In this article we will discuss about:- 1. Introduction to Phomopsis Vexans 2. Materials and Methods of Phomopsis Vexans 3. Results and Discussion.
Introduction to Phomopsis Vexans:
The Phomopsis leaf blight and fruit-rot caused by Phomopsis vexans (Sacc. & Syd.) Harter, (Tel: Diaporthe vexans Gratz) is a very destructive disease of brinjal and considered to be the major constraint for limited production and productivity of this crop. In India, PANWAR (1970) reported that the losses due to Phomopsis fruit-rot ranged to the extent of 10-20 per cent.
Literature survey revealed very limited information on the host pathogen interaction in terms of aggressiveness of the isolates of Phomopsis vexans. Keeping this in view, the present investigation was-carried- out to study differences in pathogen isolates in terms of aggressiveness to the different brinjal genotypes.
Materials and Methods of Phomopsis Vexans:
The experiment was conducted at department of Plant Pathology, G.B. Pant University of Agriculture & Technology, Pantnagar in the year 2003. More than 50 isolates of Phomopsis vexans were isolated and purified from severely infected plant parts showing typical disease symptoms of leaf blight/fruit-rot/stem canker collected from five different locations of Uttar Pradesh and Uttarakhand.
Based on the cultural characteristics, altogether 32 isolates from all the locations were selected and assigned numbers as Pv1, Pv 2, Pv 4, Pv 5, Pv 6, Pv 7, Pv 8, Pv 9, Pv 10, Pv 11, Pv 13, Pv 14, Pv 15, Pv 16, Pv 17, Pv 18, Pv 19, Pv 24, Pv 25 Pv 27, Pv 29, Pv 30, Pv 31, Pv 32, Pv 33, Pv 35, Pv 36, Pv 41, Pv 43, Pv 45, Pv 48 and Pv 50. Their pathogenicity was proved on susceptible cultivar ‘Pant Rituraj’.
Thereafter, representative isolates of each race were selected for analyzing the aggressiveness of isolates as well as genotype response against the pathogen isolates.
Younger leaves from 6 weeks old plants of 25 genotypes of brinjal were detached and surface sterilized with 0.1 NaOCl for 1 min followed by rinsing thrice with sterilized distilled water. The treated leaves 60 spores was put at the centre of each leaf. Inoculated leaves were incubated at 27±1°C.
Each treatment was replicated thrice. Each petridish was observed every day for infection and symptom appearance. The lesion area was recorded on the 8th day of inoculation by putting the trace paper on petridish down side- up. The lesions were traced. The traced areas were put on graph paper and measured.
Incubation period and lesion area were analysed by 2 factorial CRD and were given numerical values to get response value. Based on the mean response values (MRV) obtained from incubation period and lesion area, each genotype was assigned host reaction. Host response against mean response value as R (MRV upto 0.20), MR (MRV.0.21 to 0.40), MS (MRV 0.41 to 0.60) S (MRV 0.61 to 0.80) and HS (MRV 0.81 to 1.00).
Results and Discussion of Phomopsis Vexans:
In the present study, taking susceptible reaction on differential lines into consideration, the thirty two isolates of the pathogen collected from different locations were characterized into five races i.e. Race 1, Race 2, Race 3, Race 4 and Race 5 on the basis of response reaction recorded on a set of twelve differential lines in glasshouse.
Nine isolates constitute race 5 which predominates while race 3 consisted only two isolates namely Pv 4 and Pv 36 (Table 15.1). Representative isolates from each race were finally selected to characterized genotypes and aggressiveness of isolates using detached leaf technique.
The results recorded in Table 15.2 revealed that significant differences existed in time taken by individual isolate on different host genotypes to produce symptoms. Isolate Pv 10 took just 2.33 days time as incubation period with 3 host genotypes and above 4-5 days in several others.
Similarly isolate Pv 11 took just two days on certain genotypes but above 3-4 days on several other genotypes. Almost similar patterns were recorded with isolates Pv 11 and Pv 25. Isolate Pv 36 took less than 3 days to produce symptoms on 10 genotypes including PCPGR 1533 PCPGR 1593, PB 50 etc. Almost same result was observed with isolate Pv 16. The computation of lesion area too reflected the existence of differences.
The lesion areas with Pv 36 ranged from 1.67 to 16.0 mm2 whereas in Pv 11 it ranged between 1.0 to 11.67 mm2. With the same host genotype individual isolates caused significantly different size of lesions.
For example, isolates Pv 25 and Pv 16 caused lesion area measuring 2.33 to 3.0 mm2, isolates Pv 10 and Pv 11 caused 7.33 mm2 and isolate Pv 36 developed lesion of 10.0 mm2 size when inoculated on host genotype 1DBHL 50 (Table 15.3).
Similar differences existed in all the 25 host genotypes, with individual isolates. If the average of incubation period and lesion area are considered with host genotype as principal variables, significant differences existed there. If the isolates are evaluated as principal input, significant differences were observed with respect to mean IP and mean LA.
This clearly establishes the fact that there are differences in the isolates with respect to host genotypes reactions and therefore, differences in aggressiveness of isolates of Phomopsis vexans.
The response value of the pathogen isolates with regard to mean incubation period and mean lesion area recorded on 25 genotypes revealed that isolate Pv 36 was most aggressive which took minimum mean incubation period whereas isolates Pv 11 and Ps 25 took maximum time to develop symptom.
Aggressiveness of 5 isolates of Phomopsis vexans on the basis of responses shown by 25 host genotypes revealed that response value in term of IP ranged from 0.4-0.6 and in term of LA, it ranged from 0.6 – 1.0. The aggressiveness response value for isolates against all the genotypes ranged from 0.5 to 0.8 (Table 15.4).
Analysis of the data revealed that isolate Pv 36 produced lesion area 87.02 per larger than isolate Pv 11 which took 13.46 per cent less incubation period (Fig. 15.1) Further correlation analysis of the data revealed that the incubation period was negatively correlated with the lesion area having coefficient r = -0.83. (Fig. 15.2).
Cumulative response value obtained from incubation period and lesion area also resulted into differential reactions against each isolates (Table 5). Similarly, the response value and mean response value of individual genotype against all the 5 isolates of Phomopsis vexans with regard to mean incubation period and mean lesion area ranged between 0.2 to 1.0.
Mean response value gave clear characterization of genotypes into resistant, moderately resistant, moderately susceptible, susceptible and highly susceptible groups. Two genotypes namely PST and PB 46 were recorded as resistant against the pathogen whereas moderately resistant reaction was recorded in 4 genotypes viz.; PH 5, DBL 1, DBL 5 and VBL 7 (Table 15.6).
In addition, host pathogen interaction studied by detached leaf technique, the differences in incubation period and lesion area in terms of aggressiveness and host reaction were evident. The differences in incubation period required to produce lesion and lesion area by different isolates was significantly different from each other. Isolate Pv 36 was most aggressive while isolate Pv 11 was least aggressive.
These results do indicate that there are differences among the five isolates / races with respect to their aggressiveness. In different host-pathogen systems, several workers have also studied differences in virulence/aggressiveness of the isolates of pathogen and differences in host reaction.
The reactions of the genotypes obtained from mean response value on the basis of incubation period as well as lesion area confirms the reliability of detached leaf technique.
Its possible use in assessing aggressiveness of a large number of pathogen isolates/races as well as germplasm response to their pathogen within short time is worth exploring.