In providing the rationale for their study, authors Plessl et al. (2007) state that "potato late blight caused by the oomycete Phytophthora infestans (Mont.) de Bary is the most devastating disease of potato worldwide," and that "infection occurs through leaves and tubers followed by a rapid spread of the pathogen finally causing destructive necrosis." Against this backdrop, the five researchers grew individual well watered and fertilized plants of the potato cultivar Indira in controlled-environment chambers maintained at atmospheric CO2 concentrations of either 400 or 700 ppm. Four weeks after the start of the experiment, the first three fully-developed pinnate leaves were cut from the plants and inoculated with zoospores of P. infestans in Petri dishes containing water-agar, after which their symptoms were evaluated daily via comparison with control leaves that were similarly treated but unexposed to the pathogen.

In describing their results the German scientists report that the 400- to 700-ppm increase in CO2 "dramatically reduced symptom development," including extent of necrosis (down by 44% four days after inoculation and 65% five days after inoculation), area of sporulation (down by 100% four days after inoculation and 61% five days after inoculation), and sporulation intensity (down by 73% four days after inoculation and 17% five days after inoculation). Such findings, according to Plessl et al., "clearly demonstrated that the potato cultivar Indira, which under normal conditions shows a high susceptibility to P. infestans, develops resistance against this pathogen after exposure to 700 ppm CO2," while also noting that "this finding agrees with results from Jwa et al. (1995), who reported an increased tolerance of tomato plants to Phytophthora root rot when grown at elevated CO2."

Also examining the effects of atmospheric CO2 enrichment on tomato root rot were Jwa and Walling (2001), who grew tomato plants hydroponically for eight weeks in controlled-environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm. At week five of their study, they infected half of all plants growing in each CO2 concentration with the fungal pathogen Phytophthora parasitica, which attacks plant roots and induces a water stress that decreases growth and yield. At the end of the study, they found that the pathogenic infection had reduced total plant biomass by nearly 30% at both atmospheric CO2 concentrations. However, the elevated CO2 treatment had increased the total biomass of the healthy and diseased plants by the same amount (+30%), with the result that the infected tomato plants grown at 700 ppm CO2 had biomass values that were essentially identical to those of the healthy tomato plants grown at 350 ppm CO2. Thus, the extra CO2 completely counterbalanced the negative effect of the pathogenic infection on overall plant productivity.

Working on another plant, Malmstrom and Field (1997) grew individual oats for two months in pots placed within phytocells maintained at atmospheric CO2 concentrations of 350 and 700 ppm, while they infected one-third of the plants with the barley yellow dwarf virus (BYDV), which plagues more than 150 plant species worldwide, including all major cereal crops. Over the course of their study, they found that elevated CO2 stimulated rates of net photosynthesis in all plants, regardless of pathogen infection. However, the greatest percentage increase occurred in diseased individuals (48% vs. 34%). Moreover, atmospheric CO2 enrichment decreased stomatal conductance by 50% in infected plants but by only 34% in healthy ones, which led to a CO2-induced doubling of the instantaneous water-use efficiency of the healthy plants, but an increase of fully 2.7-fold in the diseased plants. Last of all, after 60 days of growth under these conditions, they determined that the extra CO2 increased total plant biomass by 36% in infected plants, but by only 12% in healthy plants. In addition, while elevated CO2 had little effect on root growth in the healthy plants, it increased root biomass in the infected plants by up to 60%. Consequently, it can be appreciated that as the CO2 content of the air continues to rise, its many positive effects will likely offset some, if not most, of the negative effects of the destructive BYDV. Quoting Malmstrom and Field with respect to two specific examples, they say in their concluding remarks that CO2 enrichment "may reduce losses of infected plants to drought" and "may enable diseased plants to compete better with healthy neighbors."

Tiedemann and Firsching (2000) grew spring wheat plants from germination to maturity in controlled-environment chambers maintained at ambient (377 ppm) and elevated (612 ppm) concentrations of atmospheric CO2 and at ambient (20 ppb) and elevated (61 ppb) concentrations of ozone (and combinations thereof), the latter of which gases is typically toxic to most plants. In addition, half of the plants in each treatment were inoculated with a leaf rust-causing fungus. Under these conditions, the elevated CO2 increased the photosynthetic rates of the diseased plants by 20 and 42% at the ambient and elevated ozone concentrations, respectively. It also enhanced the yield of the infected plants, increasing it by 57%, even in the presence of high ozone concentrations.

In another study, Matros et al. (2006) grew tobacco plants (Nicotiana tabacum L.) in 16-cm-diameter pots filled with quartz sand in controlled-climate chambers maintained at either 350 or 1000 ppm CO2 for a period of eight weeks, where they were irrigated daily with a complete nutrient solution containing either 5 or 8 mM NH4NO3. In addition, some of the plants in each treatment were mechanically infected with the potato virus Y (PVY) when they were six weeks old. Then, at the end of the study, the plants were harvested and a number of their chemical constitutes identified and quantified.

This work revealed, in the researchers' words, that "plants grown at elevated CO2 and 5 mM NH4NO3 showed a marked and significant decrease in content of nicotine in leaves as well as in roots," while at 8 mM NH4NO3 the same was found to be true of upper leaves but not of lower leaves and roots. With respect to the PVY part of the study, they further report that the "plants grown at high CO2 showed a markedly decreased spread of virus." Both of these findings would likely be considered beneficial by most people, as potato virus Y is an economically important virus that infects many crops and ornamental plants throughout the world, while nicotine is nearly universally acknowledged to have significant negative impacts on human health (Topliss et al., 2002).

In a study conducted within the BioCON (Biodiversity, Carbon dioxide, and Nitrogen effects on ecosystem functioning) FACE facility located at the Cedar Creek Natural History Area in east-central Minnesota, USA, Strengbom and Reich (2006) evaluated the effects of an approximate 190-ppm increase in the air's daytime CO2 concentration on leaf photosynthetic rates of stiff goldenrod (Solidago rigida) growing in monoculture for two full seasons, together with its concomitant effects on the incidence and severity of leaf spot disease. Although they found that elevated CO2 had no significant effect on plant photosynthetic rate in their study, they report that "both disease incidence and severity were lower on plants grown under elevated CO2." More specifically, they found that "disease incidence was on average more than twice as high [italics added] under ambient as under elevated CO2," and that "disease severity (proportion of leaf area with lesions) was on average 67% lower under elevated CO2 compared to ambient conditions."

In discussing their results, Strengbom and Reich say the "indirect effects from elevated CO2, i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher CO2," which as noted above was negligible in their study. Hence, they concluded "it may be necessary to consider potential changes in susceptibility to foliar diseases to correctly estimate the effects on plant photosynthetic rates of elevated CO2." In addition, they found that the plants grown in CO2-enriched air had lower leaf nitrogen concentrations than the plants grown in ambient air, as is often observed in studies of this type; and they say that their results "are, thus, also in accordance with other studies that have found reduced pathogen performance following reduced nitrogen concentration in plants grown under elevated CO2 (Thompson and Drake, 1994)." What is more, they say that their results are "also in accordance with studies that have found increased [disease] susceptibility following increased nitrogen concentration of host plants (Huber and Watson, 1974; Nordin et al., 1998; Strengbom et al., 2002)." It is possible, therefore, that the ongoing rise in the air's CO2 content may help many plants of the future reduce the deleterious impacts of various pathogenic fungal diseases that currently beset them, thereby enabling them to increase their productivities above and beyond what is typically provided by the more direct growth stimulation resulting from the aerial fertilization effect of elevated atmospheric CO2 concentrations.

In their report of another intriguing study, Gamper et al. (2004) begin by noting that arbuscular mycorrhizal fungi (AMF) are expected to modulate plant responses to elevated CO2 by "increasing resistance/tolerance of plants against an array of environmental stressors (Smith and Read, 1997)." In investigating this subject in a set of experiments conducted over a seven-year period of free-air CO2-enrichment on two of the world's most extensively grown cool-season forage crops (Lolium perenne and Trifolium repens) at the Swiss FACE facility near Zurich, they determined that "at elevated CO2 and under [two] N treatments, AMF root colonization of both host plant species was increased," and that "colonization levels of all three measured intraradical AMF structures (hyphae, arbuscules and vesicles) tended to be higher." Hence, they concluded that these CO2-induced benefits may lead to "increased protection against pathogens and/or herbivores."

In describing the rationale for their work, AL-Kayssi (2009) write that soil solarization "is a method of heating the soil by using polyethylene sheets as mulching over moistened soil, to retain solar radiation during the hot season," so that "soil-borne pathogens may be killed by lethal heat (>40°C) and weakened by sub-lethal heat (<38-40°C) to the extent that they are unable to cause damage to plants or they are more susceptible to chemical toxicants," which technique "has been successfully used to control soil-borne pathogens and weeds (Katan et al., 1976; Mahrer, 1979; Grinstein et al., 1979; Katan, 1981; Mahrer et al., 1984; Avissar et al., 1986; AL-Karaghouli et al., 1990; AL-Kayssi and AL-Karaghouli, 1991)."

Against this backdrop, AL-Kayssi conducted a laboratory experiment where "clay soil samples infested with Verticillium dahliae were exposed to different CO2 concentrations (350, 700, 1050, 1400, 1750 ppm air) and incubated in hot water baths at 35, 40, 45, 50 and 55°C," while "field plots were exposed to the same CO2 levels during soil solarization in three periods (1st of July to 30th of September, 1st of August to 30th of September, and 1st to 30th of September)."

According to the Iraqi researcher, higher than normal CO2 contents in the soil increased maximum soil temperatures while reducing the length of time required to kill 90% of the propagules of V. dahliae in natural field soil with moisture content at field capacity. As an example, he notes that this killing time parameter in soil heated to 35°C was reduced from 24 days at the normal ambient CO2 concentration to 15 days at 1750 ppm CO2; and he states that sub-lethal soil temperatures were raised to lethal levels as the soil's CO2 content was raised, suggesting that in a high-CO2 world of the future, soil solarization should become an even more viable method of controlling soil-borne pathogens and weeds than it is today.

In concluding this review of the literature, the data cited above clearly suggest that atmospheric CO2 enrichment asserts its greatest positive influence on infected as opposed to healthy plants. Moreover, it would appear that elevated CO2 has the ability to significantly ameliorate the deleterious effects of various stresses imposed upon plants by numerous pathogenic invaders. Consequently, as the atmosphere's CO2 concentration continues its upward climb, Earth's vegetation should be increasingly better equipped to successfully deal with pathogenic organisms and the damage they have traditionally done to mankind's crops, as well as to the plants that sustain the rest of the planet's animal life.

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