A. Interesting facts about Bacterial Blight of pomegranate:
The disease “bacterial blight of pomegranate” and isolation of its causative agent Xanthomonas was first reported by Dr. Hingorani in 1951.
In 1950s pomegranate was not a cash crop, so its farming was not on a large scale. Since the plant also does not require much water, fertilizer, and care as compared to other crops, the success stories of the good olden days of pomegranate cultivation could be seen as cases of “negligent farming” by today’s standards. Key to successful control of the particular disease lies in some such farming practices which may be seen as “neglect” by today’s farmers, but actually are the requirements to keep the crop “Happy”. There are several examples where farmer neglected his crop totally for a year or so after suffering 100% loss because of bacterial blight; and the infection was controlled 100% by the natural processes. Hence we must identify parts of our agricultural practices which are actuallyunnatural brutality for the crop and stop them at once!
The major outbreak of the particular disease and huge losses were not reported almost till 2000.
This means that the outbreak of the disease (and therefore the pathogen) can be controlled well by the pomegranate plant itself with the help of its natural defense mechanism. The year 2000 era was of rapid spread and large outbreak of the disease, and itis also marked with introduction of new varieties, and 10X hike in the rates of pomegranate.The theory of varietal correlation with the incidence of disease is mostly discarded by now.
Quick increase in the plantation area, no scientific knowledge of correct agricultural practices, decisions made only from the view point of maximizing immediate returns, and absence of scientific methods for control of spread of the disease have been the main reasons for current scenario about the disease.
A study on How the “new” agricultural practices (adopted by the farmers after high returns from pomegranate were seen) negatively affected the “natural resistance” of this plant for the particular disease can be very interesting and will throw light on the correct agricultural practices to be adopted for control of bacterial blight.
Amongst the pomegranate growing countries, outbreak of this disease is only within India.
The India specific factors such as soil characteristics, agricultural practices, and specificgeo-climate need to be critically evaluated for identification of major predisposal condition(s)that lead to the heavy losses in pomegranate yield because of the bacterial blight. A warning model along with specific preventive care options can then be built to help the farmers practically well.
A study in Thailand about organic alternative to control Xanthomonas infection revealed that pomegranates leaf extract offered the most effective solution.
Pomegranate actually produces a battery of anti-Xanthomonas metabolites. The obvious physiological changes in the crop that correlate with the lows of such biochemicals are often marked with highest damage done by the pathogen. What are these biochemicals? How is their biosynthesis regulated? What are the correct agricultural practices for the maximum expression of these in the plant tissue during the “disease sensitive stage”? What is the threshold concentration of these in different tissues of pomegranate plant that can keep the damage under control? Answers to these questions will provide meaningful solution to the problem of bacterial blight in pomegranate.
B. Biology of the Pathogen:
The disease bacterial blight is caused by Xanthomonas axonopodis puniciae(XAP). There is lots of interesting information available about Xanthomonas as a plant pathogen. The salient features are as follows:
The pathogen has a protective cover of a polysaccharide called as xanthan gum, which protects the organism from adverse climate, and it can be present as a latent infection – without causing any symptoms – for a very long period.
The xanthan gum, which is an anionic polymer, blocks several chemicals and antibiotics from reaching up to the cell. The structure of xanthan gum changes from species to species. However, exact structure of the xanthan gum of XAP has not been reported.
The pathogen in low numbers can be controlled by the plant defense mechanism. But at this stage it injects certain biomolecules inside the plant cells by T3SS system. This eliminates the capacity of the plant to defend the pathogen.
Pathogen gets internalized and may travel through vascular system or otherwise to locations distant from the site of infection.
C. Biology of the Plant:
As compared with the experience of farmers about other plant species, pomegranate is strikingly different in following aspects:
There is no correlation in pomegranate between the canopy size and fruit yield.
The crop can withstand huge water deficit without affecting the yield, but it looks sensitive to extra water.
Typically farmers apply nitrogen at the time of initiation of new leaf development, but in case of pomegranate the nitrogen needs to be restricted.
Ability to control / restrict new shoot development throughout the fruit bearing season is key very important.
D. Worldwide Trend of Disease Prediction Models:
Plant disease forecasting is a management system used to predict the occurrence or change in severity of plant diseases. At the field scale, these systems are useful to growers to make economic decisions about disease treatments for control. Often the systems ask the grower a series of questions about the susceptibility of the host crop, and incorporate current and forecast weather conditions to make a recommendation. Typically a recommendation is made about whether disease treatment is necessary or not. Usually treatment is a chemical application that kills the pathogen.
Forecasting systems are based on data about the pathogen’s interactions with the host and environment, the disease triangle. The objective is to accurately predict when the three factors – host, environment, and pathogen – all interact in such a fashion that disease can occur and cause economic losses.In most cases the host can be suitably defined as resistant or susceptible, and the presence of the pathogen may often be reasonably ascertained based on previous cropping history or perhaps survey data.
We aims at accurate analysis of the environmental conditions of the specific field, determination of the presence of the pathogen, and its load at a given point of time and simultaneous determination of disease resistance level of the plants. This analysis will provide the exact determinants for recommendation of preventive treatment, taking into account the predicted weather conditions.
Forecasting systems can only be designed if there is an understanding on the actual disease triangle parameters.
Changes in the relative shift in position of these parameters on X and Y axes should make alterations in the mechanism of infection and therefore preventive treatment should be changed suitably to match the requirements. The following diagrammatic illustration exemplifies the different parameters that require to be attacked under different intersections of the pair out of the triangle.
E. Rational Approach of Disease Prevention:
We propose a three pronged approach for prevention of any disease of crops called as BioAVert I. The program principles are as follows:
The Specific work being done in pomegranate at present is as follows:
Creating Entry Barrier for the Pathogen: Xanthomonas in general is known to inhibit one plant defense mechanism i.e. callose synthesis. By inhibiting the callose synthesis, Xanthomonas gets free entry into any cell and thereby spreads very rapidly. Dr. K.K. Mondal (Principal Scientist- Plant Pathology- IARI Pusa) and his coworkers have shown that callose synthesis is inhibited specifically by Xanthomonas (XAP) which causes Telya disease in pomegranate.