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Vol. 14 No. 4 - October 2008

Effects of Climate Change on Plant Pathogens

By: Usha Mina1 and Parimal Sinha2


Recent years have witnessed a steady increase in National and International concern over the sustainability of the global environment. Climate change has emerged as the most prominent of the global environment issues. Global climate has changed ever since industrial revolution and by now it is ascertained that major greenhouse gases especially CO2 increased by 30%. Tropospheric ozone has increased two- to five- folds since the last century. According to IPCC’s latest report, global mean temperature would rise between 0.9 and 3.5°C by the year 2100. Speed of climate change and the unpredictability of its characteristics are of great concern with respect to the pathogens, insect pests and weeds that reduce crop yield. The classic disease triangle recognizes the role of climate in plant diseases as no virulent pathogen can induce disease on a highly susceptible host if climatic conditions are not favorable. Climate influences all stages of host and pathogen life cycles as well as development of disease. Disease severity over a period can fluctuate according to climatic variation.

General impacts of climate change on plants

Climate change will influence the geographical distribution and growth of plant species around the world.  The magnitude of these impacts would vary depending upon the species involved and their growth patterns, e.g. annual vs perennials, agricultural crop vs natural vegetation, competition, migration, and recovery from disturbances. Therefore, new combinations of species are likely to evolve. Evidences are now accumulated which suggest that crop production would be affected differently depending on latitude. Although increases in yields are expected at mid and high latitudes, there may be decreases at lower latitudes where food requirements in future will be highest. There are apprehensions that yield gains caused by increased CO2 could be offset partly or entirely by losses caused by phytophagous insects, plant pathogens and weeds. It is, therefore, important to consider the effect of biotic constraints on crop yields under climate change scenario.

Impact on plant - pathogen systems

Most of the researches on how climate change may affect plant diseases has concentrated on the effects of a single atmospheric constituent or meteorological variables on the host, pathogen, or the interaction of the two under controlled condition. However, interactions are more complex in the real situation, where multiple climatological and biological factors are varying simultaneously in a dynamic environment. Climate change has the potential to modify host physiology and resistance and to alter the stages and rates of development of the pathogen. The most likely impacts would be shift in the geographical distribution of the host and pathogen, change in the physiology of host-pathogen interactions and change in crop losses. New disease complexes may arise and some diseases may cease to be economically important if warming causes a poleward shift of agroclimatic zones and host plant migrate into new regions. Pathogen would be following the migrating hosts and may infect vegetation of natural plant communities not previously exposed to the often more aggressive strains from agricultural crops. Facultative parasites with broad host range may infect plants in their proximity.  The mechanism of pathogen dispersal, suitability of the environment for dispersal, survival between seasons, and any change in host-physiology and ecology in the new environment will largely determine how quickly pathogens become established in a region. Change may occur in the type, amount, and relative importance of pathogens and affect the spectrum of disease affecting particular crop. Plants growing in marginal climate could experience chronic stress that would predispose them to insect and disease out breaks. Warming and other changes could also make plants more vulnerable to damage from pathogens that are currently not important because of unfavourable climate. For example Infection of Eucalyptus by Phytophthora cinnamomi is favored in wet soil at temperature of 12 to 30°C, hence the pathogen does not pose a serious threat to the susceptible Eucalyptus spp grown in Southeastern Australia. This situation may change with an increase in temperature due to climate change. As under climate change plants may potentially be unable to migrate or adapt as readily as environmental conditions change. But most pathogens have advantage over plants because of their shorter generation time and in many cases the ability to move readily through wind dispersal. Because of these characteristics, rate of evolution will be highest among pathogens to reduce sensitivity to climate change phenomenon.

Effects of various components of climate change on fungal pathogens

Elevated CO2: Both enhancement and reduction in disease severity under elevated CO2 has been reported. Elevated CO2 would increase canopy size and density of plants, resulting in a greater biomass production and microclimates may become more conducive for rusts, mildews, leaf spots and blights development. Decomposition of plant litter is important for nutrient cycling and in the saprophytic survival of many pathogens. Because of high C: N ratio of litter as a consequence of plant growth under elevated CO2, decomposition will be slower. Increased plant biomass, slower decomposition of litter, and higher winter temperature could increase pathogen survival on over-wintering crop residues and increase the amount of initial inoculation available for subsequent infection.

Some fungal pathosystems under elevated CO2 revealed two important trends. First, delay in the initial establishment of the pathogen because of modifications in pathogen aggressiveness and/or host susceptibility. For example, reduction in the rate of primary penetration of Eysiphe graminis on barley and a lengthening of latent period in Maravalia cryptostegiae (rubervine rust) has been observed under elevated CO2. Here, host resistance may have increased because of change in host morphology, physiology, nutrients and water balance. A decrease in stomatal density increases resistance to pathogens that penetrate through stomata. Under elevated CO2 barley plants were able to mobilize assimilates into defense structures including the formation of papillae and accumulation of silicon at sites of appressorial penetration of Erysiphe graminis.

At elevated CO2, increased partitioning of assimilates to roots occurs consistently in crops such as carrot, sugar beet, and radish. If more carbon is stored in roots, losses from soil-borne diseases of root crops may be reduced under climate change. In contrast, for foliage diseases favored by high temperature and humidity, increases in temperature and precipitation under climate change may result in increased crop loss. The effects of enlarged plant canopies from elevated CO2 could further increase crop losses from foliar pathogens.

The second important effect is an increase in the fecundity of pathogens under elevated CO2. Following penetration, established colonies of Erysiphe graminis grew faster and sporulation per unit area of infected tissue was increased several-fold under elevated CO2. It has been also observed that under elevated CO2 out of the 10 biotrophic pathogens studied, disease severity was enhanced in six and reduced in four and out of 15 necrotrophic pathogens, disease severity increased in nine, reduced in four and remained unchanged in the other two.

Elevated temperature: Increases in temperature can modify host physiology and resistance. Both temperature and the length of exposure are important in determining the effect of climate change on disease severity. Even if the temperature change may be well within the limits of current climatic variability, a modest warming can cause a significant increase in cumulative-temperature above a critical temperature threshold to affect crop physiology and resistance to a disease. Temperature change might lead to appearance of different races of the pathogens hitherto not active but might cause sudden epidemic. Change in temperature will directly influence infection, reproduction, dispersal, and survival between seasons and other critical stages in the life cycle of a pathogen.

At higher temperature, lignification of cell walls increased in forage species and enhanced resistance to fungal pathogens. Impact would, therefore, depend on the nature of the host­- pathogen interactions and mechanism of resistance. A rise in temperature above 20°C can inactivate temperature sensitive resistance to stem rust in oat cultivars. Increase in temperature with sufficient soil moisture may increase evapo-transpiration resulting in humid microclimate in crop canopy and may lead to incidence of diseases favoured under warm and humid conditions. Some of the soil-borne diseases may increase at the rise of soil temperature. If climate change causes a gradual shift of cropping regions, pathogens will follow their host. Analysis of long-term data of wheat and rice diseases in China has shown trends of an increase in minimum temperatures in association with the abundance of rice blast or wheat scab. In most locations, temperature changes had significant effects on disease development. However, these effects varied between different agro-ecological zones. In cool sub-tropical zones such as Japan and northern China, elevation of ambient temperature resulted in greater risk of blast epidemics. Situations in the humid tropics and warm humid subtropics were opposite to those in cool areas. A lower temperature resulted in greater risk of blast epidemics.

Elevated levels of atmospheric pollutants (ozone and nitrous oxide): Most air pollutants indirectly influence diseases through their effect on host. Ozone induces reactions similar to those normally elicited by viral and other pathogens. Of the 49 bacteria and fungal pathogens examined, exposure to elevated ozone concentration enhanced disease in 25, did not affect 10 and reduced 14. Pollutant concentrations, which inhibit pathogen development, also injure the host. Similarly, infection by plant pathogens can alter ozone sensitivity of plants. Exposure to 5-10 ppm ozone for a few hours can cause visible injury to sensitive crops like barley, tomato, onion, potato, soybean, tobacco, and wheat.

Plants appear to be less sensitive to nitrous oxide, however, higher concentrations can cause water-soaked lesions, which soon turn brown. Ozone and nitrous oxide injury on plants in turn may add new problem to pathologists in diagnosis. Current climate change scenarios predict a further increase of tropospheric ozone, which is well known to inhibit plant photosynthesis and growth process. Ozone can also predispose plants to enhanced biotic attack, as proposed in particular for necrotrophic fungi, root rot fungi and black beetles. However, at present it does not seem possible to predict whether increased ambient ozone will lead to higher or lower disease likelihood in particular plant-pathogen system. Several root pathogens show a preference for stressed trees, although the direct role of ozone is not always evident. Onions injured by ozone exposure were more susceptible to Botrytis cinerea, but not to B. squamosa. Increased onion yields and reduced dieback when filters removed ambient ozone has been also observed in some experimental studies.

Acid rain: Most studies on the effect of acid rain were done with simulated acid rain since it is not easy to establish experiments under field conditions. In first year of experiment no effect of acid rain has been observed on any of four pathosystems: alfalfa leaf spot, peanut leaf spot (PLS), potato late blight (PLB), and soybean brown spot. In the second year, PLS severity decreased with increasing acidity and the dose response was linear; PLB severity showed a curvilinear response to acid rain.

Elevated ultraviolet B: There is considerable information on the effects of increased UV-B on crops and natural vegetation and on the growth and life cycle of pathogenic organisms such as fungi. Studies indicate that the UV-B component of solar radiation plays a natural regulation on plant diseases. Stimulatory effect of near-UV light on reproduction of many fungi, and spore production in Leptosphaerulina trifoli peaks at 287 nm are reported. Fungi differ in their sensitivity to UV-B. Some strains of Septoria tritici are more sensitive to UV-B than others and S. nodorum, as a species, is more sensitive than S. tritici. UV-B radiation can modify the relative composition of phylloplane organisms, such as pink and white yeast. Continued exposure to enhanced UV-B radiation lowers the level of antifungal compounds in foliar parts. UV-B has been shown to reduce tolerance of rice to blast (Pyricularia grisea) and although higher UV-B reduced plant biomass and leaf area; there was no increase in blast severity. There are some evidences that sunlight can influence pathogen by causing accumulation of phytoalexins or protective pigments in plant tissues. Therefore, UV-B may affect plant diseases directly via the pathogen or indirectly via the host.

Effects of climate change on viral pathogens

Most of the climate change studies have been conducted on diseases caused by fungal pathogens. Viral diseases mostly have been ignored. Only few studies have reported the response of plants infected with viral diseases to various climate change components. It has been observed that oats infected with Barley yellow dwarf virus (BYDV) showed greater biomass accumulation to CO2 enrichment than the healthy plant. Tobacco plants grown at increased CO2 concentrations showed a markedly decreased spread of virus. It appears that CO2 rise in the air may have some positive effects, which may likely offset the negative effects of virus infection. However, generalization is difficult without much information from different virus-host infections.

Average temperature increase of 0.7°C over the last hundred years and average of 6.3 °C predicted by the year 2100 will dramatically affect both mobile (fauna) and immobile (flora) organisms, resulting in both altered and novel forms of interactions between plants, pathogens and their vectors. Most plant viruses are transmitted by vectors and majority by insects. Particularly aphids are expected to react strongly to environmental changes because of their short generation time, low developmental threshold temperatures and ability to survive mild winters without winter storms. An increase in the number of insect vectors will inevitably lead to a higher risk for viral infection of plants. The aphid transmissible complex of barley yellow dwarf viruses in cereals and potato virus Y in potato are amenable to show potential effects on the prevalence of infection because of climate change. In mild winters, high intensity of aphid movement during spring and a high frequency of PVY-infected potatoes have been reported. The severity of viral diseases is determined in large part by the amount of inoculums and the time of infection. The amount of virus inoculum is influenced by winter survival of its hosts. For some viruses, higher temperatures also cause more severe symptoms development. Aphids are expected to have increased survival with milder winter temperatures, and higher spring and summer temperatures will increase their development and reproductive rates and lead to more severe disease. Milder winters are also expected to increase survival of alternate weed hosts of viruses. Increases in frequency and intensity of summer storms with high winds, rain, and hail will increase wounding of plants and result in increased transmission of viruses by mechanical means. Therefore, with predicted changes in climate, viral diseases of plants are expected to increase in importance. Potentially of greater importance will be the effects of diseases caused by newly, introduced viruses that, because of the changed climate, will be able to persist. A warmer climate might also allow viruses that are present in greenhouses, such as Pepino mosaic virus (PepMV), to establish infection in the field. The main effect of temperature in temperate regions is to influence winter survival of vectors. Natural spread of vectors, pests and diseases is accelerated towards the North, as former climate barriers are no longer effective. This results in more severe outbreaks of plant-disease vectors like aphids, white flies, thrips or beetles, an extension of the period of disease infection further into the growing season and also introduction and establishment of new vector species. The described effects on vectors can have severe negative effects on food production or result in an increased use of plant protection products to control the vectors.

Effects of climate change on bacterial pathogens

Milder and shorter winters are expected to have little effect on soil-borne bacterial pathogens, however, survival of host or debris-borne and vector-borne primary inoculum is expected to increase. Soil-borne, bacterial plant pathogens, such as Agrobacterium tumefaciens, may build up their populations in host plants and could be released into the soil where they can survive as primary inoculum in the next season. Host or debris-borne bacteria survive on and in host tissues. On perennial hosts, bacteria, such as Erwinia amylovora on apple, overwinter on infected host tissue, and primary inoculum is spread from host to host in the next season. On annual hosts, bacteria such as Pseudomonas syringae pv. phaseolicola may survive in host debris in soil or on the soil surface. Vector-borne bacterial pathogens, such as Erwinia stewartii, survive in insect vectors, and these vectors act as the source of primary inoculum in the next season.

Bacterial pathogens, such as Pseudomonas syringae pv. tomato and Xanthomonas campestris pv. vesicatoria, arise from infected seed and possibly also survive in debris, soil, and weeds. Bacteria are spread to their host plants mainly by water, usually in the form of rain splash, and insects. In humid, wet conditions, infected plant tissues can exude masses of bacteria that are spread from host to host by rain splash and insects. Therefore, the warmer drier summers expected with climate change should limit bacterial diseases. However, bacteria often enter hosts through wounds and the expected increase in frequency and intensity of summer storms with high winds, rain, and hail will increase wounding of plants and provide moisture for the spread of bacteria.

Effects of climate change on nematodes

Majority of plant-pathogenic nematodes spend part of their lives in soil, and therefore, soil is the source of primary inoculum. Life cycle of a nematode can be completed within 2–4 weeks under optimum environmental conditions. Temperature is the most important factor, and development is slower with cooler soil temperatures. Warmer soil temperatures are expected to accelerate nematode development, perhaps resulting in additional generations per season, and drier temperatures are expected to increase symptoms of water stress in plants infected with nematodes such as the soybean cyst nematode. Overwintering of nematodes is not expected to be significantly affected by changes in climate, although for some, such as the soybean cyst nematode, egg viability may be reduced in mild winters.

Effects of climate change on disease development due to abiotic stresses

Diseases can also result from indirect effects, where plants have their defenses weakened by an abiotic factor and are predisposed to infection by plant pathogens.  Several important plant diseases are initiated by abiotic stresses, including forest decline diseases, which are an example of a disease complex caused by a combination of plant predisposition and a repetitive sequence of plant stresses that weaken a plant to become susceptible to weak pathogens that then can often infect and kill the plant These weak pathogens, called saprogens, are often ubiquitous inhabitants of soil and decaying plant material and, normally, they do not cause disease in healthy, unstressed plants. However, under conditions of environmental stress, plants can become susceptible to these saprogens and their opportunistic infections. One of the more prevalent examples of these saprogens is the girdling fungi of the genus Armillaria. As climate changes, new combinations of host–stress–saprogens will be encountered and might give rise to new types of decline diseases, particularly in tree species. In temperate climates, plants that are stressed by biotic or abiotic factors during a growing season are often predisposed to freezing damage during the subsequent winter. Plant diseases associated with interactions of biotic and abiotic stresses, or disease complexes, are a unique and important area of consideration for assessing the influence of climate change on plant diseases. In particular, forest declines are an example of plant diseases that result from a combination of interacting biotic and abiotic factors. Such diseases are characterized by a variety of disease symptoms and signs, are typically scattered in a random pattern throughout a population within a region, and are often host-specific, although more than one tree species in a region may have its own specific decline symptoms. Decline diseases are one example where a strong association between climate change and disease incidence and (or) severity has already been established in several forest species. Facts of extensive forest declines have been documented in Europe and North America in ash, birch, balsam fir and maple, and a strong relationship was evident between climate warming in the Northern Hemisphere and the onset of crown dieback in 1925, 1937, and 1981 on selected species of northern hardwoods in eastern Canada.

Impact of climate change on disease management

New dimensions of climate change may add extra uncertainty in management strategies for diseases caused by different pathogens. Delayed planting to avoid a pathogen may become less reliable. There may be problems with applications of bio-control agents in the field because of the vulnerability of bio-control agent populations to environmental variations and environmental extremes If appropriate temperature and moisture are not consistently available, bio-control agent populations may reach densities that are too small to have important effects, and may not recover as rapidly as pathogen populations when congenial conditions reoccur.

1Division of Environmental Sciences, Indian Agricultural Research Institute, New Delhi – 110012

2Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi – 110012

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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