Variability in Alternaria brassicae (Berk.) Sacc., the causal agent of black spot of rapeseed mustard and evaluation of different components of host resistance
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Date
2009-12
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G.B. Pant University of Agriculture and Technology, Pantnagar - 263145 (Uttarakhand)
Abstract
The oilseed crops especially rapeseed -mustard play a pivotal role in agricultural economy of the world. The yield potential of these crops is affected by a number of diseases. Among them, Alternaria blight caused by Alternaria brassicae (Berk.) Sacc., is most important. Control of this disease through chemicals is quite expensive, needs extra labour and also not ecofriendly. Thus one of the major challenges of research on rapeseed mustard is to develop the knowledge about the variability of A. brassicae in terms of cultural, morphological, nutritional; biochemical and induction of resistance that can minimize the losses caused by A. brassicae and help to stabilize the mustard production. The present investigation were carried out with objectives (1) to find out the extent of variation in growth rate (in vitro) among isolates of A. brassicae. (2) to find out whether the growth rates of A. Brassicae isolates in vitro is indicative of variation in aggressiveness and pathogenicity. (3) to find out the trade-off between growth rate and spore production. (4) grouping of different isolates according to preestablished groups (A, C, D) and on other, based on variation in growth and sporulation and aggressiveness. (5) to study the phenomenon of cross-protection using different combination of virulent and less virulent strains of A. brassicae. (6) to study the effect of abiotic inducers on induced resistance against A. brassicae. (7) to study comparative differences in biochemical constituents (nitrogen, proteins, phenol, sugar, etc.) in inoculated and uninoculated Brassica spp. Eighty eight isolates of A. brassicae were collected and grouped into twenty on the basis of type of symptom, growth rate, sporulation and colony characters. Isolate ABD, ABI, ABJ, ABN, ABQ, and ABT were fast growing; ABD, ABK and ABT have maximum spore body length; ABL and ABT had maximum beak length; ABL had high sporulation and isolate ABM had maximum germination of conidia. Percent germtubes per conidia was maximum in ABM and lowest in ABQ. RLEA medium supported the maximum growth and sporulation of all the isolate. The ratio of spore body length to spore beak length varied from 0.7 to 2.3. Percent spore germination varied from 10 to 80%. Most suitable temperature for growth and sporulation of all isolates was 200C and 250C. Most suitable pH for growth was 6.0 and 7.0 for most of the isolates. RLEB liquid medium supported maximum growth of all the isolates. Twenty isolates of A. brassicae showed difference in their pathogenic behaviour on different B. juncea cultivars. Minimum incubation period was in ABD isolate showing more virulence and showed maximum average number of spots as well as maximum size of spot in Varuna and Kranti. Culture filtrates of isolate ABD, ABI, ABJ and ABT showed very low seed germination and low plumule, radical length and maximum disease reaction. Four categories viz. least virulent, mildly virulent, virulent and highly virulent, were formed with virulence index. According to this, isolate ABA, ABM and ABO were least virulent; ABD, ABI, ABJ and ABT were highly virulent. Nitrogen and protein content were more in susceptible genotypes and was highest in uninoculated disease free condition. Phenol content was higher in resistant genotype and it increased with age. Total sugar content was high in resistant genotypes and disease free plants. It decreased with increase in age. Resistant and disease free genotypes contained higher content of reducing sugar and non- reducing sugar. Among the Brassica cultivars, B. rapa had highest and B. alba cultivar had low disease severity at 60 DAS. B. juncea cv. PHR-1 showed very less disease severity at 60 and 90 DAS. B. rapa cultivars showed high disease severity on pods at 90 DAS. No disease on pods was observed in case of PHR-1 (B. juncea), PBN-9501, GSL-1 (B.napus), PPCS-1 (B. carinata) and PPBA-1 (B. alba). Maximum disease was at 30-60 cm height at 60 DAS. Disease at 90 DAS increased upto 90 cm. In B. rapa high disease severity was at 90- 120 cm. Varuna and Kranti gave highest average yield. B. alba (PPBA-1) had a very low seed yield but had highest 1000 grain weight. Size of spots increased from 60 to 90 DAS. It was found to be smallest in PPBA-1 (B. alba) and largest in Kranti (B. juncea). Prior inoculation of least virulent isolate and mildly virulent isolate reduced the disease caused by virulent isolate. Salicylic acid gave reduced number of spots and average disease score against least virulent, mildly virulent and virulent isolates of A. brassicae.
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Thesis-PhD
Keywords
variability, Alternaria brassicae, causal agents, rapeseed, mustard, host plants, disease resistance, fungal diseases, oilseed crops, blight, virulence