184 Marine Macro- and Microalgae: An Overview
et al. 2001; Sultana et al. 2005; Arunkumar et al. 2010). Furthermore, the rich composition of various
crude algal extracts may themselves promote the growth of favourable microbes, which could further
out-compete or antagonize pathogenic organisms (Dixon and Walsh 2002; Sultana et al. 2005). It is
most often hypothesized that plant responses are elicited via seaweed components such as: oligo- or
polysaccharides, peptides, proteins, lipids, cell wall debris, carrageenans, laminarin, β 1–3 glucans or
sulphated fucans, or combinations thereof (Kobayashi et al. 1993; Lizzy et al. 1998; Mercier et al. 2001;
Subramanian et al. 2011). A selective literature review on seaweed extract-induced plant resistance is
discussed in the following section:
Bacteria: The use of various seaweed extracts has been shown to suppress infections in plants
caused by certain bacteria. Polysaccharides, such as green algal ulvans, when sprayed on tomato leaves
(0.1–10 mg/ml), protected the plants against the bacterium Pseudomonas syringae (Jaulneau et al. 2010).
The same treatment was also shown to impart protection against a mite (Tetranichus urticae) in strawberry
and a parasitic nematode (Meloidogyne incognita) also in tomato; the forgoing observations indicated a
broad spectrum activity of the seaweed extracts. Subramanian et al. (2011) also reported that infection
with Pseudomonas syringae pv. tomato DC3000 on Arabidopsis plants was reduced with root treatment
of commercial A. nodosum extract. Another study reported the suppression of bacterial spot of tomatoes
caused by Xanthomonas campestris, using two different formulations of A. nodosum seaweed extract.
Further field studies with these two formulations in Virginia, USA, in 2007, demonstrated suppression
of bacterial spot in tomatoes, under low disease pressure conditions, although the results did not show
a significant difference (R. Ross unpublished data). Caccamese et al. (1980) studied extracts from more
than twenty different algae on the growth of the plant pathogenic bacterium Xanthomonas malvacearum
and reported a considerable suppression of that plant pathogen.
A commercial, aqueous seaweed extract from Sargassum wightii, was evaluated against bacterial blight
caused by X. campestris pv. malvacearum in cotton (Raghavendra et al. 2007). Cotton seeds were soaked
with commercial extract before being sown and subsequently, the seedlings were sprayed at 10 day intervals,
starting 10 days after sowing. These applications resulted in a reduction in the incidence of bacterial blight
on plants (ranging from 66–74%), that was observed between 40–80 days after sowing. Seeds of soybean:
cv. SibNIIK-315, treated with aqueous solutions of laminarin (from Laminaria cichorioides; most recent
name - Saccharina cichorioides), fucoidan (from Fucus evanescens), and polymannuronic acid to soybeans
resulted in a reduced incidence of both bacterial and fungal diseases on the seedlings, buds, and flowers
in the field (Zaostrovnykh et al. 2009). The incidence of seedling bacteriosis (caused by Pseudomonas
savastanoi pv. glycinea) on seedlings treated with oligosaccharides from Laminaria was the lowest
(i.e., 10.2%) as compared to 25.9% in the control. Kumar et al. (2008) screened the anti-microbial activity
of 12 different seaweeds, including the greens: Chaetomorpha antennina, Halimeda tuna, Ulva lactuca,
the reds: Laurencia obtusa, Gracilaria corticata, G. verucosa, Grateloupia lithophila and the browns:
Padina boergesenii, Sargassum wightii, and Turbinaria conoides, against the phytopathogenic bacterium
Pseudomonas syringae (i.e., causative agent for leaf spot), in the medicinal plant Gymnema sylvestre. The
results showed that methanolic extracts, followed by ethyl acetate fractions, of Sargassun wightii were
more effective than the other organic solvent extracts when tested in this study.
Fungi: The suppression of plant diseases, caused by fungal pathogens, by various seaweed extracts has
been observed on several ornamental, agricultural and horticultural plants, both in the field and also
greenhouse studies. Foliar applications of commercial A. nodosum extract on Capsicum (peppers)
decreased infection caused by Phytophthora capsici (Lizzi et al. 1998). The same study also reported
that the incidence of downy mildew (Plasmopara viticola) on grapes was decreased with seaweed
extract application. In greenhouse experiments, a biostimulant formulation, containing seaweed extract
(of A. nodosum), showed strong suppressive activity against tomato late blight caused by P. infestans,
although, interestingly it was found that the A. nodosum extract alone was not effective (Portillo
et al. 2007), indicating the roles of synergism and integrated disease management. Interestingly, this
biostimulant was also fungistatic in vitro and suppressed mycelial growth. A commercial product
(i.e., BioFeed Basis) containing a complex mixture of plant proteins and extracts of seaweeds, especially
A. nodosum and Fucus spp., when used to fertilize tomatoes, was effective in reducing late blight
et al. 2001; Sultana et al. 2005; Arunkumar et al. 2010). Furthermore, the rich composition of various
crude algal extracts may themselves promote the growth of favourable microbes, which could further
out-compete or antagonize pathogenic organisms (Dixon and Walsh 2002; Sultana et al. 2005). It is
most often hypothesized that plant responses are elicited via seaweed components such as: oligo- or
polysaccharides, peptides, proteins, lipids, cell wall debris, carrageenans, laminarin, β 1–3 glucans or
sulphated fucans, or combinations thereof (Kobayashi et al. 1993; Lizzy et al. 1998; Mercier et al. 2001;
Subramanian et al. 2011). A selective literature review on seaweed extract-induced plant resistance is
discussed in the following section:
Bacteria: The use of various seaweed extracts has been shown to suppress infections in plants
caused by certain bacteria. Polysaccharides, such as green algal ulvans, when sprayed on tomato leaves
(0.1–10 mg/ml), protected the plants against the bacterium Pseudomonas syringae (Jaulneau et al. 2010).
The same treatment was also shown to impart protection against a mite (Tetranichus urticae) in strawberry
and a parasitic nematode (Meloidogyne incognita) also in tomato; the forgoing observations indicated a
broad spectrum activity of the seaweed extracts. Subramanian et al. (2011) also reported that infection
with Pseudomonas syringae pv. tomato DC3000 on Arabidopsis plants was reduced with root treatment
of commercial A. nodosum extract. Another study reported the suppression of bacterial spot of tomatoes
caused by Xanthomonas campestris, using two different formulations of A. nodosum seaweed extract.
Further field studies with these two formulations in Virginia, USA, in 2007, demonstrated suppression
of bacterial spot in tomatoes, under low disease pressure conditions, although the results did not show
a significant difference (R. Ross unpublished data). Caccamese et al. (1980) studied extracts from more
than twenty different algae on the growth of the plant pathogenic bacterium Xanthomonas malvacearum
and reported a considerable suppression of that plant pathogen.
A commercial, aqueous seaweed extract from Sargassum wightii, was evaluated against bacterial blight
caused by X. campestris pv. malvacearum in cotton (Raghavendra et al. 2007). Cotton seeds were soaked
with commercial extract before being sown and subsequently, the seedlings were sprayed at 10 day intervals,
starting 10 days after sowing. These applications resulted in a reduction in the incidence of bacterial blight
on plants (ranging from 66–74%), that was observed between 40–80 days after sowing. Seeds of soybean:
cv. SibNIIK-315, treated with aqueous solutions of laminarin (from Laminaria cichorioides; most recent
name - Saccharina cichorioides), fucoidan (from Fucus evanescens), and polymannuronic acid to soybeans
resulted in a reduced incidence of both bacterial and fungal diseases on the seedlings, buds, and flowers
in the field (Zaostrovnykh et al. 2009). The incidence of seedling bacteriosis (caused by Pseudomonas
savastanoi pv. glycinea) on seedlings treated with oligosaccharides from Laminaria was the lowest
(i.e., 10.2%) as compared to 25.9% in the control. Kumar et al. (2008) screened the anti-microbial activity
of 12 different seaweeds, including the greens: Chaetomorpha antennina, Halimeda tuna, Ulva lactuca,
the reds: Laurencia obtusa, Gracilaria corticata, G. verucosa, Grateloupia lithophila and the browns:
Padina boergesenii, Sargassum wightii, and Turbinaria conoides, against the phytopathogenic bacterium
Pseudomonas syringae (i.e., causative agent for leaf spot), in the medicinal plant Gymnema sylvestre. The
results showed that methanolic extracts, followed by ethyl acetate fractions, of Sargassun wightii were
more effective than the other organic solvent extracts when tested in this study.
Fungi: The suppression of plant diseases, caused by fungal pathogens, by various seaweed extracts has
been observed on several ornamental, agricultural and horticultural plants, both in the field and also
greenhouse studies. Foliar applications of commercial A. nodosum extract on Capsicum (peppers)
decreased infection caused by Phytophthora capsici (Lizzi et al. 1998). The same study also reported
that the incidence of downy mildew (Plasmopara viticola) on grapes was decreased with seaweed
extract application. In greenhouse experiments, a biostimulant formulation, containing seaweed extract
(of A. nodosum), showed strong suppressive activity against tomato late blight caused by P. infestans,
although, interestingly it was found that the A. nodosum extract alone was not effective (Portillo
et al. 2007), indicating the roles of synergism and integrated disease management. Interestingly, this
biostimulant was also fungistatic in vitro and suppressed mycelial growth. A commercial product
(i.e., BioFeed Basis) containing a complex mixture of plant proteins and extracts of seaweeds, especially
A. nodosum and Fucus spp., when used to fertilize tomatoes, was effective in reducing late blight