4 Climate change and plant diseases

Transcript

Hello! I welcome you back to this course. In the last class we have seen that what are the different components of plant disease development and in that we have seen that climate or environment is one of the reason of plant disease development. So in our today’s topic we will talk about the change in climatic patterns and how it helps in development of plant diseases. We know that climate has changed from the previous time and the change in climate is recorded from the pre industrial age and we can consider three important components of climate such as atmospheric carbon dioxide, temperature and relative humidity or rainfall pattern are the three major components of climate that are considered to be changed over a period of time and that has lead to several changes on the earth. We can say that atmospheric carbon dioxide concentration has increased significantly by approximately 30%, the temperature has increased say from 0.3 to 0.6o C and there is a major change in precipitation pattern. All these things are leading to changes in the agricultural practices and crop development.

If we just see into the Global food security scenario overall in the world we have come across the situation where there is threat of global food security in mostly the African countries were including India also is not spared from it. In comparison to other parts of the world these are the major spots or major area of the world where food security is under threat. And under this food security threat situation if climate is favoring development of certain diseases we can imagine that it can lead to even disastrous effect in a longer period of time. So there were flare of researches that has seen or they have tried to estimate the importance of these climatic factors on plant, growth and development. When the temperature was increased from 2 to 4o C under greenhouse condition and the precipitation pattern was varied by + or – 20% and increase in carbon dioxide concentration to be doubled and the effect of this variation in the climate was observed on the biomass production of wheat, maize, rice and soyabean and it was observed that average crop yield at +2o C increase in temperature mostly increase the productivity 10 to 15% in wheat and soyabean and by 8% in rice and maize. So what it means is that if the temperature goes increase upto 2o C there won’t be any impact on yield of this particular crop like soya bean, rice and maize. Then if the temperature is increase up to 4o C then the effect becomes reverse. That means that 4o C along with elevated carbon dioxide which normally helps in enhancement in biomass production in this crop at 4o C the effect becomes reverse. That means the yield starts reducing from this particular temperature onwards. So 4o C can be taken as a threshold level of temperature on the atmosphere. If the current atmospheric temperature increases by 4o C we can expect there is reduction in yield. Similarly, in sorghum, yield increases due to carbon dioxide concentration and it is again if we just look just into the temperature the increase in yield due to enhance carbon dioxide concentration is again masked by the elevated temperature. So, temperature beyond a threshold level can negate the effect of increase in of carbon dioxide concentration that is upto the double than the current concentration. For instance, a doubling of carbon dioxide has been conclusively shown to increase C3 crop yield by approximately 33% and C4 crop yield by about 10% in approximately 100 of studies. Now all these studies has been done taking only these three environmental conditions into criteria. But what was minus in those resources were the effect of plant, pest and pathogens. So, what our increase in yield that we were experiencing that we are observing in or taking these three climatic conditions alone may not be the same if the scenario is different in presence of pest and diseases.

So we know that Disease triangle in our last class we talked about Disease triangle where we talked about there should be a Susceptible host, there should be a Virulent pathogen and there should be a Conducive environment for successful establishment of a pathogen and causing disease. But what we also know that fungal pathogens normally benefits from more precipitation. So in presence of Susceptible host, Virulent pathogen and Conducive environment if the precipitation or rainfall pattern is changed then there may be a favorable situation for fungal pathogens to cause more disease in case of more wet period. Therefore, most likely impact of climate change will be felt in three areas. First one is losses from plant diseases, in the efficacy of disease management strategies and in the geographical distribution of plant diseases.

So, climate change in our previous experience what we have seen is that it is stimulating the crop growth leading to more yield but when the food component of climate that is the pest or pathogen when it was introduced then we have seen a reversed scenario. So, climate change effect will be more realizable when we take this food component that is pest or pathogen into consideration. So, we can take an example of Barley powdery mildew. Normally, what we see is that when we altered the host physiology and morphology under elevated carbon dioxide concentration then there would change in the interception of light and precipitation, and modify canopy structure and microclimate to influence disease epidemiology. So, under higher carbon dioxide concentration crop canopy will be more dense because the plant biomass will increase more and because of this dense biomass condition there will be a change in the plant micro canopy and the change in the plant micro canopy may lead to either that may favor disease development in certain cases or in certain cases it may lead to even non establishment of certain diseases. So some disease may cause more severe reduction in plant growth under twice ambient compared to ambient CO2 concentration. So it is because of the indirect effect of carbon dioxide on plant biomas the condition may indirectly benefit the pathogen o cause disease. We can take the example of Barley powdery mildew where we can normally understand that photosynthesis at increase carbon dioxide concentration in barley and if we consider infection-induced reduction in net photosynthesis then we can say that there is larger reduction in plant growth than at elevated carbon dioxide concentration. So what we can conclude here is that if we just consider one parameter that powdery mildew reduces the photosynthesis ability of barley plants and its photosynthesis ability under a elevated carbon dioxide concentration if it is increase to some extent then it’s effect of benefit can be reduced or can be lost if the leaf is covered by more powdery mildews. Here you can see if the leaf is covered by more powdery mildew fungal growth then the photosynthesis rate will be reduced even after effect of the carbon dioxide concentration. So environment may favor one in one way by increasing the net photosynthesis ability of barley but at the same time if the disease development is enhanced at under elevated carbon dioxide concentration then the elevated carbon dioxide induce net photosynthesis ability will be lost because of minimal area that will be left out for photosynthesis in barley due to coverage of the powdery mildew pathogen.

Now if you just consider the late blight of potato pathogen, it stimulate yield loss in potato based on a 3 year long controlled environment study of late blight at ambient temperature and 3o C higher than ambient. So, at 3o C higher temperature the late blight infection is enhanced incase of potato and this enhancement will lead to a low increase in dry matter of potato by 2 ton/hectare per degree of warming but this enhancement of disease will have a negative impact on this biomass production or yield enhancement because of loss of more healthy plants. So to protect under elevated temperature although the biomass or yield is increased at the same time what is increased is the ability of the late blight pathogen to cause disease. So, the extended benefit of higher temperature is minimized by extended damage of the crop by the pathogen. So we may require more or you can say the synthetic chemical spray by 10 to 20 days more beyond the extend or desired time period so that we can get the desired benefit out of the temperature increment. So there was another study where people have worked on both Biotrophs and Necrotrophs and they have studied 10 Biotrophs and around 15 Necrotrophs where the effect of elevated carbon dioxide was measured and it was seen that pahtogen were more active or dead they cause more out of 10 Biotrophs 6 pathogens were able to cause more disease under elevated carbon dioxide concentration. 9 Necrotrophs they were able to cause more disease or increase disease in higher carbon dioxide concentration. 4 of them the disease causing ability were reduced in case of Biotrophs and 4 Necrotrophs disease causing ability were reduced and 2 pathgens of Necrotrophs the disease causing ability were not change under elevated concentration. So if we just summarize it is more number of Biotrophs and more number of Necrotrophs their disease causing ability was enhanced under elevated carbon dioxide concentration. So the benefit of carbon dioxide concentration enhancement to agronomical parameters of crop plants were negated by the ability of the pathogens to cause disease.

So, impact on host – pathogen interactions when it was seen on elevated carbon dioxide concentration it was observed that the initial establishment of a pathogen maybe delayed because of modifications in pathogen aggressiveness and or host susceptibility but its fecundity is increased. So even the initial onset of the disease may be delayed but if its reproductive ability is more or its reproductive ability is high then they have certainly it will go and cause more disease or it will lead to a epidemic situation. Let us take the example of Soybean rust. Here we can see that the combination of increased fecundity and more humid microclimate within dense crop canopies associated with increased carbon dioxide concentrations might provide more opportunities for severe infection. So although the disease development or establishment initially may be slow or less but if the disease are reproductive ability of the pathogen is high because of the increase carbon dioxide concentration and that favors the micro climate for the soyabean canopy that means this cumulative effect is going to have a negative impact on the plant because of enhanced or more disease.

Similarly, if we just talk about rice blast pathogen, there in Asian countries they caused disease due to this particular pathogen Magnaporthe oryzae may not have a greater impact because asian conditions are more suitable for the pathogen to cause disease. But if we take or if you talk about cool subtropical zones, where if the temperature is elevated or high then there is increase in disease severity because there will be high risk of epidemics occurs under higher temperatures. High temperature in cool subtropical zones will favor or it will lead to a temperature where the pathogen will be able to cause more disease. Originally, since the temperature was low the pathogen was unable to cause disease. But once the temperature was increased in those regions then what will happen the pathogen will get a favorable condition to cause disease. But in humid tropical and humid warm subtropical zones, say for example the Southern China, Philippines and Thailand under natural condition the temperature is high so if in these conditions if the temperature becomes lower by 1 degree or more then this lowering of temperature will favor the pathogen to develop and cause blast disease in this particular regions. So in directly we can say that change in temperature, precipitation pattern and associated climatic condition factors like carbon dioxide concentration, this all have a greater impact on disease development and this aspect is even important in comparison to the agronomical parameters that are derived benefits by change in this three climatic patterns. So we can say that climatic change in this in this major atmospheric parameters will have less impact on agronomical parameters in comparison to the disease development processes. So we can now again conclude talk, with that the climatic change is going to have a severe impact on crop produce because of non agronomical parameters rather due to increase development of plant pathogens. And with this we conclude today’s talk and in the next class we will talk about emerging plant pathogens that is causing new diseases in new areas.

Thank You.