Wheat is the most widely grown and consumed food crop of the world. It is estimated that to maintain self-sufficiency in the availability of wheat in India, annual production of wheat should increase by 2 percent annually. Recent climate changes have aggravated the disease scenario in India which leads to increase in the incidence of Helminthosporium leaf blight of wheat i.e. spot blotch caused by the Bipolaris sorokiniana. In India, foliar blight is considered to be a complex issue due to presence of Alternaria sp. and Bipolaris sorokiniana in blighted leaf samples which make it difficult to identify the cause of disease in the field. However, B. sorokiniana dominates in Bihar, Delhi, Gujarat, Haryana, Karnataka, Maharashtra, Rajasthan and Uttar Pradesh. Alternaria triticina is predominant in Jammu & Kashmir, Himachal Pradesh, Madhya Pradesh and Punjab (Goyal et al., 1999)
This disease is of much economic importance in humid and warmer areas especially in North eastern plains zone (NEPZ), but due to climate change and crop intensification this disease is also gaining attention in north western, peninsular zone and central zone of India, posing new challenges to farmers. The rice and wheat rotations over larger areas in Punjab, Haryana, western U.P. and thereby causing delayed sowing of wheat. Crops get exposed to warm and humid weather in February which is conducive for spread of spot blotch disease. For this, three hundred and seventy five diverse wheat genotypes of India were evaluated for estimates of heritability, genetic advance and magnitude of association and contribution of important morphological and yield attributes with Helminthosporium Leaf Blight (HLB) disease resistance to assess genetic variability (Singh et al., 2008). In 1990-91, the disease appeared in an epidemic form in some western districts of U.P. on wheat varieties HD2329 and HD 2285, mainly due to late sowing and warm humid environment in March (Singh et al., 1993). Since then, this disease has emerged as a major problem in north-western as well as in the peninsular India and has acquired the status of national importance instead of regional one.
Spot blotch is a disease of importance mainly in warm, humid wheat growing areas where the mean temperature of the coolest month is higher than 17.5˚C. It can cause serious yield losses to wheat crop in South East Asia, North and Latin America, Africa, India, China and Brazil. In India, the disease frequently occurs in the in warm and humid areas of Bihar, West Bengal, Uttar Pradesh, Orissa, Assam, Madhya Pradesh, Maharashtra and Karnataka. More recently, it is shifting to the cooler, non-traditional irrigated rice-wheat production areas of northern India.
The destructive capacity of this pathogen is evident from the reports around the world. Grain yield reductions due to spot blotch are variable but are of great significance in warmer areas of South Asia. On an average, a South Asian country loses 20 percent of crop yield through leaf blight disease (Saari, 1998). In Nepal, under rice-wheat cropping system, spot blight severity went up to 100 percent and 70 percent in 2004 and 2005 respectively. Grain yield loss due to spot blotch in South Asia ranged from 4 percent to 38 percent and 25 percent to 43 percent in the year 2004 and 2005 respectively (Sharma and Duveiller, 2006). Yield loss was estimated to be 18-22 percent in India (Singh et al., 1997), which can be devastating for farmers in the eastern gangetic plains. In India, there are reports indicating the losses are variety dependent. Nema and Joshi (1971) reported that at flag leaf in Sonora 64, the loss incurred was hardly 3 percent, while S-227 showing maximum susceptibility suffered loss of about 20 percent. However, when the disease occurred only at flag leaf, the losses of grain/ear were up to 24.2 percent. Yield losses of 10-30 percent have been reported, especially when warm temperatures and moist conditions favour disease development during and after heading. Under natural conditions, losses as high as 20 percent and 22 percent in wheat varieties UP262 and HP 1633 respectively were reported (DWR 1995). Due to widespread losses, this disease is considered as the most significant disease of wheat not only in north-eastern plain zone of India (Saari, 1998) but in all wheat growing regions. Spot blotch has been reported to cause 15 percent grain yield reduction in Bangladesh (Alam et al., 1998) and China (Xiao et al., 1998). The pathogen also causes grain yield losses up to 10, 15, and 20% through common root rot and seedling blight in countries like Scotland, Canada, Brazil etc. (Murray et al., 1998).
The spot blotch pathogen is capable of producing disease symptoms in all parts of plant i.e. leaves, nodes, internodes, stem, awn and glumes (Fig.1). Early lesions are characterized by small, dark brown lesions varying 1 to 2 mm long without chlorotic margin (Fig. 2). Spot blotch symptoms reduce the photosynthetic area and lead to premature drying of the infected leaves and lead to death of the plant in severe conditions.
In susceptible genotypes, these lesions extend very quickly in oval to elongated blotches, light brown to dark brown in colour. They may reach several centimetres before coalescing and inducing the death of the leaf. If spikelets are affected, it can result in shrivelled grain and black point, a dark staining of the embryo end of the seed. Root and crown infections in severe form may lead to complete drying of infected plants without seed production.
Fig. 1 Wheat leaves showing symptoms of spot blotch infection in field.
Fig. 2 Symptoms of spot blotch disease after 10 days of inoculations in Polyhouse
Initial infection by the spot blotch fungus results in small brownish spots on leaf blades and sheaths. As these spots or lesions enlarge they are typically oval in shape, brown to dark brown in colour with distinct margins. Chlorosis or yellowing of the leaf may occur around individual lesions. Individual lesions may be up to 1.0 cm in length. Small brown to dark brown spots and discolouration may be observed on head tissues and individual seeds. Although they are similar in colour, spot blotch lesions do not have the typical long, narrow appearance of net blotch lesions. However, spot blotch may be confusing with the spot-form of net blotch, which is characterized by small circular to oval, dark brown spots.
Variability for resistance in wheat genotypes
Wheat genotypes have been investigated by wheat workers from time to time and some sources of resistance have been established. Chaurasia et al. (1999) evaluated spring wheat lines belonging to Indian and CIMMYT gene pool and found 43 lines as resistant; and CIMMYT lines showed better tolerance than Indian lines. Sources of spot blotch resistance that have been identified over the years broadly falls into three categories; Latin American, Chinese and wild relatives of wheat or alien species (Van Ginkel and Rajaram, 1998). The Latin American sources are mainly derived from Brazil and may trace back to Italian ancestry. Older resistant Brazilian commercial varieties are BH-1146 and CNT-1. Chinese source of resistance used at CIMMYT include Shanghai#4, Suzhoe#8 and Yangmai#6. Thinophyrum curvifolium has also been used as alien resistance sources at CIMMYT. Singh et al.(2005) evaluated two hundred and ninety one promising wheat genotypes and out of these 10 genotypes namely, CB (BW)-351, CB (BW)-355, MRANGALD/AN’SS MON’s’, UHU, BAU4, K9204, BW 14989, HW 2012 and HW 2014 were rated as resistant.
Major spot blotch resistant variety in India
In India, resistance to B. sorokiniana is low among commercial wheat cultivars. The development of recently identified spot blotch resistant varieties like HS 490, TL 2942, VL 829, in northern hills zone, DBW 17 and PDW 314, in NWPZ, DBW 14 and DBW 39 in NEPZ, HI 1531, HI 8498 in central zone (CZ), MACS 6222, UAS 304 and HD 8663 in peninsular zone, HW 5207 in Southern hills zone (SHZ), KRL 210 and MACS 2496 for saline and alkaline soils have helped in better management of spot blotch in field and results in higher productivity and better grain quality. Dhaliwal et al. (1986) have found very little resistance to spot blotch in the germplasm of wild wheat and Aegiolopes species.
- Commercial cultivation of improved and disease resistant varieties is the safest cheapest and eco-friendly approach.
- Use clean seed and seed treatment to reduce the potential for poor germination, seedling blight, and early season root rot.
- Use a crop rotation of at least 2 years between cereal crops or grasses. Rotation will help to reduce the amount of infested residue as well as the level of soil-borne inoculum.
- If weather conditions are favourable and spot blotch symptoms are readily present at flag leaf emergence, a fungicide application may be considered. Spray of Tilt (propiconazole) at 0.1 % is recommended around heading or the appearance of disease.
Other major control methods
Disease situations will vary according to geographical areas and cropping conditions, therefore for timely 2-3 spray at one- to two- week intervals may be needed to control the disease. Three sprays of propiconazole @ 0.1 percent after appearance of disease were found significantly effective against leaf blight at multi-location and resulted in 30.9 per cent increase in grain yield over unsprayed check (Singh et al., 2005). An integrated management of leaf blight has been worked out, using resistant varieties, fungicides, botanicals fertilizers and bioagents in three agroclimatic zones of India. Use of seed treatment with carboxin, carboxin+thiram (1:1) @ 0.25 per cent and thiram @ 0.3 per cent provide effective controlling of seed borne infection of B. sorokiniana and increased the seed germination percentage.
Biological control of spot blotch
Natural resistance of wheat towards this pathogen is found to be low. However, there is a possibility of biological control of this disease. The saprophytic ascomycetous fungus, Chaetomium globosum is a potential antagonist of several soil and seed borne plant pathogens and recent studies have emphasized the role of C. globosum in controlling spot blotch of wheat caused by C. sativus. A thorough study made by Agarwal et al. (2004), has highlighted the potential antagonism of an antifungal metabolite produced by C. globosum against C. sativus both in vitro and in vivo conditions.
- Agarwal R, Tewari AK, Srivastava1 KD, Singh DV (2004). Role of antibiosis in the biological control of spot blotch (Cochliobolus sativus) of wheat by Chaetomium globosum. Mycopathol. 157: 369-377.
- Alam KB, Banu SP, Shaheed MA (1998). The occurrence and significance of spot blotch disease in Bangladesh. In:Duveiller E, Dubin HJ, Reeves J and McNab A (eds) Proc.Int. Workshop on Helminthosporium Disease of Wheat: Spot Blotch and Tan Spot, CIMMYT, El Batan, Mexico, 9-14 February 1997, pp 63-66.
- Chaurasia S, Joshi AK, Dhari R, Chand R (1999). Resistance to foliar blight of wheat: A search. Genet Resour Crop Evol 46: 469-475.
- Dhaliwal HS, Gill KS, Singh PJ, Multani DS, SinghB (1986). Evaluation of germplasm of wild wheats, Aegilops and Agropyron for resistance to various diseases. Crop Improv. 13: 107-112.
- DWR (1995). Report of coordinated experiment, 1994-95. Vol.1Crop Protection (Pathology) (Eds. Parasher M, NagarajanS, Goel LB, Kumar J). pp. 43-54. AICWIP. Directorate of Wheat Research, pp 206.
- Goel LB, Nagarajan S, Singh RV, Sinha VC and Kumar J (1999). Foliar blights of wheat: Current status in India and identification of donor lines for resisatnce through mulilocational evaluation. Indian Phytopathol 52:398-402.
- Murray TD, Parry DW, Cattlin ND (1998). In: A color handbook of diseases of small grain cereal crops. Iowa State University Press, Ames, Iowa.
- NemaKG,Joshi LM (1971). The Spot blotch disease of wheat caused by Helminthosporium sativum. Proc. 2nd Int.Symp. Plant. Path., IARI, New Delhi, pp 42.
- Saari EE (1998). Leaf blight diseases and associated soil borne fungal pathogens of wheat in south and south east Asia. In: Duveiller E, Dubin HJ, Reeves J, McNab A (eds) Helminthosporium blights of wheat: spot blotch and tan spot. CIMMYT, Mexico, DF, pp 37–51.
- Sharma RC, Duveiller E (2006). Spot blotch continues to cause substantial grain reductions under resource- limited farming conditions. J Phytopathol 154:482-488.
- Singh DV, Srivastava KD (1997). Foliar blights and Fusarium scab of wheat. Present status and strategies for management. In: Management of Threatening Plant Diseases of National Importance. Malhotra Publishing House, New Delhi. pp. 1-16.
- Singh DV, Srivastava KD, Aggarwal R, Bahadur P (1993). Wheat Disease Problems: The changing scenario.In: Pest and Pest Management in India- The Changing Scenario.(Eds. H.C. Sharma and M. Veerabhadra Rao) Plant Protection Association of India, Hyderabad, pp. 116-120.
- Singh D, Singh RV, Singh AK, Singh RN (2005).Sources of resistance against foliar blight of wheat caused by Helminthosporium sativum. Indian Phytopath. 58: 111-113.
- Singh G, Tyagi B S, SinghG P, ChatrathR, Singh D P, ShoranJ (2008). Genetic analysis and association of spot blotch resistance caused by Bipolaris sorokiniana with morphological and yield attributes in bread wheat Indian Journal of Agricultural Sciences 78 (11):957-61.
- van Ginkel M, Rajaram S (1998). Breeding for resistance to spot blotch in wheat: Global per spective. In: Duveiller E, Dubin HJ, Reeves J, McNab A (eds) Helminthosporium Blights of Wheat: Spot Blotch and Tan Spot. CIMMYT, Mexico DF, pp 162-169.
- Xiao Z, Sun L, Xin W (1998). Breeding for resistance in Heilongjiang province, China. In: Duveiller E, Dubin HJ, Reeves J, McNab A (eds) Proc. Int. Workshop Helminthosporium Disease of Wheat: Spot Blotch and TanSpot. 9-14 February 1997, CIMMYT, El Batan, Mexico, DF, pp 114-118.
Rajita, Gyanendra Singh, Virender Singh, Ashish Ojha, R. SelvaKumar, Sonia Sheoran
Directorate of Wheat Research, Karnal-132001