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The disease appears in the field in patches at both seedling and adult stages. Seedling wilt is characterized by sudden drooping, followed by drying of leaves and seedling death. The roots appear healthy, with reduced proliferation and nodulation and usually no internal discoloration of the vascular system.
Adult wilt symptoms appear from flowering to late pod-filling stage and are characterized by sudden drooping of top leaflets of the affected plant, leaflet closure without premature shedding, dull green foliage followed by wilting of the whole plant or of individual branches. Seeds from plants affected in mid-pod-fill to late pod-fill are often shrivelled (Beniwal et al., 1993).
Cultural Control and Sanitary Methods
Sowing date affects wilt incidence because it determines the growth stage of the crop that is at an optimum or near-optimum temperature for fungal growth. In India, delayed sowing reduces disease incidence, but late sowing dramatically reduces yield potential and its effect on disease development differs over locations and seasons (Kannaiyan and Nene, 1975a; Mittal, 1997). In Syria, early sown crops are usually less affected. This is mainly due to differences in temperature at sowing, in the two countries (See Biology and Ecology; Epidemiology). A crop rotation of 4-5 years reduces inoculum density in the field, but does not completely eradicate the disease. In India, cultivation of paddy or sorghum in the rainy season reduced lentil wilt incidence the following winter (Kannaiyan and Nene, 1979). Soil amendment with organic matter (wheat or barley straw) enhances antagonism by other soil microorganisms.
Resistance in lentil is considered the most practical and environmentally sound means of vascular wilt management for lentil. Screening for wilt resistance is done in the field (Kannaiyan and Nene, 1976; Khare et al., 1993; Bayaa et al., 1994), greenhouse (Bayaa and Erskine, 1990), or in the laboratory (Bayaa et al., 1994; Omar et al., 1988). Screening methods have been compared and a strong correlation was found between field and greenhouse disease reaction (Bayaa et al., 1994). Plant age has a dramatic effect on resistance; for example, many lines exhibiting resistance at the seedling stage are not as resistant at the adult stage (ICARDA, 1990). Cultivars with resistance to wilt have been released. These include Naslada, Zhana, Anicia and Tadzhikskaya 95 in Bulgaria (Mihov et al., 1987), Talya 2 in Lebanon (Abi Antoun et al., 1990) and Pant L 406 (Pandya et al., 1980) and Pant 4 (Singh et al., 1994) in India. Sources of resistance to wilt have been found by many authors in the cultivated lentil (Nene et al., 1975; Kannaiyan and Nene 1976; Khare et al., 1979; Khare 1980; Tiwari and Singh, 1980; Hossain et al., 1985; Bayaa and Erskine, 1990; Hamdi et al., 1991; ICARDA 1993, 1994) and made available through the Lentil International Fusarium Wilt Nursery. Resistance to wilt has also been found among lentil wild relatives in three accessions (ILWL 79 and ILWL 113 of L. culinaris ssp. orientalis and ILWL 138 of L. nigricans ssp. ervoides) (Bayaa et al., 1995).
Biological control is a desirable alternative to chemical control of vascular wilt. In Syria, several antagonistic bacteria isolated from soil were detected and identified as species of Bacillus. The isolates did not affect seed germination but stimulated plant growth in pots and reduced disease severity (El-Hassan et al., 1997). In India, Trichoderma viride, Streptomyces gougeroti and some species of bacteria were antagonistic to F. oxysporum f.sp. lentis (Mehrotra and Claudius, 1972). Similarly, Trichoderma harzianum and T. koningii showed antibiosis and mycoparasitism (Mukhopadhyay et al., 1989) and, in Pakistan, Arachniotus sp. and T. harzianum have been studied (Akhtar, 1989; Aslam, 1989). The possibility of their use as biocontrol agents on a large scale should be explored through field trials and the development of a suitable delivery system. The addition of organic matter to the soil enhances the activity of the antagonists. Complete control of wilt was achieved when Arachniotus sp. or T. harzianum were used in combination with chopped wheat straw along with 1% N2 urea + 1% glucose +1% K2SO4 or 1% N2 urea + 1% K2SO4 + 0.001% MgSO4.
Leaf extracts from different plant species including Ranunculus sceleratus, Impatiens balsamina, Lawsonia inermis, Mentha spicata, Adenocalymma allicium, and Artabotrys hexapetalus were reported to be fungitoxic against the causal agent in-vitro (Jaspal-Singh et al., 1994, 1995a, b)
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Wilt is an important disease of lentil that can cause complete crop failure, especially in a warm spring and dry, hot summer (Izquierdo and Morse, 1975; Bayaa et al., 1986; Agrawal et al., 1993). Baraimer and Izquierdo (1977) found that the degree of F. oxysporum infection ranged from 25-95% depending on the cultivar tested.
Wilt incidence during reproductive growth was correlated with yield loss estimates, with a reduction in seed yield per unit change in wilt incidence of 0.846 + 0.118% in northern Syria. In the laboratory, disease reaction was positively correlated to inoculum density (Erskine and Bayaa, 1996). In the field, inoculum density was not correlated to disease incidence in susceptible lentils, precluding the possibility of predicting disease incidence from inoculum density.