Potential Use of Extracts of Seaweeds Against Plant Pathogens 181
seaweed extracts, as applied towards the priming or stimulation of plant responses against pathogens, we
present the following information from in vitro and in vivo experiments on the biostimulatory activity of
certain seaweed extracts; including their direct and indirect effects on microbes.
Direct activity of seaweed extracts related to plant/microbial activity
Though whole seaweeds directly, or as various extracts, have been used in agriculture since ancient
times, documented anti-microbial activity attributed to the algal component, has only been presented
since the mid-20th century (Pratt et al. 1951; Ross 1957; Welch 1962). The number of reports of in vitro
anti-microbial activities attributed to crude seaweed extracts increased tremendously after including antibacterial (Fenical and Paul 1984; Gonzalez et al. 2001; Yi et al. 2001; Esserti et al. 2017), anti-fungal
(Fenical et al. 1973; Sultana et al. 2009; Esserti et al. 2017) and anti-viral activities (Klarzynsci et al.
2003; Jiménez et al. 2011). This was probably due to increasing interests in new sources of ecologically,
safe anti-microbial compounds. Although many of these studies targeted human pathogens, some plant
pathogens, were also equally susceptible to certain seaweed extracts. Nevertheless, marine algal extracts
demonstrated activity against a wide range of potential plant pathogenic bacteria, fungi, and viruses.
Bacteria: A wide range of extracts from red, brown, and green algae have been shown to suppress
activity of Gram positive and Gram negative bacterial pathogens in vitro. It is known that whole marine
algae produce their own defensive compounds enabling them to reduce the effects of colonization of
their thalli by epiphytes including marine bacteria, thereby helping to protect the host. The same, or
similar, compounds in hydrolysed seaweed extracts may subsequently protect plants from microbial
attack (Weinberger 2007; Sangha et al. 2010; Jiao et al. 2011). Amongst the various seaweeds, extracts of
the browns appeared to have the highest levels of anti-bacterial activity (Subramanian et al. 2011). The
phenolic and halogenated compounds synthesized by members of the Phaeophyta have been demonstrated
to have properties that may have potential applications as anti-fouling agents (see Armstrong et al. 2000).
Wang et al. (2009) studied the bacteriostatic and bactericidal effects of phlorotannins, as isolated from
Ascophyllum nodosum on Escherichia coli strains and found a strong inhibition at 50 or 100 µg/ml;
their anti-bacterial efficacy was greater than tannins purified from the extracts of Quebracho (Schinopsis
lorentzii) and sumach (Rhus semialata) plants, known for their anti-bacterial activity (Costabile et al.
2011). Various aqueous and organic extracts of A. nodosum inhibited growth of Pseudomonas syringae
pv. tabaci (Pst) DC3000 (Subramanian et al. 2011). However, no anti-microbial activity was observed
against this pathogen when aqueous extracts of brown seaweeds Cystoseira myriophylloides, Laminaria
digitata, and Fucus spiralis were tested in vitro, although the disease was suppressed in planta (Esserti et
al. 2017). Seaweed extract treatment could also be useful to reduce the virulence factors of the bacterium,
which in itself may potentiate disease suppression by the seaweed extracts (Prithiviraj et al. 2005).
Organic extracts (e.g., 80% ethanol, methanol, and acetone) of several green algae: that is, Ulva
fasciata, Ulva (Enteromorpha) intestinalis, and Chaetomorpha sp. inhibited the growth of several Gram
negative bacteria, that is, Pseudomonas aeruginosa and Klebsiella pneumoniae and Gram positive bacteria
such as Staphylococcus aureus (Seenivasan et al. 2010). Additionally, extracts of the red algae Hypnea
valentiae and Rosenvingea intricata and the green U. intestinalis were applied against the bacterium
Xanthomonas oryzae, a causal organism of leaf blight disease on rice (Manimala and Rangasamy 1993).
The U. intestinalis extract strongly inhibited the bacterium, followed by R. intricata and H. valentiae.
Among the six different extracts of U. intestinalis, the diethyl ether extract demonstrated the greatest
inhibition, followed by those prepared with ethanol and methanol. Interestingly, the extracts obtained
with water, acetone, and chloroform were not effective. In another in vitro experiment, Arunkumar and
Rengasamy (2000) rated the anti-bacterial potential of several seaweeds extracts against Xanthomonas
oryzae pv. oryzae. Three seaweeds, viz. Gracilaria edulis, Sargassum wightii, and U. flexuosa showed the
highest anti-bacterial activity from the eleven species tested. In a recent investigation, various extraction
procedures were tested to determine the anti-microbial activity of extracts from the red alga Kappaphycus
alvarezii against Xanthomonas oryzae pv. oryzae (Venkatesh et al. 2011). The results revealed a considerable
inhibition of the pathogen growth in vitro.
seaweed extracts, as applied towards the priming or stimulation of plant responses against pathogens, we
present the following information from in vitro and in vivo experiments on the biostimulatory activity of
certain seaweed extracts; including their direct and indirect effects on microbes.
Direct activity of seaweed extracts related to plant/microbial activity
Though whole seaweeds directly, or as various extracts, have been used in agriculture since ancient
times, documented anti-microbial activity attributed to the algal component, has only been presented
since the mid-20th century (Pratt et al. 1951; Ross 1957; Welch 1962). The number of reports of in vitro
anti-microbial activities attributed to crude seaweed extracts increased tremendously after including antibacterial (Fenical and Paul 1984; Gonzalez et al. 2001; Yi et al. 2001; Esserti et al. 2017), anti-fungal
(Fenical et al. 1973; Sultana et al. 2009; Esserti et al. 2017) and anti-viral activities (Klarzynsci et al.
2003; Jiménez et al. 2011). This was probably due to increasing interests in new sources of ecologically,
safe anti-microbial compounds. Although many of these studies targeted human pathogens, some plant
pathogens, were also equally susceptible to certain seaweed extracts. Nevertheless, marine algal extracts
demonstrated activity against a wide range of potential plant pathogenic bacteria, fungi, and viruses.
Bacteria: A wide range of extracts from red, brown, and green algae have been shown to suppress
activity of Gram positive and Gram negative bacterial pathogens in vitro. It is known that whole marine
algae produce their own defensive compounds enabling them to reduce the effects of colonization of
their thalli by epiphytes including marine bacteria, thereby helping to protect the host. The same, or
similar, compounds in hydrolysed seaweed extracts may subsequently protect plants from microbial
attack (Weinberger 2007; Sangha et al. 2010; Jiao et al. 2011). Amongst the various seaweeds, extracts of
the browns appeared to have the highest levels of anti-bacterial activity (Subramanian et al. 2011). The
phenolic and halogenated compounds synthesized by members of the Phaeophyta have been demonstrated
to have properties that may have potential applications as anti-fouling agents (see Armstrong et al. 2000).
Wang et al. (2009) studied the bacteriostatic and bactericidal effects of phlorotannins, as isolated from
Ascophyllum nodosum on Escherichia coli strains and found a strong inhibition at 50 or 100 µg/ml;
their anti-bacterial efficacy was greater than tannins purified from the extracts of Quebracho (Schinopsis
lorentzii) and sumach (Rhus semialata) plants, known for their anti-bacterial activity (Costabile et al.
2011). Various aqueous and organic extracts of A. nodosum inhibited growth of Pseudomonas syringae
pv. tabaci (Pst) DC3000 (Subramanian et al. 2011). However, no anti-microbial activity was observed
against this pathogen when aqueous extracts of brown seaweeds Cystoseira myriophylloides, Laminaria
digitata, and Fucus spiralis were tested in vitro, although the disease was suppressed in planta (Esserti et
al. 2017). Seaweed extract treatment could also be useful to reduce the virulence factors of the bacterium,
which in itself may potentiate disease suppression by the seaweed extracts (Prithiviraj et al. 2005).
Organic extracts (e.g., 80% ethanol, methanol, and acetone) of several green algae: that is, Ulva
fasciata, Ulva (Enteromorpha) intestinalis, and Chaetomorpha sp. inhibited the growth of several Gram
negative bacteria, that is, Pseudomonas aeruginosa and Klebsiella pneumoniae and Gram positive bacteria
such as Staphylococcus aureus (Seenivasan et al. 2010). Additionally, extracts of the red algae Hypnea
valentiae and Rosenvingea intricata and the green U. intestinalis were applied against the bacterium
Xanthomonas oryzae, a causal organism of leaf blight disease on rice (Manimala and Rangasamy 1993).
The U. intestinalis extract strongly inhibited the bacterium, followed by R. intricata and H. valentiae.
Among the six different extracts of U. intestinalis, the diethyl ether extract demonstrated the greatest
inhibition, followed by those prepared with ethanol and methanol. Interestingly, the extracts obtained
with water, acetone, and chloroform were not effective. In another in vitro experiment, Arunkumar and
Rengasamy (2000) rated the anti-bacterial potential of several seaweeds extracts against Xanthomonas
oryzae pv. oryzae. Three seaweeds, viz. Gracilaria edulis, Sargassum wightii, and U. flexuosa showed the
highest anti-bacterial activity from the eleven species tested. In a recent investigation, various extraction
procedures were tested to determine the anti-microbial activity of extracts from the red alga Kappaphycus
alvarezii against Xanthomonas oryzae pv. oryzae (Venkatesh et al. 2011). The results revealed a considerable
inhibition of the pathogen growth in vitro.