Abstract: The assessment of severity with chronological development of disease of mesta (Hibiscus cannabinus, cv. HC-583) on a particular diseased plant was studied. Based on the symptomatology and intensity of diseases caused by Begomovirus an empirical formula was developed. A methodology for assessing the disease intensity of yellow vein mosaic disease of mesta grown under natural field condition was proposed and it was validated by comparing with infected plants grown under field as well as glasshouse conditions. Quantitative assessment of the disease indicated that the disease index value at different stages of pathogenesis varied from 1.82 to 49.44 under field condition whereas the same varied from 2.78 to 50.83 under glasshouse condition. Mortality of infected leaves was noticed when the index value reached maximum. With the present methodology scoring of gradual development of disease has become possible for a particular plant and even for an individual leaf of a particular cultivar.
INTRODUCTION
The occurrence of yellow vein mosaic disease of mesta (Hibiscus cannabinus L; family Malvaceae) is a serious threat to disease scenario in India (Chatterjee and Ghosh, 2007a, b). It was found in endemic form in different parts of India during the last few years and cause great reduction in fibre and seed yield and thus a major threat to production (Chatterjee et al., 2005). The association of a novel Begomovirus (family: Geminiviridae) namely Mesta yellow vein mosaic virus with the disease was confirmed by electron microscopy and molecular techniques, sequence information and southern hybridization (Chatterjee and Ghosh, 2007a, b; Chatterjee et al., 2006). The disease is characterized by yellowing of veins and veinlets followed by complete chlorosis of the leaves of affected plants with the advancement of infection (Fig. 1). If the plants are infected by the virus at early stage of growth they do not flower even. The infected plants in general show stunted growth with reduced leaf size. This disease is transmissible by whitefly (Bemisia tabaci) under natural condition (Chatterjee et al., 2005). Now it is spreading at a faster rate at different parts of India like Andhra Pradesh, Uttar Pradesh, Bihar, Orissa and West Bengal and attracted immediate attention for in-depth study.
Many disease scoring techniques for assessing incidence and intensity caused by viruses have been reported in plants (Anonymous, 1976; Anjaneyulu, 1975; Furumoto and Mickey, 1967a, b; Joshi, 1973; Ghosh et al., 1981), but assessment of severity with chronological development of disease on a particular plant and even in a particular leaf of a diseased plant was never reported. In recent days these studies are neglected and for this purpose disease scoring was not attempted. It results into wrong information of disease intensity and disease incidence pattern. Moreover, for assessing intensity of diseases caused by viruses on fibre crops especially mesta has not yet been attempted. Beside this the typical yellow vein mosaic disease symptom caused by Begomovirus is unique and thus for chronological assessment of severity of the disease, development of an empirical formula was needed. Hence, the present investigation was an attempt to develop a methodology for assessing the disease intensity of Yellow Vein Mosaic (YVM) disease on mesta plants grown under field condition and to validate the method it was also compared with infected plants grown under field as well as glasshouse conditions.
MATERIALS AND METHODS
Inoculation of host and virus transmission at glasshouse condition: Thirty healthy mesta plants (H. cannabinus cv. HC-583), a highly susceptible cultivar to yellow vein mosaic (YVM) disease, were raised in earthen pots containing four seedlings per pot and filled with 4 kg of well-sieved field soil mixed with 1.5 g of urea in the glasshouse.
A virus-free stock of whitefly was reared on healthy tobacco plants (Nicotiana tabacum) in large wooden cages, covered with insect proof, galvanized wire mesh at 30°C in a temperature-controlled glasshouse with a photoperiod of 18:6 h (L:D) and 60% RH. Adult whiteflies that emerged from nymphs were used in this study.
| Fig. 1: | Yellow vein mosaic disease development in Hibiscus cannabinus (cv. HC-583) plant; H: healthy leaf, 1-11: development of symptoms |
Acquisition access of the virus-free whiteflies was given for 12 h on yellow vein mosaic infected mesta plants, maintained under glasshouse conditions. Twenty-day-old healthy plants at 6-leaf stage, maintained in glasshouse, caged individually with mylar polyester tubes (2” X 6”) with two nylon screen side windows, were inoculated by confining 5 viruliferous insects in each cage for 12 h. Control plants were treated similarly with non-viruliferous insects. All plants were then kept in insect-proof temperature controlled glasshouse.
Disease transmission at field condition: Healthy seedlings of the same cultivar were grown 6 cm apart in small field plot (2H2.5 m) containing 100 plants in 3 replications maintained in randomized block design. The germinated plants were then allowed to grow under natural environment for disease development. After the appearance of first symptom the gradual development of the disease was noted at every 10 days interval in each plant and compared with those of grown and artificially infected with the virus under glasshouse condition for assessing the usefulness of the proposed scale.
Development of disease intensity index formula: Observations on plants grown under glasshouse condition were recorded at weekly intervals starting from 11 to 46 days of plant age after inoculation for successive four years and a disease intensity index formula was developed based on the symtomatology. Disease development was noted chronologically by considering different grades, viz. (A) development of foliar symptom, (B) nature of discoloration, (C) extent of yellowing of veins, (D) area of discoloration and (E) extent of stunting. Total gradation was based on a numerical value of 100 (Table 1). In the scale, each symptom syndrome was given equal importance and according to disease severity the symptom syndrome was divided into different gradations. Each grade was given a numerical value and noted with respect to each infected leaf (Table 1, 2). When the chlorotic spots started developing irregularly, the size of the individual area of discolorations was noted and their average was used to grade the area of discoloration (Table 1). In this way numerical values for each leaf of the individual plant were recorded separately and indexing was done according to the following formula:
Where:
| L1, L2, L3, Ln | = | No. of affected leaves on a plant varying from 1 to n |
| s | = | Score of a diseased leaf varying from A to D |
| Lis | = | Total score on diseased leaf of a plant in a particular stage/ period (i = 1/2/3/Y./n) |
| G | = | Growth habit of infected plant (i.e., extent of stunting = E) |
| TN | = | Total number of leaves in a single infected plant |
| Table 1: | Distribution of numerical values based on the importance of the factors in the proposed scale |
| Table 2a: | YVM disease index at different stages of pathogenesis in cultivar HC-583 under field condition |
| *Mean average of 100 plants±SE | |
| Table 2b: | YVM disease index at different stages of pathogenesis in cultivar HC-583 under glasshouse condition |
| *Mean average of 30 plants±SE | |
RESULTS
The present investigation revealed that under glasshouse condition the disease development on inoculated mesta (HC-583) plants was early as compared with those grown under natural field condition. But the disease symptoms and its chronological development under both the conditions appeared to be of similar nature. In a single plant not all the leaves had the symptoms due to infection. Symptom started in the emerging leaves in the form of scattered pin-head like light-yellow coloured irregular shaped spots, either one or both halves of the lamina including veins. The spots gradually increased in size by coalescing with each other leading to the yellow-chlorotic flecks on veins and yellow mosaic symptoms on inter-veinal regions. With the advancement of the disease, the infected lamina showed yellow chlorotic discoloration. Gradual yellowing of petiole and crown portion was also noticed in infected plants. In case of severe infection stunting of plant with defoliation of complete chlorotic leaves was observed. In addition to the above, the survival rate and life span of infected plants in glasshouse were also found less than those in field condition.
A successive four years epidemiological study on the YVM disease index at different stages of pathogenesis revealed that the infection severity varied from 1.82 to 49.44 in mesta (HC-583) under field condition, whereas under controlled glasshouse condition the same varied from 2.78 to 50.83 (Table 3a, b). Longer incubation period (nearly one month) for symptom expression was noticed under field condition, whereas under artificial inoculation in glasshouse the same observed to be around eleven days (Table 2a, b).
| Table 3a: | Distribution of different numerical values and calculations of YVM disease index values using equation at different stages of pathogenesis under field condition (cultivar HC-583)* |
| *Mean average of 100 plants | |
Gradual development in severity of the disease leading to the index value of 49.44 and 50.83 at field level and glasshouse condition, respectively, was noticed with the age of the plant. Mortality of the infected leaves was also noticed at the maximum infection level.
DISCUSSION
Present investigation highlighted the useful application of empirical formula developed for assessing disease intensity of yellow vein mosaic disease of mesta (HC-583). Ascribing of numerical values to some characters have been made in conducting the experiments and the sum total of those provided the index value on disease intensity. Variation in symptom expression in plants grown in glasshouse condition and under natural condition indicated that the lower index value obtained under field condition might be due to interaction of different epidemiological factors on plant growth and presence of lower population of the vector. Highest intensity or 100% infection was never noticed under natural and glasshouse condition and as a result YVM disease index value during this experiment never reached to 100%, as because all the infected plants died before it under both conditions. Hence, we stopped gradation at maximum value.
| Table 3b: | Distribution of different numerical values and calculations of YVM disease index values using equation at different stages of pathogenesis under glasshouse condition (cultivar HC-583)* |
| *Mean average of 30 plants | |
Here, severity of 49.44 and 50.83 index value was sufficient enough to consider the whole plant totally infected at field level and glasshouse condition, respectively. As by 100% infection, quantifications is not clearly emphasized, so the gradation of either 49.44 or 50.83 infection value was clearer for maximum infectivity, in a particular plant as evident by the validation of the formula in fields as well as glasshouse condition.
The present investigation showed that in a single plant not all the leaves had yellow mosaic discolorations and even if discolorations were present, they were not necessarily of the same type because cells vary in their susceptibility. This type of symptom development was also noticed in previous study on another diseases (Furumoto and Mickey, 1967a, b). Furthermore, susceptibility of one leaf differs from another and the infected areas also differ between leaves as was observed by early studies (Kado, 1972). Hence, gradual development of the disease and the cumulative effect of all factors are the most important criteria to designate a single plant about its severity of infection. Consideration of symptom expression as determined by visual observation may lead to incorrect assessment of severity (Joshi, 1973). This present experiment took account of all such possible factors associated with YVM disease incidence and gradual disease development in a particular plant and even in the individual leaf. So, it is not improbable to conclude that the empirical formula developed during the present investigation may be helpful for assessing severity of YVM disease on mesta.
CONCLUSION
The present study is an attempt towards the understanding of chronological disease development of mesta yellow vein mosaic caused by a Begomovirus. It is a unique approach to contribution as because such type of basic study is rarely attempted now-a-days, though the experts of plant pathology and agriculture are regularly shouting in this direction to attract the researchers mind. Without having a correct disease scoring technique, which is applicable in all situations, we can not be in position to declare the disease intensity and incidence on universal level. Moreover, it will resist to claim a disease an epidemics. From the corner of disease management and information to farmers the disease indexing also has immense importance.
ACKNOWLEDGMENT
Author is thankful to University Grants Commission, Govt of India to provide financial support to do this research under a project No.F. PSW-178/11-12(ERO), at Department of Botany, Barasat College, West Bengal, India.