. (2012a)} in the antifungal evaluation of four plant extracts (tabaquillo, cinnamon, pirul and chirimoya) in the control of three species of Fusarium, of which the extracts of pirul and tabaquillo showed an increase in the production of conidia. Like ^{Montes and Prado (2006)} who report the increase in the production of sclerotia of S. cepivorum when exposed to four extracts (parsley, alfalfa, black pepper and marjoram) of the 15 evaluated extracts; showing increases of 296.41%, 192.67%, 140.6% and 117.35%.

The vigor of in vitro growth of the sclerotia showed no significant difference between the treatments in the strains PBGTO7, PBGTO52, PBGTO61, PBZAC and MBREP (Table 4). While in the MBLEC strain, it was reduced to 4.75 at the 25 000 ppm concentration; it should be mentioned that the ED₅₀ calculated for this strain is higher (57 900 ppm).

^{Pérez-Moreno et al. (2009)} evaluated the ability of five fungicides to inhibit growth, sclerotia production, viability and growth vigor, with Tebuconazole and TCMTB, which showed the lowest growth vigor (without growth), while Procimidone, Thiabendazole and Iprodione showed a growth vigor of 4.87, 5.58 and 5.73 respectively. It should be mentioned that in the strain PBGTO52, PBGTO61 and MBLEC showed a vigor of growth lower than those reported by ^{Pérez-Moreno et al. (2009)} for Procimidone, Thiabendazole and Iprodione products.

Conclusions

The extract of H. longipes showed an effect on mycelial growth, reduced the number of sclerotia and the vigor of growth was only affected in one of the strains used in this study.