Literature Review on Oilseed Crops and Its Infections in India

Rapeseed-mustard is the third most important oilseed commodity in the world after soybean (Glycine max) and palm (Elaeis guineensis Jacq) in world agriculture and India is the third largest producer with global contribution of 28. 3 percent acreage and 19. 8 percent production (Shekhawat et al. , 2012; Bandopadhyay et al. , 2013). Brassica juncea L. (Czern & Coss. ) is an important oilseed crop cultivated traditionally in all the Indian states with particular reference to marginal and sub-marginal soils of the eastern, northern and northwestern states as a pure crop as well as intercrop (mixed crop). Low temperature and humid climate of winter is a significant factor for lavishing growth and productivity of mustard in these regions (Rathi and Singh, 2009). Despite the considerable increase in productivity and production, the crop is still facing multiple abiotic and biotic challenges in farmer’s fields. The destructive diseases of rapeseed-mustard include those caused by fungi, bacteria, viruses, and phytoplasma. Among them, Sclerotinia stem rot is the most serious fungal disease that causes maximum damage in Indian mustard (Rakesh et al. , 2016).

Sclerotinia rot or white rot caused by Sclerotinia sclerotiorum (Lib. ) de Bary, is a cosmopolitan and destructive soil inhibiting plant pathogen. The pathogen shares a broad host range comprised of more than 500 plant species which includes 278 genera in 75 families of dicotyledonous, and several monocotyledonous plants (Boland and Hall, 1994; Willetts and Wong, 1980; Purdy, 1979; Steadman, 1983; Sharma, 2014; Saharan and Mehta, 2008; Sharma et al. , 2015). Sclerotinia stem rot, although, occurs more frequently in cool and moist regions (Purdy, 1979; Saharan and Mehta, 2008). It occurs primarily in its destructive nature in temperate and sub-temperate zones of the world. It is reported to inflict considerable yield reduction in economically important crops (Boland and Hall, 1998; Fernando, 2004; Malvarez et al. , 2007; Parveen et al. , 2007).

Extensive literature is available on various aspects of S. sclerotiorum related to several crop plants, but its information on Rapeseed mustard concerning the present study is reviewed here in the following paragraphs. Causal pathogen: Sclerotinia rot or white stem rot of mustard is caused by a soil-borne, homothallic, nonsporic fungus, i. e. , Sclerotinia sclerotiorum (Bolton et al. , 2006). The pathogen was first described by Madame M. A. Libert (1837) as Peziza sclerotiorum. Later in 1870, Fuckel created a new genus Sclerotinia and renamed it as Sclerotinia libertinia. Whetzel in 1945 proposed a new family Sclerotiniacae and also provided the key for diagnosis of belonging genera. However, the name and the authority for the fungus has generally been accepted to be Sclerotinia sclerotiorum (Lib. ) de Bary because of the significant contribution of de Bary in his literature. S. sclerotiorum is a necrotrophic pathogen, comprises of hyaline, septate, branched and multinucleated hyphae; the mycelium may appear white to tan in culture plates (Boltan, Thomma, and Nelson, 2006). The pathogen undergoes four stages in its life cycle viz. sclerotia, apothecium, ascospore, and mycelium (Purdy, 1979). It may from thick mycelial mat in restricted colonies, which later produces white mounds of mycelium, covered with small liquid droplets, these mounds later fuse together to form dark-colored structures i. e. sclerotia, which are resistant to environmental stresses (Willetts and Wong, 1971), and also act as primary survival structures (Coley-Smith and Cook 1971; Willetts and Wong 1980). The fungus spent 90% its lifespan as dormant sclerotia (Adams and Ayers 1979), the sclerotia may survive in one or more of three ways, i. e. , myceliogenic, corpogenic and sporogenic, but corpogenic and myceliogenic germination occurs in S. sclerotiorum (Tourneau, 1979).

The germinative sclerotia give rise to 2-5 columnar structure, stipes or primordial which are dichotomously branched. Apothecial initially arise in the cortex or medulla as brown to hyaline cluster or nest of interwoven hyphae and at the tips minute, brownish, funnel-shaped cups or apothecia are produced, measuring 6-9 mm across. They are generally borne 6-10 mm above the soil surface and become darker in color with age. The apothecia contain cylindrical asci, measuring 108-153 x 45-10 micron. Each ascus forms eight hyaline one-celled and ovate ascospore, measuring from 7-16 x 3. 6-10 micron. These ascospores are ejected violently through the apical pore of ascus and help in the spread of disease during the crop season. Initiation of disease cycle takes place by means of sclerotia, a dormant, multihyphal structure which can tolerate a varied range of unfavorable weather condition for several years. The method of germination depends upon environmental conditions (Saito, 1973; Joens, 1974; Kosasih and Willetts, 1975; Steadman, 1983; Sharma and Meena, 2011; Willets and Wong 1980), and crop canopies (Bardin and Huang 2001). Infection of the crop is associated with senescent blossoms, which provide a source of energy for germinating ascospores (Cook et al. 1975), suggests that crop phenology plays a significant role in the onset of this disease in some crops (McLean 1958; Natti 1971; Abawi and Grogan 1975; Abawi et al. 1975; Kruger 1975;).

The pathogen survives (overwinters) in soil and also in the host by means of hard dormant structure, i. e. , sclerotia. It consists of a light colored interior called medulla, and a black exterior protective covering which is known as rind. The rind is highly resistant to degradation due to the presence of melanin; sclerotia enclose fungal cells which bears abundant of -glucans and proteins. In S. sclerotiorum, development of sclerotia development may be in three distinct stages (Townsend and Willetts, 1954): (i) Initially, the formation of small mounds, i. e. , interwoven hyphae developed due to the repeated branching of long areal primary hyphae. (ii) Development, increase in size, and (iii) Finally, the sclerotia get matured which is characterized by the appearance of dark colored or melanized, internally consolidated hard structures, which may often bear droplets at maturity. Both morphological and biochemical differentiations accompany these phases.

All the stages of developmental stages of sclerotia are affected by numerous factors as photoperiod temperature, aeration, and nutritional status, etc. , production of growth hormones like Oxalic Acid (OA) may show some extent of correlation with development and play an essential role in disease development (Zhou and Boland, 1999; Chet and Henis, 1975; Donaldson et al. , 2001). Development of sclerotia is a complex and multistage process reported to be regulated by signal transduction pathways as MAPK and PKA (Rollins and Dickman, 1998; Chen et al. , 2004; Chen and Dickman, 2005; Harel et al. , 2005;). Kafadar and Cyert, 2004 reported that two more pathways namely, calcineurin – MAPK and calcineurin - PKA associated pathways are also associated with pathogenicity of this pathogen. The germination of overwintered sclerotia may be of three types depending upon the weather condition. Carpogenic germination is the most common method of sclerotial germination which tends to development of minute mushroom-like apothecium. This type of germination is mainly observed in the winter season when there is plenty of rain or in irrigated fields because it needs relatively low temperature along with soil wetness of 1 – 2 weeks.

The apothecia bear ascospores which get liberated in the environment. These ascospores may have different fates, i. e. , few may fall on the nearby susceptible host and lead to the development of the disease or may travel to a long distance by the help of blowing wind. The pathogen also shows myceliogenic germination, in this type of germination sclerotia, produces mycelium instead of apothecium. This mycelium often occurs at or beneath the soil – line and known to cause infection in healthy susceptible host nearby the germinating sclerotia. This type of germination supposed to occur when there is plenty of nutrients (Saharan and Mehta, 2008). The soil temperature and moisture are the key factors affecting the carpogenic germination of S. sclerotiorum (Carpenter et al. , 1999; Huang and Kozub, 1991; Schwartz and Steadman, 1978; Phillips, 1986; Dillard et al. , 1995; Thaning and Nilsson, 2000; Sun and Yang, 2000; 2002; Hao et al 2003; Ekins et al. , Clarkson et al. , 2004). Numerous factors determine the survival of sclerotia as prevailing environmental conditions, previous crop and soil type. According to Mehta et al. , 2009, the production of apothecia from sclerotia was affected by various soil types, and he mentioned that the sclerotia germination and apothecia production was least in sandy soils, while the same was maximum in the sandy loam soil. He also reported that flooding of the field before seed sowing resulted in the least disease incidence and minimum lesion length.

According to his findings, irrigation should be given once with a three or seven days intervals, had exhibited low disease intensity as compared to control. The elevated soil temperature and moisture are probably the most harmful environmental factors to S. sclerotiorum. Apothecia formation: Significant documentation of literature regarding the structure and development of apothecium has done by a number of researchers like Saito, (1973), Jayachandran et al. (1987), Kosasih and Willetts (1975) and Jones, (1974). According to Abawi and Gorgan (1979), Apothecia generally produced after a certain dormancy period during which the sclerotia are chilled or frozen. Cold temperature seems to be a predominant factor in “conditioning” sclerotia to produce apothecia.

Hence, preconditioning of minimum two weeks at 10 - 15 ºC in moist soil (at >50% field capacity) and presence of organic material in rhizosphere within the top 2 cm of the soil surface is essential for apothecia production. Sharma and Meena (2011) also observed apothecial development in B. juncea in field condition while the prevailing weather was quite favorable, i. e. , maximum and minimum temperature and relative humidity (RH) were 17. 5 ºC, 4. 4 ºC, and 98. 3% respectively, with low sunshine hours of 4. 0, and 16% soil moisture. In carpogenic germination, the sclerotial cortex or medulla are the regions of active fungal growth. In this type of germination, the tough rind of the sclerotia gets busted due to the continuous growth of enclosed fungal cells; growing primordia form a tube-shaped stalk called stipes. These stipes after emerging from the soil continue to grow upward up to a height of 1 cm, an exposure to ultraviolet light of