Pigeonpea (
Cajanus cajan (L.) Millsp.) is one of the major grain legume crops of the tropics and subtropics and accounts for about 5% of the world’s legume production. The pigeonpeas are rich in protein. Soil- borne diseases are important in pulses causing heavy losses in seed yield.
Macrophomina phaseolina (Tassi) Goid., a soil-inhibiting fungus is an important root pathogen and causes dry root rot/stem canker, stalk rot, or charcoal rot of over 400 plant species including pigeonpea (
Mahrshi, 1986).
Macrophomina phaseolina has been recently reported as an emerging phytopathogen
(Kaur et al., 2012). The disease development is favoured by high temperature (30-35°C) followed by moisture stress
(Amrit et al., 1999) and a good source of inoculum (
Lodha, 1998). This is a serious problem in late sown or summer crops and in perennial or ratooned pigeonpea. The pathogen poses a greater problem in cultivation and causes considerable loss
(Bajpal et al., 1999). There is growing concern that environmental pollution caused by imbalanced use and misuse of chemical fertilizers and pesticides is directly or indirectly related to human health problems. Consequently, farmers in developed countries began to shift from chemical-based conventional farming methods towards organic, alternative or low-input, sustainable agriculture (
Bhatia, 2002). The seaweed concentrates are applied to crops as root dips, soil drenches or foliar sprays. Seaweed concentrates are effective biostimulants in many crops including vegetables, trees, flowering plants and grain crops
(Stirk et al., 2004). Compounds extracted from different macroalgae (seaweed and seagrass) families like green, brown and red algae
(Vallinayagam et al., 2009) were confirmed earlier for their antifungal activity
(Khanzada et al., 2007; Bhosale et al., 2002). Extracts of the brown algae
Ascophyllum nodosum applied as a soil drench and foliar sprays have been shown to improve growth rates and reduce pests, consequently increasing crop yields, as well as the overall quality of the product
(Blunden et al., 1997). The present investigation was undertaken to evaluate different seaweed and seagrass extracts for their antifungal activity against
Macrophomina phaseolina in red gram and compatibility with antagonistic bacteria and fungi.
The pathogen,
Macrophomina phaseolina, was isolated from the diseased tissues of red gram by tissue segment method (
Rangaswami, 1958). Seaweeds and sea grasses were collected from the Mandapam coast, Tamil Nadu. After washing of seaweeds and seagrasses with water they were shade dried for 2 weeks followed by oven drying at 40°C for 24 h and powdered. A total quantity of 150 ml of alcohol was added to 20 g powder and kept overnight with intermittent stirring and extracted through a rotary evaporator at 40°C and 45 rpm. The extract was collected and stored in an airtight container. The different concentrations of 5, 10, 15 and 20% were prepared. A Poisoned food technique (
Schmitz, 1930) was employed to screen the antifungal efficacy of seaweed extracts. Radial growth was calculated (
Reeslev and Kjoller, 1995) and inhibition percentage was calculated
(Harlapur et al., 2007). Finally, compatibility between antagonistic bacteria, fungi and seaweed extracts were tested. The data from various experiments were analyzed statistically adopting the procedure described by
Panse and Sukhatme (1985). Wherever necessary, the percentage values were transformed to arc sine values before carrying out the statistical analysis.
Results indicated that a 20% concentration of all seaweeds and sea grasses showed better performance in general. Significant difference
s were observed in the seaweed extract of
S.
wightii (20%) compared to seagrass and
S.
myriocystum. Mycelia growth of
Macrophomina phaseolina was lowest in
S.
wightii (20%) (10, 25 and 38 mm after 24, 48 and 72 h) mm followed by
S.
myriocystum (20%) with 20, 28 and 45 mm whereas control recorded the highest mycelial growth of 30, 71 and 90.0 mm after 24, 48 and 72 h respectively (Table 1). The inhibition over control was also highest (63%) for
S.
wightii (20%) followed by in 20% for
S.
myriocystum (56%). The bacterial (
P.
fluorescens) and fungal antagonist (
T.
viride) were found to be compatible with seaweed and sea grass extracts. The presence of growth was not affected by the extracts (Table 2; Fig 1). The compatibility test shows that the seaweed and seagrass extracts can be applied to the plants in combination with biocontrol agents. Bacterial strains of
P.
flourescens,
P.
putida and
P.
aeruginosa have been reported as effective bio-control agents of various soil fungi
(Validov et al., 2005). The use of antagonistic organisms against
Macrophomina root rot has been well-documented in several crops
(Raguchander et al., 1998). Cotton seeds soaked in seaweed solution (1:500
Sargassum wightii for 12 h) provided considerable resistance to seedlings against
Xanthomonas campestris (Raghavendra et al., 2007). In carrot application of SLF (seaweed liquid fertilizer) enhanced the activities of chitinase, B-1-3 flucanase, polyphenol oxidase and lipoxynase which are factors regulating plant disease. Similar results were found in cucumber which showed enhanced activities of various defense-related enzymes including chitinase, B-1, 3-glucanase, peroxidase, polyphenol oxidase, phenylalanine ammonia lyase and lipoxygenase due to SLF application
(Jayaraman et al., 2011). The commercial extract from the brown seaweed
Ascophyllum nodosum was found to reduce fungal diseases in cucumbers (Jayaraman
et al., 2011). Brown algae have shown effectiveness in controlling plant diseases. The laminarin polysaccharide isolated from
Laminaria digitata is able to elicit host defense responses in plants
(Klarzynski et al., 2000). Brown seaweeds contain high amounts of flavonoid and phenolic compounds could be the reason for antifungal activity
(Cowan et al., 1999). Seaweed could also affect cell metabolism through the induction of the synthesis of antioxidant molecules which could favor plant growth and plant resistance to stress (
Zhang and Schmidt, 2000).
