Epidemiological and Biochemical Components of Host Resistance To Blackspot Disease of Rapeseed-Mustard
Department of Plant Pathology, G. B. Pant University of Agriculture and Techonology,
Pantnagar 263145, India
Epidemiological components of infection-rate reducing resistance to Alternaria blackspot (Alternaria brassicae) and the disease progression characteristics of 11 oilseed Brassica genotypes were quantified. Significantly lower AUDPC values and apparent infection rates were observed in six genotypes viz, Brassica alba, B. campestris ssp rapifera, B. carinata (PPSC 1), B. juncea cv orna rai, B. juncea cv PHR-1 and B. napus cv target compared to five other commonly cultivated susceptible genotypes. Number of spots on stem 15cm-1 length was found to be positively correlated with size of lesion (r=0.732), percent defoliation (r=0.833), percent siliqua infection (r=0.939) and overall disease score (0.814) and hence number of spots on stem at 15 days prior to maturity was the best component for evaluation of resistance to disease under field conditions. Total chlorophyll and epicuticular wax contents, presence of higher amount of gallic acid, synergic acid and vanillic acid indicated the possibility of their significant role in infection rate reducing resistance of oilseed brassicas.
KEYWORDS : Alternaria brassicae, Infection-rate reducing resistance, partial resistance
Alternaria blackspot (ABS) is one of the most important diseases of rapeseed-mustard in India (Kolte, 1985). Losses in yield attributed to this disease range from 10 to >50% depending on the brassica species cultivar planted and prevailing weather conditions in a crop season. In the absence of transferable genetic sources of resistance to ABS in commercially cultivated genotypes, infection rate-reducing resistance to ABS is a viable breeding strategy. But only limited studies have been done to identify this type of resistance in rapeseed-mustard. The overall objective of the present study was to quantitatively evaluate the effect of different epidemiological components in relation to the rate of disease progress in some known field resistant genotypes of Brassica species in comparison to susceptible genotypes. Attempts were also made to determine biochemical components of most promising infection rate-reducing resistance of some selected genotypes.
Six known ABS-field resistant genotypes viz; Brassica alba, B. carinata cv PPSC 1, B. campestris ssp rapifera, B. napus cv target, B. juncea cv orna rai and B. juncea cv PHR-1 and five ABS-susceptible genotypes viz., B. campestris var. toria cv PT 303 , B. campestris var yellow sarson cv YST 151, B. juncea cvs Krishna and Varuna and B. napus cv regent were grown both under greenhouse and field conditions from 1990-91 through 1996-97. Each genotype was replicated four times using a randomized complete block design. The field plot size was 4x3m or 5x3m. In order to precisely study the effect of epidemiological components, the pot-grown plants were also separately inoculated with conidia obtained from 20-day-old pure culture of A. brassicae isolate A. For inoculation 1x105 conidia ml-1 was used and inoculation was done on 30-day-old plants of each genotype on the middle leaf by spraying adaxial surface with conidial suspension. All the inoculations were made in the evening and inoculated plants were kept in a polythene-made moist chambers for 72 hrs. Observations on epidemiololgical components as shown in Table 1 were recorded using the standard methods. Analysis of variance for resistance components data as affected by the genotypes performed and correlation coefficients ( r ) were computed for individual components of resistance.
Presence of phenolic acids such as gallic acid, synergic acid and vanillic acid was determined in plant leaf samples at 50-days after sowing using HPLC (Charpentier and Cowles,1981). Chlorophyll content and epicuticular wax content were determined as per the prescribed methods.
The results revealed that number of spots per unit area of leaf did not show significant difference between resistant and susceptible genotypes, and there was no correlation with the overall field disease score. With regard to size of spots on leaf there was a significant difference in the rate of expansion of spot between resistant and susceptible genotypes. For example, sarson cv YST 151, B. juncea cv Krishna and toria cv PT 303 showed significantly higher rate of expansion of the spot than resistant genotypes of B. campestris ssp rapifera, B. alba and B. carinata. Usually in resistant genotypes B. campestris ssp rapifera, necrotic spots were surrounded by dark purple-red area and in B. alba and B. carinata minimum size of spot was observed without chlorotic area. The variation in lesion diameter was positively correlated with sporulating ability of the host (r= 0.894) and field disease score (r=0.798).
It was interesting to note that number of lesions on stem appeared to be an important criterion for evaluation of resistance to the ABS. Number of lesions per unit length of stem was found positively correlated with most other components of resistance (Table 1) including the number and size of lesion on siliqua particularly under field conditions about 15 days before harvesting. Photosynthates from siliquae are known to contribute significantly to the development of seeds in rapeseed-mustard.
Higher content of chlorophyll in the range of 1.6 to 1.8 mg g-1 fresh weight of leaf and low frequency of open stomata (21%) were found associated with resistance of B. alba as against lower chlorophyll (0.3 to 0.7 mg g-1) and higher frequency of open stomata (67%) in the most susceptible B. campestris cv YST 151. But in the case of B. carinata cv PPSC 1 and B. napus cv target, besides higher content of gallic acid, synergic acid and vanillic acid, higher epicuticular wax content in the range of 0.13 to 0.23 mg /cm2 leaf area also played role in guarding the infection against A. brassicae. The susceptible genotype, however, had epicuticular wax content of 0.10 to 0.11 mg/cm2 leaf area.
1. Charpentier, B. A. and Cowles, J.E. 1981. Rapid method of analysing phenolic compounds in Pinus ellioti using high performance liquid chromatography. Journal of Chromatography 208: 132.
2. Kolte, S. J. 1985. Diseases of Annual Edible Oilseeds Crops. Vol. II: Rapeseed-Mustard and Sesame Diseases, CRC Press, Boca Raton, Florida PP135.