Cowpea [
Vigna unguiculata (L.) Walp.] is an important food legume which belongs to family
Fabaceae. Cowpea originated in Africa and widely growing in tropical and subtropical regions of Africa, Asia and Central and South America (
Quinn, 1999). Like other legumes cowpea also have ability to fix atmospheric nitrogen with its nodules and performs well even in poor soils with more than 85 per cent sand, less than 0.2 per cent organic matter and low levels of phosphorus (
Bilatu Agza et al., 2012) and also serves as a residue, which benefits the succeeding crops. Coupled with these attributes, its quick growth and rapid ground cover have made cowpea an essential component of sustainable subsistence agriculture in marginal lands and drier regions of the tropics, where rainfall is scanty and soils are sandy (
Sheahan, 2012). Cowpea is also known as vegetable meat due to high amount of protein in the grain with better biological value on dry weight basis. The grain contains 26.61 per cent protein, 3.99 per cent lipid, 56.24 per cent carbohydrates, 8.60 per cent moisture, 3.84 per cent ash, 1.38 per cent crude fibre, 1.51 per cent gross energy and 54.85 per cent nitrogen free extract
(Owolabi et al., 2012). In India the major cowpea growing states are Karnataka, Kerala, Tamil Nadu and Madhya Pradesh. It is cultivated in
Kharif season, especially for grain and summer season for fodder.
Despite being an important pulse crop its productivity has been quite low probably due to various biotic and abiotic constraints. This global crop, encounters a number of operational constraints, including pests and diseases that limit its production and yield potentials from seedling to harvest (
Asiwe, 2006) and often prooking grain yield loss of over 35 per cent (
Amadioha, 2003). Cowpea is attacked by atleast 35 diseases; the incidence and severity of these individual diseases differ from place to place and the stage of plant growth. Diseases hampers crop establishment, spoil forage quality and reduces green fodder and seed yield. Besides causing direct yield losses they also suppress nodulation and consequently negating the maximum nitrogen fixation. Among diseases, web blight has become a serious problem in cowpea growing areas. The anamorph or imperfect stage of pathogen which cause web blight in cowpea is
Rhizoctonia solani. Cowpea is especially predisposed to this seedling disease when planted in moist soils coupled with high temperature and humid conditions (
Thies et al., 2005). Very meagre information is available on the web blight disease of cowpea so this review article is an effort to incorporate information regarding distribution, etiology, biology and yield losses of web blight diseases cause by
Rhizoctonia solani and deals with disease management strategies in the light of present outlook.
Distribution and yield losses
Web blight is one of the foremost constraints in the production of many pulses in warm humid tropic zones of the world. In India first report of its occurrence on cowpea was given by
Lakshmanan et al., 1979 and they identified the causal organism as
R. microsclerotia (synonym,
R. solani), the perfect state,
Thanatephorus cucumeris. The disease has been known to occur in India on other leguminous crops
viz., blackgram (
Saksena and Dwivedi 1973), urd bean (
Sharma and Tripathi, 2001), mungbean (
Dwivedi and Saksena, 1974), pigeonpea (
Dwivedi and Saksena, 1975), Groundnut (
Dwivedi and Dubey, 1986), Soybean (
Verma and Thapliyal, 1976) and ricebean (
Jalali, 1989).
Saikia (1976) gave an account of the occurrence and etiology of the web blight disease of
Phaseolus aureus (
Vigna radiata) and found that a high disease incidence (85-90%) resulting into about 30 per cent plant mortality.
Emechebe and Florini (1997) reported that web blight of cowpea (
Vigna unguiculata) caused by
R. solani (teleomorph:
Thanatephorus cucumeris (Frank) Donk) is a destructive foliar disease in the hot, humid tropics.
Jhamaria and Sharma (2002) reported that web blight is a common and wide spread disease on mungbean. The incidence of disease varied from 17-90 per cent in India and 30-40 per cent in Rajasthan.
Singh (2006) reported up to 40 per cent yield losses at 60 per cent severity of web blight disease in mungbean in eastern Uttar Pradesh.
Shailbala and Tripathi (2007) reported that web blight caused by
Rhizoctonia solani (AG-1A) is one of the foremost fungal diseases in India causing up to 20-30 per cent loss in grain yield in urdbean.
Priyanka et al., (2017) reported that cowpea web blight caused by
Rhizoctoma solani is an emerging disease in cowpea growing areas of Rajasthan and causes considerable yield losses.
Pathogen
The genus
Rhizoctonia [
Rhizoctonia solani [anamorph, telemorph
Thanatephorus cucumeris (Frank) Donk] was first described by
De Candolle (1815) to accommodate the non sporulating root pathogen
R. crocorum. Kuhn (1858) reported the pathogen on diseased potato tubers and named it as
Rhizoctonia solani kuhn. Fungi generally grouped as
R. solani occurs all over the world and are capable of attacking a broad range of hosts (ca. 250 plant species), combined with competitive saprophytic ability and lethal pathogenic potential, earn
R. solani its status as formidable pathogen. The pathogen has a propensity for attacking seedlings (
Lewis and Lumsden, 2001) and causes pre- and post-emergence damping-off, brown-girdling and seedling blight (
Benhamou and Chet, 1993) resulting in yield losses worldwide. Genus
Rhizoctonia is considered as a heterogeneous assemblage of filamentous fungal taxa belonging to a group of fungi called the “Mycelia Sterilia”.
R. solani is a basidiomycete fungus that does not produce asexual spore and occasionally produce sexual spores called basidiospores which are formed on specialized structures called basidia.
Rhizoctonia species also produces specialized hyphae composed of compact cells called monilioid cell.
R. solani are genetically diverse in their cultural, physiological and morphological characteristics as well as in their pathogenic range of host plants. Due to high degree of diversity in pathogenicity and morphology as well as in cultural, physiological characteristics the species complex of
R. solani has been classified into diverse anastomosis groups (AGs) of which 13 have been described
Gonzalez et al., (2006).The characters of the organism when compared with an isolate of
R. solani causing sheath blight of rice; they are closely related morphologically and pathogenically. The colonies of
R. solani are yellowish white to light yellowish brown and later becoming pale brown to dark brown in colour. Sclerotia are white but later turning chestnut brown, globose to sub globose, oval or cushion shaped, often less than 1 mm in diameter to thin crusts several centimeters across (
Saikia and Roy, 1976 and
Singh, 2006). On the host, the sclerotia are superficial, variable in shape and size, dark brown to black, scab like and accompanied with dark short celled, abundantly branched, stout mycelium without clamp connections. Inside the host tissues, the mycelium is slender, hyaline and with longer cells. The perfect stage
Thanatephorus cucumeris develops on healthy tissue adjacent to the lesion on cowpea as well as
in vitro (Lakshmanan
et al., 1979). The sexual stage is induced on the host under warm and humid conditions with heavy dew formation during the night. Fructifications are resupinate, creamy to greyish white, loosely attached to the substratum, composed of arachnoid, repent hyphae giving rise to thin hypochnoid or sub membranous fertile patches. 4.5 to 17.0 μm wider hyphae without clamp connections often anastomosed; basal hyphae are widest with branching at angles while median and subhyaline hyphae are narrow, thin walled and hyaline. Mostly basidia are 9-25 × 5-12.4 μm in size but sometimes shorter, wider, barrel shaped to sub cylindrical without a median construction. Basidiospore are hyaline smooth thin walled, oblong to ellipsoid and dorsally fattened or broad ovoid and commonly widest at the distal end form secondary basidiospores (
Saksena, 1973).
Anderson (1982) stated that hyphal anastomosis in
R. solani is a manifestation of somatic or vegetative incompatibility between hyphae.
Host range
R. solani has prolonged saprophytic endurance ability hence able to infect a wide range of host. Literature reveals that the fungus
R. solani can infect twenty one host species belonging to different families (Gramineae-4 Solanaceae-3, Leguminosae-11 and Malvaceae-3). However, typical web blight symptoms were observed on plants belonging to family Leguminosae followed by Gramineae. Disease infection Minimum was reported on the family Solanaceae.
Sharma and Tripathi (2001) also worked on the host range of urdbean isolates of
R. s
olani and found the wide host range belonged to different families’
viz. Leguminosae, Solanaceae, Brassicaceae, Malvaceae and Cucurbitaceae.
Tiwari (1993) also reported the host range of web blight fungus of rice bean and observed typical web blight symptom on plants belonging to family Leguminoseae and banded blight symptom on plants of family Gramineae and Cyperaceae.
Singh and Malhotra (1994) worked on the host range of
R. solani causing web blight of winged bean and observed that lobia, guar, urd, soybean, groundnut, sem, arhar, french bean, paddy, jowar, moong, caster, bottle gourd, bitter gourd, brinjal, chillies, tomato and okra were also infected by this fungus.
Symptoms
Web blight caused by
Rhizoctonia solani is the most serious soil borne disease due to favorable environmental conditions like high temperature and humidity causing severe yield loss. This ubiquitous fungus is very much virulent in cowpea causing stand loss and subsequent yield loss. Collar rot is most severe at seedling stage and web blight is severe at vegetative stage. Living infected seedlings have cankers, which are reddish-brown with lesions on the stem and roots (
Ceresini 1999). The collar rot is characterized by oval or spindle shaped brown-black lesions having length ranging from 0.2-8 cm at soil level near collar region, girdling the basal portion of the stem. The leaves turn yellow followed by shedding of leaves and finally the entire plant become dry. In affected plants root development is very poor. White mycelial growth often studded and small sclerotia were seen on basal part of the affected stem. Symptoms of web blight appear on leaves as small circular, light grayish brown spots which later enlarge. The affected regions were surrounded by irregular water soaked area. On leaves also there is mycelial growth accomplished by sclerotial formation over the affected areas. Cobweb like symptoms also noticed on the leaves and hence the name web blight
(Vavilapalli et al., 2014). Rhizoctonia solani produced reddish brown lesions on the leaves and also on the stem near soil level resulted in stem girdled which agrees with the report of (
Olsen and Young 2018) that seedling infected with
R. solani had reddish brown lesions on leaves and cankers on the stem and roots. Splashes of soil during rainfall could easily get into contact with the branches and leaves of the plants which could lead to increased population of the pathogen hence increased severity of web blight. Besides causing direct yield losses to the crop, pathogen also suppresses nodule formation and consequently negating the maximum nitrogen fixation.
Epidemiology
Epidemiological factors play a vital role in the development of web blight disease caused by
R. solani.
R. solani infection and disease development can occur over a wide range of soil moisture levels.
R. solani is known to prefer warm wet weather and its outbreaks occur typically during an early summer months in tropics and subtropics region.
Lakshmanan et al., (1979) reported that collar rot and web blight caused by
R. solani are the soil borne diseases of cowpea, particularly under high temperature and relative humidity this disease cause severe yield losses. Damage caused by
R. solani occurs at any time during the growing season; however, it is more severe on young seedlings
(Dorrance et al., 2003). Moisture is an important factor which effect on inoculum potential of
R. solani and epidemiology of the resultant disease. Higher aerial temperature (26 to 32°C) relative humidity near 100% and soil temperature 30-33°C also favoured the development of high disease severity. Rainfall (91-97 mm) had a significant role in severe development of web blight during early stage of crop in urdbean (
Sharma and Thripathi, 2001).
Upmanyu and Gupta (2005) also evaluated the effect of different temperature regimes, which showed that web blight of french bean was not observed at 15°C but with increase in temperature, it increased and reached maximum at 25°C, after which it decreased. In case of web blight of legumes, the plant are susceptible at seedling stage until maturity and the severity is maximum in 30-70 days old plants probably because of dense canopy of the crop which promote the early spread of the pathogen through contact of plants and leaves with one another, forming mycelial bridges. The sources of inoculum for
R. solani are natural soil, contaminated weed or rotation crops, plant debris and infected seeds (
Parmeter, 1970). Survival and inoculums potential are influenced by soil factors such as temperature, moisture, pH and competitive activity with associated organisms
(Jones et al., 1997). The pathogen invades the seed while still in the pod, decomposing the pod, or may infect from infested soil during planting. The problem of seed decay starts immediately after planting before germination (
Beker 1947,
Neergard 1958 and
Parameter 1970).
Kumar et al., (2013) reported that optimum temperature at 26-28°C and 90-100% relative humidity favours disease development chronically.
Disease cycle
R. solani is primarily soil-borne and source of initial inoculum are soil splashes on to the leaves during heavy rains. The pathogen is accomplished of extensive growth through soil and survives on crop debries. It can persist in soil for many years. The first phase is soil borne and the second is leaf borne
(Yang et al., 1990). R. solani also survive as mycelium by colonizing soil organic matter as saprophytes, particularly as a result of plant pathogenic activity. The highest inoculum potential is recorded in the top 10 cm soil and no inoculum is found below 40 cm
(Kaiser et al., 1970). The fungus can grow in the soil very fast to a longer distance and can survive there in the form of mycelia or sclerotia. Because of their proximity to the soil carrying primary inoculum and their special micro climate, the weed hosts are first to take the infection and facilitate the production of basidiospores. Thereafter, the pathogen becomes air-borne and cause leaf, stem or pod infection and produces fresh cycle of basidiospores on the main host.
Variability
Variability in
Rhizoctonia solani has been reported by many authors with respect to its cultural, morphological, biochemical and molecular aspects (
Susheela and Reddy, 2013;
Prasad et al., 2015). This variability includes different virulence patterns, fungicide resistance, optimal growth temperature and epidemiology. Studies on variability provide information regarding pathogenicity and pathological behaviour under both controlled and field condition.
Cultural and morphological Variability
Variation in colony colour ranges from creamy white, light whitish brown to dark brown in colour (
Tiwari and Khare, 1998;
Singh et al., 1999;
Mughal et al., 2017). In same anastomosis group (AG-1), Colony colour of different isolates were also varied from light to dark brown. The isolates also differed in their growth rate. Variation in hyphal width within an anastomosis group was observed, range of hyphal width was found 4.68 to 13.25 μm (
Singh et al., 1999). Morphological and cultural variation in
R. solani have been reported by many workers
viz., Singh
et al.,
(1999),
Panja et al., (2011), Prasad et al., (2015) etc. It produces small number of globose sclerotia, which are initially white, later turn to brown or black in colour. The hyphae are 4-15 μm wide and tend to branch at right (90°) angles and usually possess more than three nuclei per hyphal cell.
Pathogenic variability
Isolates of
R. solani show greater variability in pathogenecity and categorized into poorly, moderately and highly pathogenic on the basis of the degree of their pathogenicity (
Tiwari and Khare, 1998;
Singh, 2006). Generally
R. Solani is considered as a non-specialized plant pathogen; however, host specificity has been recognized at various levels. For example, isolates AG-1 and AG-2 could be divided into sub group based on pathogenicity (
Anderson, 1982;
Liu et al., 1991). Parmeter et al., (1969) also reported that mode of pathogenicity appears to be quite variable within the groups. Different isolates of
R. solani showed wide variation in their host range and were classified according to their host specificity.
Sumner (1985) reported that
R. solani AG-4 and AG-2 type 2 were highly virulent on cultivar of cowpea.
Disease management
Management of soil borne diseases is relatively more laborious than that of airborne ones, since the pathogen starts to do the damage while buried in the soil and the disease outbreak may become visible too late for effective plant protection.
Host resistance
Resistance is fundamental attributes of all living systems. Resistance is an important characteristic of a plant which suppresses pathogen development. The magnitude of resistance can range from very small to very large. Host plant resistance (HPR) is the most efficient and ecofriendly way of management and also compatible with other disease management. For exploitation of HPR, reliable field and controlled environment screening techniques are essential. Very meagre information is available on the resistant variety of cowpea against web blight. The web blight disease severity is recorded by
Priyanka et al., 2018 using 1-9 rating scale (Table 1) (
Anonymous, 2016). They screened twelve cowpea genotypes for their reaction to web blight caused by
R. solani Kuhn. Among 12 genotypes screened, two genotype DC 7-15 and CPD 229 were free from web blight incidence. One genotype DCS 47-1 showed highly resistant reaction and two genotype TC 161 and VCP 12-007 showed resistant reaction. Two genotype CPD 240 and RC-101 were found moderately resistant. One genotype GC 1304 was highly susceptible to web blight. Four genotype GC 3, GC 1203, KBC 10 and VCP 09- 019 showed susceptible reaction. The details of rating scale are given in Table 1.
Cultural practices
Crop rotation can be used as an effective control measure for the disease (
Parmeter, 1970;
Dorrance et al., 2001). Use of mulches and preventing direct contact of the plant with the soil under warm, moist conditions can control the disease effectively
(Jones et al., 1997). Burying the infected leaves immediately after harvest will reduce the primary inoculum. For proper control of disease, the diseased material should be destroyed and crop rotation be practiced.
Biological management
Biological management is the reduction of inoculum potential or disease producing activity of a pathogen accomplished by or through one or more living organism other than man (
Cook and Baker, 1983). It comprises use of living microorganisms to control plant pathogens (
Mukhopadhayay, 1996).
Pan and Das (2011) evaluated the efficacy of two organic formulations of
Trichoderma harzianum along with other treatment combinations. They noted that seed priming with mycelia preparation of the antagonist
Trichoderma harzianum of seed and organic formulation of the antagonist in vermicompost along with neem cake gave best disease control along with yield of cowpeas. The basis of biological management is exploitation of the antagonistic potential of microorganism; such microorganisms are known as bioagents. These antagonists are the microorganism that adversely affects the target pathogen. The disease control efficacy of bioagents was reported to be increased considerably when these were used in combinations such as
T. harzianum plus
P. fluorescens (Agrawal et al., 2002; Khan and Gangopadhyay, 2014;
Meena and Gangopadhyay, 2016;
Priyanka et al., 2017 and
Singh et al., 2017).
Chemical control
Chemicals play a very important and significant role in disease management because of fact that they are easy to apply, economical, quick acting and effective to manage crop losses incurred by plant diseases. Thiophanate methyl was found best controlling seedling mortality of mung bean caused by
R. solani (
href="#singh_1995">Singh et al., 1995).
Meena et al., (2018) demonstrated treatments consisting with tebuconazole 50% + trifloxystrobin 25% WG seed treatment (1.5 g/kg
-1 seed) and soil drenching (1.5 g/ lt
-1 water) was found most effective in minimizing the web blight incidence (10.76%), disease inhibition (83.70%) and gave highest seed yield (14.20 q/ha
-1) in mungbean followed by seed treatment of carbendazim 12% + mancozeb 63% WP @ 2 g/kg
-1 and soil drenching of carbendazim 12% + mancozeb 63% WP @ 2 g lt
-1 water (12.60%), (80.91%) and (13.50 q/ha
-1)as compared to control (66.00% and 5.70 q/ha
-1) respectively.
Akhtar et al., (2014) revealed that propiconazole @ 0.1%, which is marketed as Tilt 25 EC, proved to be the best as it reduced severity index of web blight of greengram and blackgram and provide higher grain yield.
Basandrai et al., (2016) evaluated more than ten fungicides as foliar spray against web blight of urd bean. Two foliar spray of fungicides namely; hexacanazole 5 EC (Contaf 5 EC) @ 0.1%, difenconazole 25 EC (Score 25 EC) @ 0.05%, carbendazim 50 WP (Bavastin 50 WP) @ 0.1% and propiconazole 25 EC (Tilt 25 EC) @ 0.1% significantly reduced the disease severity and highest grain yield.
