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5 Emerging plant pathogens


Hello everybody I again welcome you back to the fifth and final topic of this week and that is Emerging plant pathogens. In the last class we talked about change in climatic scenario and these factors and we have seen that how these climatic change patterns has influenced crop disease development due to plant pathogens. So today we will talk another aspect of the same topic that is emerging plant pathogens under changing climatic condition. So change in geographical distribution that is the major concern for plant pathogens because it is migrating to new areas because of the climatic factors. Climate is favoring pathogens to migrate to new distribution or you can say the geographical distribution has been widened up for many pathogens because of the changing climatic scenario. If we talk about Phytophthora infestans we have seen that sudden oak disease in certain parts of the world including United States has become a problem in forest oak’s. Due to increase in polar temperature we have seen that the infestation of Phytophthora related sudden oak disease has become more in these oak trees. it is because of the warming that is causing a pole-ward shift of the plants and because of these the plants along with the plants the pathogen is also migrating towards the pole because the temperature in the pole is getting warmer and along with the shift in the vegetative vegetation the pathogen is also migrating along with the host. It is Phytophthora cinnamomi which is currently seen as a major threat in many parts of the world including the United States where it is causing a serious threat to the forest oaks plants. If you just see the scenario of United States and that is the coastal California, in 2000 the occurrence of sudden oak disease was limited to only certain parts of the western California by 2010 the area has increased to a significant amount and it is expected by 2020 the area will be doubled because of this particular pathogen and it is by 2030 it’s going to be very very severe threat for the oak plantation in this part of the world. So we have seen that how a pathogen is progressing and it is causing a severe and extensive damage to a particular vegetation along with the change in the climatic pattern. So increase in wet periods with soil temperature that is ranging between twelve to thirty would favour Phytophthora diseases in southwest Australia but the currently it is not present there but once the temperature the soil will increase up to 30 degree Celsius in Southern West Australia we can evident the infestation by Phytophthora diseases in this part of the world. Similarly fast growing popular cones that grow in much northern Europe are generally susceptible to thermophilic Melampsora species but currently this person does not constitute a major threat to polar production. But if the climate is changed in the current ratio which will cause warmer climate in those areas and because of this warmer climate in those areas the threat to the populous plant because of Melampsora will be very very high and we can expect a serious damage of popular plants by Melampsora species. So migration of pathogens to other hosts is another concern. More aggressive strains of pathogens with broad host range such as Rhizoctonia, Sclerotinia, Sclerotium and many other necrotrophic pathogens they may migrate from agricultural crops to natural plant communities. So this is another aspect the host species may wide enough for certain polyphasic pathogens like Rhizoctonia, Sclerotinia, sclerotium and so on. We can take an example of the fire blight pathogen that is a bacterium, Erwinia amylovora, and it is basically a native of United States of America. But it was present in the Rosaceae family without causing serious threat to the agricultural crops but once the apple plantation was introduced from Europe then the pathogen that was originally present in Rosaceae family has migrated to this apple plants and it started causing severe damage to the apple plantation and thereby causing an epidemic like serious situation causing serious loss to the apple plantation. Similarly, pathogens which are normally less aggressive in natural plant communities could devastate crop monocultures growing in close proximity. Another example for this we can take that in Sri Lanka or you can say in the South Asian countries the Hemileia vastatrix, aesthetics it’s the rust pathogen of many plant species where it was present in countries like Sri Lanka in wild and other alternative host but once the coffee plantation was introduced in this areas particularly in Sri Lanka this pathogen migrated from this alternative host to the coffee plantation which was done in an extensive way in Sri Lanka it’s devastated this crop after a few years and coffee plantation has to be abolished in Sri Lanka thereafter.

Similarly, in the coast of Oregon, USA winter temperature and spring precipitation is increasing by 0.4% degree Celsius and seven to one point five centimeter respectively per decade since 1970. Swiss needle cast is a disease caused by a fungal pathogen which causes chlorosis of the needle when retention of the needle is also reduced and this also caused reduction in growth of the Douglas fir plants. Severe epidemic of this disease began on the Oregon coast in early 1990s. This was positively correlated with warmer day accumulation during winter and duration of leaf wetness during spring and autumn. So these are the major factors that is the warmer day along with duration of leaf wetness during spring and autumn. This condition has led the Swiss needle cast to cause extensive damage in Douglas fir and the most of the plantation in Oregon state of USA is being affected by this particular climatic condition. So it is kind of a new disease that has emerged in this area after increase in temperature and precipitation pattern. You can see here that these are the Douglas fir plants infected with the needle cast disease. So introduction of novel plant pathogens have already occurred in major regions but climatic changes are likely to often facilitate their further establishment and spread. So migration of pathogen is not a new issue but the changing environmental factors or climatic factors is favoring them to establish in newer region and that is what is causing serious threats to the crop growers. The fire blight pathogen was introduced into Switzerland from South Germany in somewhere in 1980s. It affects tree and shrub species of the family Rosaceae and is favored by humid and mild springs as was the case in 2007 when the epidemic reached unprecedented levels. So the pathogen was not present in Switzerland and it was basically introduced from West Germany during the 1980s. So you can see that how the pathogen has affected Switzerland seriously. In 1995 it was present only in a very negligible amount which increased by 1999 to a great extent and by 2003 it was or covering almost half of the country and by 2007 it has developed an epidemic form. So entering of this pathogen to Switzerland from Germany coupled with favorable climatic factors has led this particular disease to develop an epidemic condition in Switzerland. Coming to the examples of India where we can evident that in 2008 it was reported that Phytophthora late blight in tomato was causing severe damage to the tomato crop. Either although it was present in a mild form but suddenly the extent of damage due to Phytophthora blight in tomato has increased after 2008. Onwards systematic studies has led to the understanding that the original Phytophthora infestans strains that is US-1 was replaced by another strain 13A_2 and this new strain is causing the severe late blight in tomato and it was more virulent in comparison to the original US-1 strain in tomato. So a similar outbreak of tomato late blight was also reported from the northern part of India in 2016.

Another example of such cases is the dried root rot in chickpea it is caused by a reject Rhizoctonia bataticola and all of a sudden after 2010 and onwards the extent of damage by this pathogen was seen to be very very severe in central and southern part of India. Otherwise it was not a serious threat to chickpea growers by the time. So it was assumed that systemic and systematic studies has led to the understanding of the fact that the moisture contained during the maturity period in chickpea has gone down in this part of the India particularly central and southern India and this particular condition warm and dry condition helping the pathogen to infect more on chickpea plants causing extensive damage thereby. It was reported that the damage may be up to 5 to 50% in some parts of the growing areas.

So we can imagine that how serious a threat it might be in coming years to come. So what we could see today is that climate is changing and because of the change in the climatic pattern we are facing some new challenges in terms of expansion of geographic distribution of certain bad plant pathogens. We have taken some example across the globe in case of Phytophthora cinnamomi from United States, in case of fire blight from Switzerland and in India we could see the emergence of new virulent strains as well as new diseases in particularly southern part of India. So we can now conclude that systematic studies on this aspect is also required how climate change is affecting pathogen development and this geographical expansion. So this is a very very important part of our study and it needs more focus in future years to come. So in this week we talked about background and significance of plant diseases, then we talked about introduction of plant pathogens, what are plant pathogens what are the diseases they caused, what are the symptoms to produce, then we also talked about the component of plant disease development we have seen that climatic factors including the disease triangle how disease is developed then we also talked about climate change and how it is impacting the crop production and distribution of agricultural plant pathogens. Similarly, at the end we talked about new diseases or new pathogens that are emerging in new areas and what are the major threats they are causing to agricultural crops globally. So all together in these last five talks we talked about the principles of plant disease development.

Thank you



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