190 Marine Macro- and Microalgae: An Overview
B. cinerea infection (Jiménez et al. 2011). Increased activity of superoxide dismutase (SOD), was observed
in bentgrass (Agrostis stolonifera) given a combined application of A. nodosum extract and humic acid
that resulted in reduced dollar spot (S. homoeocarpa) disease (Zhang et al. 2003). Peroxidase activity was
enhanced in rice seeds treated with a commercial seaweed extract of Sargassum wightii that resulted in
enhanced resistance to pathogens, Alternaria padwickii and Bipolaris oryzae (Sathyanarayana et al. 2006).
The activity of defence-related enzymes, viz, peroxidase, phenylalanine lyase and polyphenol oxidase
was also increased in potato, with a treatment of the commercial extract of S. wightii (Raghavendra et al.
2008). Using the Arabidopsis model, the role of reactive oxygen species derived from the respiratory burst
oxidase homologue D (RBOHD) NADPH oxidase was implicated in resistance against A. brassicicola
(de Freitas and Stadnik 2015). Higher peroxidase activity and phenylalanine ammonia lyase activity was
also attributed to the resistance of rice blast (Pyricularia oryzae) when given foliar applications of aqueous
concentrates of Padina pavonia, Acanthophora spicifera and Ulva lactuca (Flora and Rani 2012). Foliar
and root applications of A. nodosum extract were reported to suppress P. capsici infections of peppers as
a result of the increased activity of soluble peroxidases, phytoalexin and capsidiol, indicating an elicited
defence response (Lizzy 1998).
It has been postulated that various seaweeds and their extracts can stimulate beneficial microbial
antagonists, as revealed from the efficacy of various extracts against different plant root pathogens. It
has also been suggested that components of the seaweed extracts may serve as food sources and promote
the proliferation of favourable microbes which out-compete or antagonize pathogenic organisms (Dixon
and Walsh 2002), in a manner similar to a gut health prebiotic in animals and humans. Direct antagonism
of the pathogen (Sultana et al. 2005; Arunkumar et al. 2010) and components of some extracts, such as
betaines, may also suppress plant disease (Blunden et al. 2010).
Seaweed extracts as biostimulants and plant health agents in integrated disease
management
Biological control and induced resistance are part of an integrated pest management approach, which has
emerged as a promising system for disease control which exerts low environmental impacts and reduces
the need for synthetic chemicals. Certain seaweed extracts contain bioactive, natural products which
are able to elicit molecular and biochemical defence responses in plants to protect against pathogens
(Subramanian et al. 2011). As a result, bioactive seaweed products can be integrated with disease
management strategies. This is further simplified with the availability of commercial products that have
an elevated “eco-friendly” potential for successful application in the integrated management of crop
diseases.
There is an increasing trend towards the use of milled seaweeds and their extracts as soil amendments
to control soil-borne plant diseases. This may be due to improvements in the activity of beneficial
microorganisms in the soil which are antagonistic to plant pathogens (Sultana et al. 2005; Sultana et al.
2008). For example, the use of Solieria robusta as a soil amendment to suppress the root-rotting fungi,
Fusarium solani, Macrophomina phaseolina, and Rhizoctonia solani which infect soybeans (Sultana et
al. 2011a). Integration of extracts of green (Halimeda tuna), brown (Spatoglossum variabile), and red
(Melanothamnus afaqhusainii) seaweeds with fungicides reduced the infection of M. phaseolina, R. solani,
and F. solani on sunflower (Sultana et al. 2011b). However, S. variabile alone or H. tuna, in combination
with Topsin-M or carbofuran, completely inhibited M. phaseolina infection on sunflower roots. Kuwada
and co-workers demonstrated that seaweed extracts helped to improve soil mycorrhizal activity which
further contributed to improved plant growth (Kuwada et al. 1999, 2006). The application of methanolic
extracts of the red seaweeds, Gracilaria verrucosa, Gelidium amansii, and Eucheuma cottonii, and a green
microalga, Chlorella pyrenoidosa either in vitro, or to the soil stimulated the growth of the arbuscular
mycorrhizal fungi, Gigaspora margarita and Glomus caledonium which form a symbiotic relationship
with the roots of their host. Root colonization of papaya and passionfruit with these mycorrhizal fungi
was markedly stimulated and the plant growth was improved.
The efficacy and compatibility of selected seaweed extracts have been tested in combination with other
disease control methods. Tuber soaking and three foliar sprays of a commercial product of a Sargassum
B. cinerea infection (Jiménez et al. 2011). Increased activity of superoxide dismutase (SOD), was observed
in bentgrass (Agrostis stolonifera) given a combined application of A. nodosum extract and humic acid
that resulted in reduced dollar spot (S. homoeocarpa) disease (Zhang et al. 2003). Peroxidase activity was
enhanced in rice seeds treated with a commercial seaweed extract of Sargassum wightii that resulted in
enhanced resistance to pathogens, Alternaria padwickii and Bipolaris oryzae (Sathyanarayana et al. 2006).
The activity of defence-related enzymes, viz, peroxidase, phenylalanine lyase and polyphenol oxidase
was also increased in potato, with a treatment of the commercial extract of S. wightii (Raghavendra et al.
2008). Using the Arabidopsis model, the role of reactive oxygen species derived from the respiratory burst
oxidase homologue D (RBOHD) NADPH oxidase was implicated in resistance against A. brassicicola
(de Freitas and Stadnik 2015). Higher peroxidase activity and phenylalanine ammonia lyase activity was
also attributed to the resistance of rice blast (Pyricularia oryzae) when given foliar applications of aqueous
concentrates of Padina pavonia, Acanthophora spicifera and Ulva lactuca (Flora and Rani 2012). Foliar
and root applications of A. nodosum extract were reported to suppress P. capsici infections of peppers as
a result of the increased activity of soluble peroxidases, phytoalexin and capsidiol, indicating an elicited
defence response (Lizzy 1998).
It has been postulated that various seaweeds and their extracts can stimulate beneficial microbial
antagonists, as revealed from the efficacy of various extracts against different plant root pathogens. It
has also been suggested that components of the seaweed extracts may serve as food sources and promote
the proliferation of favourable microbes which out-compete or antagonize pathogenic organisms (Dixon
and Walsh 2002), in a manner similar to a gut health prebiotic in animals and humans. Direct antagonism
of the pathogen (Sultana et al. 2005; Arunkumar et al. 2010) and components of some extracts, such as
betaines, may also suppress plant disease (Blunden et al. 2010).
Seaweed extracts as biostimulants and plant health agents in integrated disease
management
Biological control and induced resistance are part of an integrated pest management approach, which has
emerged as a promising system for disease control which exerts low environmental impacts and reduces
the need for synthetic chemicals. Certain seaweed extracts contain bioactive, natural products which
are able to elicit molecular and biochemical defence responses in plants to protect against pathogens
(Subramanian et al. 2011). As a result, bioactive seaweed products can be integrated with disease
management strategies. This is further simplified with the availability of commercial products that have
an elevated “eco-friendly” potential for successful application in the integrated management of crop
diseases.
There is an increasing trend towards the use of milled seaweeds and their extracts as soil amendments
to control soil-borne plant diseases. This may be due to improvements in the activity of beneficial
microorganisms in the soil which are antagonistic to plant pathogens (Sultana et al. 2005; Sultana et al.
2008). For example, the use of Solieria robusta as a soil amendment to suppress the root-rotting fungi,
Fusarium solani, Macrophomina phaseolina, and Rhizoctonia solani which infect soybeans (Sultana et
al. 2011a). Integration of extracts of green (Halimeda tuna), brown (Spatoglossum variabile), and red
(Melanothamnus afaqhusainii) seaweeds with fungicides reduced the infection of M. phaseolina, R. solani,
and F. solani on sunflower (Sultana et al. 2011b). However, S. variabile alone or H. tuna, in combination
with Topsin-M or carbofuran, completely inhibited M. phaseolina infection on sunflower roots. Kuwada
and co-workers demonstrated that seaweed extracts helped to improve soil mycorrhizal activity which
further contributed to improved plant growth (Kuwada et al. 1999, 2006). The application of methanolic
extracts of the red seaweeds, Gracilaria verrucosa, Gelidium amansii, and Eucheuma cottonii, and a green
microalga, Chlorella pyrenoidosa either in vitro, or to the soil stimulated the growth of the arbuscular
mycorrhizal fungi, Gigaspora margarita and Glomus caledonium which form a symbiotic relationship
with the roots of their host. Root colonization of papaya and passionfruit with these mycorrhizal fungi
was markedly stimulated and the plant growth was improved.
The efficacy and compatibility of selected seaweed extracts have been tested in combination with other
disease control methods. Tuber soaking and three foliar sprays of a commercial product of a Sargassum