- Background of 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 the 1950s, the pomegranate was not a cash crop. So, its farming was not done on a large scale. Since the plant also does not require much water, fertilizer, and care as compared to the 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 actually unnatural brutality for the crop and stop them at once!
- The major outbreak of the particular disease and huge losses were not reported almost CH 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 the large outbreak of the disease, and it is also marked with the introduction of new varieties, and 10X hike in the rates of pomegranate. The theory of varietal correlation with the incidence of the disease is mostly discarded by now.
A quick increase in the plantation area, no scientific knowledge of correct agricultural practices, decisions made only from the viewpoint of maximizing immediate returns, and absence of scientific methods for control of the spread of the disease have been the main reasons for the 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 throw light on the correct agricultural practices to be adopted for the control of bacterial light.
- Amongst the pomegranate growing countries, the outbreak of this disease is only within India.
The Indian factors such as soil characteristics, agricultural practices, and specific geo-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 the specific preventive care options can be built to help the farmers practically well.
- A study in Thailand about an 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
biochemical are often marked with the 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 a meaningful solution to the problem of bacterial blight in pomegranate.
- As of today, no genes of disease resistance have been identified in Pomegranate, and significant varietal differences in disease susceptibility have not been proven.
Solving the problem of bacterial blight disease through the selection of resistance gene does not look a viable option. This is because the bacterium has over 300 genes responsible of causing the disease and knocking effect of most – if not all of these effector genes – by single or a few plant genes is a tall task. The bacterial blight of pomegranate is very similar to citrus canker, and canker of Florida Oranges has also not been resolved in spite of available genetic approaches.
- Background of pomegranate cultivation in India:
Pomegranate research in India dates back to 1932. At the Ganeshkind Fruit Experiment station, Dr. G. S. Cheema, Father of Indian Horticulture, started working with local wild type pomegranate variety known as Alandi having deep pink colored, highly acidic and hard seeded arils.
In the year 1936, a selection from the seedling population of Alandi, bearing sweet aril type fruit with soft seeds was identified and released for commercial cultivation in Maharashtra in the name of cv.GBG-1. In the year 1970, the cultivator GBG-1 was renamed as Ganesh.
During the last three decades, innovative farmers took real initiative and pomegranate crop has been acclimatized from typically temperature zones to semiarid and arid areas of India. The most promising variety for exports from India is “Bhagwa”. This variety is believed to be developed from an accidental selection they happened in a farmer’s field. The variety is being cultivated since 1996. MPKV released further selections from the variety as Phule Bhagwa and Super Bhagwa.
In the absence of much of the theoretical base and practical guidance, the farmers used their experience from other crops and many times the experience of grapes to develop farming practices for pomegranate. Lower capital need for cultivation (as compared to grapes) and higher returns and/or low labor requirement were lucrative reasons for farmers to shift to the pomegranate in masses. Additionally, low water requirement and the possibility of using otherwise useless land for pomegranate brought masses of even BoP farmers into pomegranate cultivation. These masses of farmers lacked knowledge and resources, and often imitated their idols recklessly, partly and at times only at some convenient steps to expect the best possible outcome. Another boon for this country is that the flowering of a pomegranate can be initiated irrespective of the season. Therefore we can, in fact, become the world leader in pomegranate market. However, since the farming was being done without proper knowledge, this boon got converted into a curse. Farmers tried multiple flowering cycles within one single year, and many of them had to uproot their thousands of plants without getting any returns at all.
Along with the control measures for bacterial blight, education of the farming community, handholding for 2-3 years to demonstrate that scientific measure alone could help them get good returns is absolutely essential to save this industry.
C. Biology of the Pathogen:
The disease of bacterial blight is caused by Xanthomonas axonopodis punitive (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 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 the T3SS system. This eliminates the capacity of the plant to defend the pathogen.
- Pathogen gets internalized and may travel through the vascular system or otherwise to locations distant from the site of infection.
- Biology of the Plant:
As compared with the experience of farmers about other plant species, pomegranate is strikingly different in the 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 the case of pomegranate, the nitrogen needs to be restricted.
- Ability to control/restrict new shoot development throughout the fruit-bearing season is very important.
- Existing Methods of Management / Treatment of Bacterial Blight:
Existing methods of management work around the cleanliness of orchard, burning the infected plant parts, and disinfection of the agricultural implements.
The treatments recommend the use of chemicals: Bronopol and Captan, and antibiotics Streptocycline. Typically, 12 gm of streptomycin per 200 L spray water controls Xanthomonas infections in other crops in a single spray. However, in pomegranate, about a dozen sprays of 100 gm. /200 L is recommended. In-spite of use of the chemicals as per advice (> 6 kg/acre per season), losses may not be under control.
- Worldwide Trend of Disease Prediction Models:
Plant disease forecasting is a management system used to predict the occurrence or change in the severity of the plant diseases. At the field scale, these systems are useful for 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 the environment, the disease triangle. The objective is to accurately predict when the three factors – host, environment, and the pathogen – all interact in such a fashion that the disease can occur and cause economic losses. In most of the 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.
Swasti aims to accurately analyze 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 a recommendation of preventive treatment, taking into account the predicted weather conditions.
Forecasting systems can only be designed if there is an understanding of the actual disease triangle parameters.
Triangular parameters: Susceptible Host, Conducive environment and the pathogen.
Changes in the relative shift of these parameters and 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. Know more about our approach to disease prevention in the upcoming blog.