Effects of plant extracts and sodium hypochlorite on lettuce germination and inhibition of Cercospora longissima in vitro

Highlights

• Clove, cinnamon and coriander extracts have allelopathic effect for germination of lettuce seeds.

• Cinnamon extract at 5% concentration resulted in total inhibition of in vitro growth of Cercospora (pathogen in lettuce).

• NaOCl increased lettuce seed germination with total inhibition of in vitro growth of Cercospora.

• The aqueous extracts of cinnamon or NaOCl at 5% can be used as fungicide in treatment of lettuce seeds.

• The results obtained showed a potential products for use in organic horticulture.

Abstract

Vegetables that are propagated by seeds are considered to be commercially and nutritionally valuable. However, there is a need to overcome the present challenge of developing alternative technologies for organic horticulture. This paper evaluates the potential use of the aqueous extracts of clove, cinnamon, and coriander and the sodium hypochlorite in the germination of lettuce (Lactuca sativa) seeds and in the control of Cercospora disease. Two experimental groups were thus designed: 1) the effects of the different aqueous extracts on germination and initial development of lettuce seeds were observed and; 2) the effects of these extracts on the mycelial growth of the fungus Cercospora longissima (isolated from lettuce plants) were evaluated. The treatments used in the first group were control (with water only), 5% (v/v) sodium hypochlorite (NaOCl) and 5% (w/v) aqueous extracts of clove, cinnamon, and coriander. The pretreatment of seeds was performed just before placing them for germination by immersing in these solutions. In the second set of the experiment, the inhibition of mycelial growth of the Cercospora longissima, isolated from lettuce plants presenting cercosporiosis symptoms, was evaluated. In the seeds treated with clove and cinnamon extracts, the germination speed index was negatively affected, while those treated with coriander extract showed an increase in the fresh mass of the aerial part and in the number of leaves of lettuce seedlings. In the second experimental group, it was observed that the growth of Cercospora longissima was inhibited completely by the cinnamon extract and by sodium hypochlorite.

Introduction

Most of the cultivated plant species used for food are propagated by seeds. The health of such seeds is of utmost importance as there might be possibility of the survival and transmission of phytopathogens. The pathogens are present as inocula within or on the surface of the seeds and sometimes even in the seed lot, thereby introducing the diseases into previously clean areas (Henning, 2005). According to Souza et al. (2007), fungi are the biggest group of pathogenic agents associated with seeds and grains.

The usage of certain products in seed treatment has been found to effectively reduce or annul mycelial growth on the surface of grains and seeds (Souza et al., 2007). A great part of the seed commercialization involves treatment of the contact fungicides (Strandberg, 1984; Kaewkham et al., 2016), which are banned in some horticulture production systems. Therefore, it is necessary to develop alternative methods, like physical treatment, microbial treatment and treatment with natural agents like plant extracts, which are capable of preventing or controlling the microorganisms in the seeds (Koch and Roberts, 2014).

According to Venturoso et al. (2011), herbal byproducts, in the form of crude extracts and essential oils, are alternatives to the synthetic products. These extracts contain microbiocide active substances that give them a potential to be used against phytopathogens in the plant defense. Hong et al., (2015) suggested that the potent fungicidal nature of the plant extracts and essential oils makes them useful in the control of various phytopathogens.

It is important, therefore, to emphasize, that high-quality seeds promote rapid germination and give rise to normal and healthy seedlings. The seedling, which are free from contamination and have essential structures developed, are extremely important for horticulture (Minami, 2003; Mancini and Romanazzi, 2014; Lima et al., 2016; Kaewkham et al., 2016), especially in organic farming.

In addition, the use of natural substances replaces and diminishes the potential toxic effect of pesticides on human beings (Pereira et al., 2006). The investigations that have employed essential oils and plant extracts for the treatment of seeds of vegetable crops have reported a significant reduction in the seed pathogens (Lima et al., 2016). However, it is important to point out that some of the herbal compounds may hinder the development and germination of other plants (Mairesee et al., 2007). In this context lettuce (Lactuca sativa) is one of the most important vegetables of the world to be propagated by seeds and its leaves are mostly consumed fresh. Lettuce is used as a bioindicator model in different allelopathy tests (Lima et al., 2016).

Among the plant species, clove (Syzygium aromaticum) and cinnamon (Cinnamomum sp.) have a high potential to control pathogens. Besides clove, eugenol is also found as one of the constituents of essential oil of cinnamon, whose fungicide potential is reported in the literature (Rao and Gan, 2014; Wang et al., 2010). According to Mandal and Mandal (2015), coriander contains linalool, a substance with antifungal and antioxidant potential.

The aim of this paper is to evaluate the possible effects of plant extracts and of sodium hypochlorite in the treatment of lettuce seeds and in the inhibition of mycelial growth of the Cercospora longissima fungus, which was isolated in vitro from diseased lettuce plants.