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Environmental Effects on Soil – Plant – Pathogen Interactions

Paul R. Bullock, Department of Soil Science, University of Manitoba, Winnipeg, MB R3T 2N2 email: bullockp@ms.umanitoba.ca

Introduction The Soil Environment Soil is a complex and dynamic medium with a variable range of many physical (texture, density, water content), chemical (pH, nutrient status, solution chemistry) and other (cation exchange, organic matter content) properties. This is the world in which soil micro-organisms (MO) and plant roots must survive. The properties of the soil affect the makeup of the soil microbial population as well as their rate of metabolism. Likewise, the properties of the soil affect plant root vigor, length, density and uptake of water and nutrients. However, neither MO nor roots are merely passive tenants. They exert their own influence on the properties of the soil. Exudates, secretions and mucilage from roots impact soil properties adjacent to the root, as well as the roots from other plants and the microbial population. Similarly, MO modify their environment with exudates, mucilage and lyzates, which impact soil properties as well as plant roots and other MOs. There is a complex interaction between the soil and the roots and MO that inhabit a soil volume. Changes in soil properties can affect the type of MO that can survive in the soil, which, in turn will impact the health of plant roots. The changes in the soil environment can be either detrimental or beneficial depending on the type of MO involved. The detriment or benefit to the MO can be either positive or negative for a plant root and will vary with plant species. Thus, changes in the soil environment can cause either a direct effect on the growth and productivity of an agricultural crop or an indirect effect through the impact on the microbial population around the plant root. Climate Change Impacts The changing surface energy balance that is driving climate change and warmer air temperatures also impacts the temperature of the soil. It is expected that prairie soils will warm earlier and more rapidly in the spring and that the warmth will penetrate to deeper soil depths earlier each growing season. The impacts that this will have on agriculture has been completely ignored in any discussion of climate change impacts to this point in time. It is possible that, in some cases, the subsurface impacts will be more significant than those occurring above ground. Better knowledge of soil – plant – pathogen interactions will provide a basis from which to make better predictions about climate change impacts on agriculture. Agricultural Production: From Technology to Holism The technical revolution that started midway through the 20th century sparked a green revolution that saw crop yields increase phenomenally. However, there is a growing awareness of many long term negative effects of high-input-monoculture crop production including pest resistance to chemicals, pollution of water, compromising food safety and soil degradation. It is becoming clearer that a more holistic approach to agricultural production would be desirable to sustain long term soil productivity. However, we currently understand very little about the processes that conserve soil stability and enhance soil productivity above and beyond the application of chemical fertilizers, pesticides and certain well-established crop rotation and residue management practices. The soil-plant-pathogen interactions that are a fundamental to these processes and affect the productivity of the soil are extremely complex and largely unknown. In fact, agricultural researchers a century ago likely had better empirical understanding of these interactions than we do now through their investigation of agricultural management systems that espoused rotation of crops as an integral component of soil conservation, pest control and maintenance of soil productivity.

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In order to shift into more holistic agricultural management, we must be able to do so while maintaining and enhancing agricultural output in the short term. The world has grown accustomed to the high crop yields achieved through the technological revolution and we now have a very large population dependent on that productivity. We have an opportunity to improve crop yield through manipulation of soil properties that benefit the crop either directly or indirectly. However, manipulation that provides benefits on one soil type may have no effect on another or, worse, cause a detrimental effect. In the short term, we cannot afford to see a reduction in crop yield and agricultural productivity. In order to maintain food production levels, holistic approaches to soil management will require techniques targeted to specific soil types with specific properties. This is much different than the technological management systems where a particular chemical or fertilizer is expected to provide the same benefit across all soil types and landscape positions. Manipulating Soil-Plant-Pathogen Interactions Soil Solarization A good example of modifying the soil environment to enhance agriculture productivity is soil solarization. This technique was pioneered in Israel (Katan, 1980) and involves covering the soil with a thin clear polyethylene sheet during the hottest month of the year. Leveling and wetting the soil prior to covering makes this more effective by increasing the heat sensitivity of the MO and the thermal conductivity of the soil (Mahrer et al, 1984). The technique is most effective in areas with a hot temperatures sometime during the annual climate such as the Mediterranean (Katan, 1980), Australia (Porter and Merriman, 1985) and the southern USA (Martyn, 1986). However, trials in cooler climates of New Zealand have indicated that it has potential as a plant disease control mechanism there as well (Mclean et al, 2001). The clear, polyethylene cover increases the soil temperature over several cm of depth (Figure 1). This can have a significant impact on the survival of various microbial plant pathogens in the soil including Verticillium dahliae (Figure 2), which, in turn, can cause a significant reduction in certain diseases such as Fusarium wilt in watermelon (Figure 3). This technique provides an opportunity to improve crop yield through better disease control without the use of fumigants or chemical control. However, its success is dependent upon the heat treatment reducing the population of the problem pathogen. Solarization could also decrease the levels of a soil microbe that is beneficial to a particular crop or it could reduce the levels of a soil microbe which controls the population of other microbes that are pathogenic to plants. Therefore, its success is not assured in every situation. Soil Inoculation Microbial inoculants have been in use for decades. These include rhizobacteria to enhance nitrogen fixation as well as microbial pathogens for disease control. Even though these have been in wide use, the response can still be variable. In some cases, what appears at first to be a promising microbial inoculant in laboratory tests sometimes fails when taken to the field. Pseudomonas sp. Strain PsJn has been shown to improve root branching, root and shoot weights, node number, stem length and chlorophyll content in some varieties of tissue culture grown potato plantlets. This suggested that it might provide an advantage to potatoes that are subjected to heat stress. The ability of Pseudomonas sp. Strain PsJn to enhance potato yield response under heat stress was tested on 18 different potato genotypes in a growth chamber study that compared a low temperature (20oC daytime, 15oC nighttime) and a high temperature (33oC daytime, 25oC nighttime) environment with potato plantlets

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Figure 1. Impact of solarization using an ultraviolet-stabilized, 1.5 mil, clear, polyethylene film on

maximum and minimum soil temperature profiles for July 17-19. Temperatures are the mean of three replicates. o---o min temp non-solarized, •---• min temp solarized, o__o max temp non-solarized, •__ • max temp solarized (Martyn, 1986).

either bacterized or not bacterized with Pseudomonas sp. Strain PsJn in each environment (Bensalim et al, 1998). Water and nutrients were not limiting. At low temperature, the tuber fresh weight yield after 12 weeks was not significantly impacted by bacterization. At high temperature, one potato genotype showed a significant increase in tuber fresh weight with bacterization and another genotype showed a significant decrease in the same. The impact on all other genotypes was not significant. The inoculation of potatoes with Pseudomonas sp. Strain PsJn simply is not warranted based on ex vitro testing of the plant response, even though the in vitro tests showed significant beneficial effects. This demonstrates that potential beneficial effects of inoculation in a controlled environment may not be exhibited outside that environment.

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Figure 2. Effect of solarization on control of Verticillium dahliae. Microsclerotia were buried at three

depths and removed after various periods. Results are expressed as percent control of the pathogen at each layer in the mulching treatment (Katan, 1980).

Inoculation is also used to enhance plant nutrient uptake (e.g. Penicillium bilagi for enhancing plant uptake of phosphorus). However, the benefits of this practice are not demonstrated consistently. This may be a result of having a natural population of the microbe already present or the microbe may not thrive in a certain soil. Likewise, the inoculation of microbial control agents have been shown to be affected by variation in soil temperature and moisture conditions (O’Callaghan et al., 2001). The success of these management practices, therefore, will be restricted by the conditions of the soil environment and the ranges for key soil properties for successful results should be specified. Green Manure Impacts on Soil Pathogens Cultural practices are known to directly or indirectly affect populations of soilborne pathogens and the severity of their resultant root diseases. A number of cover crops and green manures can be effective in suppressing nematode populations and infections. Selected cultivars were incorporated into soil infested with Pratylenchus penetrans and sown with bean (Phaseolus vulgaris) to test for suppressive behavior of the crops as green manures (Figure 4). The most efficient green manures were those of sudangrass, rapeseed, and ryegrass. These results demonstrate that the use of cover crops/green manures can be highly effective in managing lesion nematodes. Two field studies were conducted to investigate the effects of green manure on Verticillium wilt (Verticillium dahliae) on potato. Verticillium wilt was best controlled after green manure treatments of either sudangrass or corn (Figure 5). Wilt was most severe when potato followed the fallow treatment. However, the effect was not consistent. During 1991 in study 2, sudangrass did not provide a suppressive effect either early or late in the growing season (Figure 5b). The reasons for the lack of effect in the 1991 study 2 are unknown.

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Figure 3. Incidence of Fusarium wilt in solarized and non-solarized soil. * = infested, nonsolarized

soil; o = infested, 30-day solarized soil; • = infested, 60-day solarized soil; and = noninfested, nonsolarized soil (Martyn, 1986).

Figure 4. The influence of various green manures on the population of Pratylenchus penetrans per

gram of bean root, 8 weeks after seeding (Abawi and Widmer, 2000).

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a. Study 1

b. Study 2 Figure 5. Incidence of Verticillium wilt. A. Early growing season 1990; B. Late growing season 1990;

C. Early growing season 1991; D. Late growing season (1991); (Davis et al, 1996).

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Wilt incidence was positively correlated with V. dahliae colonization in apical stems but was not significantly related to other pathogens or to effects of green manure treatments on preplant nutritional effects of N, P or K. The effect of green manures is not completely understood but other lines of evidence have suggested biological control. First, the direct effects of treatments on soilborne V. dahliae populations alone do not account for disease suppression. Second, root-colonization data have shown V. dahliae suppression on potato roots with green manure treatments to be highly correlated with both disease resistance and potato yield even though treatments have had no effect on inoculum densities of V. dahliae in the soil. Nitrogenous Soil Amendments Soil amendments can be used to suppress disease but the success varies with soil type. The effect of a soil amendment (urea @ 200 kg N ha-1 plus CaO @ 5,000 kg ha-1) on the survival of bacterial wilt (Ralstonia solanacearum) was investigated (Figure 6). Depending on the soil, the amendment had different effects on R. solanacearum. The population did not decline until 3 weeks after amendment in the first two soils. This coincided with the appearance of nitrite. This did not happen in soil MMSU because at the higher pH of this soil, nitrite is not as toxic (less nitrous acid). In the first two soils, the pH dropped below 7 on the second and last sampling dates, thereby increasing the toxicity of nitrite. Ammonia was not considered to be toxic even though all soils showed an accumulation of ammonium. The first two soils did not show a significant decline in R. solanacearum when a nitrification inhibitor was added. This kept the ammonium levels high and prevented buildup of nitrite. Therefore, the toxic effect was attributed to nitrite. The initial decrease in R. solanacearum in the MMSU soil was attributed to a high pH effect, which is known to strongly reduce growth of nitrifying bacteria. The BRCI soil did not accumulate nitrite, therefore, there was no toxic effect on R. solanacearum. The lack of amendment effect on the BRCI soil illustrates the importance of understanding the soil environment interactions in order to utilize these amendments successfully. Addition of swine manure to field soils killed Verticillium dahliae microsclerotia and reduced verticillium wilt in potato but only at one of several fields (Conn and Lazarovits, 2000). When swine manure was added to the soil, the efficacy increased with the concentration of swine manure added, indicating that one or more components were directly toxic to the microsclerotia. The toxicity of swine manure was reduced with increasing soil moisture indicating that the active component was undergoing dilution. When the pH of the soil was adjusted from 5.0 to 6.5, the toxicity of the swine manure was eliminated (Figure 7). Conversely, in a soil where swine manure initially had no effect, an adjustment of the pH from 7.5 to below 6 caused mortality of microsclerotia. Increasing the soil temperature slightly increased the toxicity of the swine manure. The results were similar in a range of soil textures from sand to loam when they were made equal with respect to pH and swine manure concentration. It was also equally effective in soils with organic carbon contents from 1.4 to 6% when soil pH and moisture levels were made equal. Tenuta et al (2002) found that a mixture of volatile fatty acids (VFAs) comparable to that found in the liquid swine manure above had a similar toxicity to V. dahliae microsclerotia. Acidity promotes the protonation and generation of non-ionized forms of short-chain VFAs, which they found were the most effective at killing the microsclerotia. This explained why the effectiveness of the manure was greater at lower soil pH. Acetic acid was the predominant VFA in the liquid swine manure with propionic acid also present in significant concentration. It is not known why the non-ionized forms of short-chain VFAs are more effective at killing microsclerotia.

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Figure 6. Populations of Ralstonia solanacearum (log10[(CFU/g dry soil) + 1], concentrations of

ammonium-N (NH4+-N), nitrite-N (NO2--N) and nitrate-N (NO3--N) (mg of N/kg dry soil)

and pH in four Philippine soils at 0, 7 and 21 days after adding a soil amendment (SA). Data are means of two experiments. _•_ with SA; _o_ without SA. ns = no significant difference; *, **, *** = significant difference at P<0.05, P<0.01 and P<0.001, respectively (Michel and Mew, 1998).

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Figure 7. Effect of soil pH on the efficacy of swine manure (SwM; % concentration mass/mass soil

water) to kill Verticillium dahliae microsclerotia (MS). Microsclerotia were placed in soil and removed 1 week later. Error bars represent standard error of the mean of two experiments (n=6) (Conn and Lazarovits, 2000).

Adding meat and bone meal to a sandy soil resulted in the death of microsclerotia of the fungal wilt pathogen Verticillium dahliae (Tenuta and Lazartovits, 2003). No effect was found when the same amount was added to a loam soil. Microsclerotia mortality was attributed to the accumulation of ammonia or nitrous acid products that did not accumulate to lethal levels in the loam soils. Twelve different soils were tested for the toxicity of meat and bone meal amendment to microsclerotia and their ability to accumulate ammonia and nitrous acid. Addition of meat and bone meal resulted in killing microsclerotia in four soils due to ammonia accumulation. The inhibition of nitrification caused the accumulation of ammonia. In six soils, meat and bone meal amendment resulted in moderate accumulation of ammonia and a reduction in germination of microsclerotia from 35 to 90%. In those soils, nitrification was rapid and ammonia was quickly removed from solution. In three of the soils, when the ammonia dissipated, the germination of microsclerotia returned to the same level as that in non-amended soil. The addition of meat and bone meal killed microsclerotia in two soils by nitrous acid accumulation. These soils had an acidic pH (< 7). Nitrous acid accumulated to significant levels in three other soils where the pH was basic (> 7) but it was not lethal at the higher pH. Meat and bone meal amendment can also suppress verticillium wilt in potatoes, but again, it does not work on all soils. It is most effective on sandier soils with low organic matter content. High ammonia content with soil pH >8.5 suppresses the MOs that normally convert ammonia to nitrite, which causes high ammonia levels to persist and kill the verticillium microsclerotia. Nitrous acid, conversely, is most toxic to the microsclerotia at pH levels below 5. The results indicated that organic C and soil bulk density were potential predictors of the ability of soil to accumulate ammonia. This is very different from the soil

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properties that affect the ability of swine manure to control the same pathogen where soil texture and organic matter content had little effect. Summary There are many other examples of soil-plant-pathogen interactions that could be cited. Modern research techniques that allow us to manipulate the DNA of microscopic organisms and detect subtle changes in root-microbe interactions are revealing just how little we actually know about the factors controlling the relationship between the soil environment and the plant-pathogen interactions and feedback mechanisms that occur within. These interactions will be impacted by a warming climate that will change the present range of both temperature and moisture status for many soils. The impact of these changes is completely unknown at this point. It is clear that attempting to modify the soil environment to advantage for agriculture is a complex task. We can only continue to investigate carefully the impacts of various modifications of the soil environment on a case-by-case basis to build a better knowledge base from which we can design soil management practices that can benefit agricultural productivity. This will take much more research than we currently have to date. But no matter how daunting the task may now appear, it is clear that we must invest the time and resources needed to gain a fuller understanding of soil-plant-pathogen dynamics. If we want to take a more holistic approach to managing our soil resource without heavy use of chemical inputs, we must exploit this knowledge to meet the dual challenge of sustaining high crop yields in the short term and soil productivity in the long term. References

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temperature effects on performance of 18 clones of potato. Amer. J. Potato Res. 75: 145-152. Conn, K.L. and Lazarovits, G. 2000. Soil factors influencing the efficacy of liquid swine manure added

to soil to kill Verticillium dahliae. Can. J. Plant Path. 22: 400-406. Davis, J.R., Huisman, O.C., Westermann, D.T., Hafez, S.L., Everson, D.O., Sorensen, L.H. and

Schneider, A.T. 1996. Effects of green manures on Verticillium wilt of potato. Phytopathology 86: 444-453.

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