 113 μg SO2 m-3 were applied in 1984-85, 18 (ambient), 37, 76 and 126 μg SO2 m-3 in 1985-86 and 13 (ambient), 64, 73 and 100 μg SO2 m-3 in 1986-87, throughout the growing season. A general increase in powdery mildew infection in plots exposed to SO2 and decreased leaf blotch infection was recorded at the time of sampling. However, there was no effect of SO2 treatment in any year on the incidence of sharp eyespot, Fusarium foot rot, brown rust, glume blotch and net blotch on the flag leaves but variable effects were observed for eyespot and black ear moulds. Khan et al. (1998) investigated the effects of intermittent exposures of SO2 at 143 μg m-3 on the development of powdery mildew of cucumber (Cucumis sativa L.) caused by Sphaerotheca fuliginea, using pre, post and concomitant inoculation exposures in closed-top chambers. Fungus colonization was relatively greater on the plants exposed to 143 μg SO2 m-3. Gas injury on fungus-infected plants was also less in the other treatments. Conidia of S. fuliginea collected from exposed plants varied in size. Conidial germination was considerably greater at 143 μg SO2 m-3. The number of fibrosin bodies declined at all the concentrations. Synergistic relationship between 143 μg SO2 m-3 and S. fuliginea was recorded on plant growth and yield of cucumber. Intermittent exposures of mustard plants to SO2 at 25 and 50 ppb promoted pathogenesis of A. brassicicola resulting to greater blight and plant growth reductions (Khan and Khan, 2010).
Inhibition of pathogens: Since long germicidal nature of SO2 has been known to man but probably such observation in ambient condition was first made by Kock (1935) when he noticed absence of powdery mildew fungus, Microspheaera alni on oak trees in the vicinity of a paper mill in Australia. The disease was quite prevalent on oaks in an area which was agroclimatically identical but away from the smokes (SO2) of the mill. Industrial emissions containing SO2 may decrease incidence of the disease caused by various fungi viz. Alternaria solani, Hypodermella juniper, Hysterium pulicara, Rhystisma acerinum, Sphaerotheca fulginea, Venturia inequalis etc. (Przybylski, 1967). Scheffer and Hedgcock (1955) observed low intensity and incidence of the diseases caused by species of Coleosporium, Cronartium, Melampsora, Peridermium or Puccinia on trees grown in SO2 polluted area (near an ore smelter) and sustained pollution injury. With a gradual increase in distance from the smelter, there was a corresponding decrease in plant injury and increase in the incidence of fungal diseases. Lilacs grown in polluted air (SO2) of New York City and other urban areas of USA developed substantially less infection of the powdery mildew fungus than the lilacs grown in rural areas (Hibben and Walker, 1966). Similar inhibitory effects of SO2 have also been reported in India (Rai, 1987; Khan et al., 1991). Incidence and severity of early blight of potato caused by Alternaria solani were low in fields about 3-4 km away from a coal fired thermal power plant compared to unpolluted area (Rai, 1987). Khan et al. (1991) observed a low intensity and incidence of powdery mildew caused by Sphaerotheca fulginea on cucurbits grown around ceramic and pottery industries at Khurja, India (upto 2 km left to Khurja-Delhi road). With increasing distance from the source, SO2 injury gradually decreased but incidence of powdery mildew increased.
Stimulation of pathogens: Some researchers have reported diseases in severe form on plants growing in SO2 stressed environments (Heagle, 1973). The incidence of Armillaria mellea was remarkably greater on trees injured by ambient SO2 (Kudela and Novakova, 1962; Donaubauer, 1968). An increased incidence of wood rot (Jancarik, 1961) and needle rot caused by Lophodemium piecae (Kudela and Novakova, 1962) was recorded in localities polluted with SO2. Chiba and Tanaka (1968) also observed high disease incidence of Rhizosphaera kalkhoffii on red pine in ambient condition. Greater infection of Trametes heteromorpha, T. serialis, Glocophyllum and Melampsora species was noticed on different trees grown in SO2 polluted areas (Scheffer and Hedgcock, 1955; Jancarik, 1961).
Plant diseases caused by nematodes: Plant parasitic nematodes constitute an important group of parasites that are considered as a constraint in the cultivation of agricultural crops. Plant parasitic nematodes are typically obligate parasites and can not survive for a longer duration in the absence of host plant. Majority of plant parasitic nematodes are root feeders but a few, like Anguina sp., Aphelenchoides spp., Ditylenchus spp. etc., attack and invade above ground parts such as stem, leaf or inflorescence. Plant nematodes on the basis of parasitism are categorized as ectoparasites and endoparasites. Ectoparasites feed on the surface of root or shoot whereas endoparasites penetrate the plant tissue and feed internally (Khan, 2008). Both groups of plant nematodes are agriculturally important, but endoparasites such as Meloidogyne spp. and Pratylenchus spp. are more damaging and inflict tremendous yield losses to susceptible crops. Effects of SO2 on the diseases caused by nematodes have been tested in some studies however, they are not adequate enough to categorize under simulated and ambient condition, hence are clubbed together and presented under following subheadings of inhibitory and stimulatory effects (Table 3).
Inhibition of nematode pathogenesis: Enough evidences exist to indicate that SO2 may inhibit parasitism of nematodes (Table 3). The effect of SO2 may be indirect either through altered host physiology or through some chemical alterations in soil as SO2 is unlikely to penetrate the soil and affect the inhabiting nematodes directly.
Bassus (1968) was probably the first to correlate nematode population to atmospheric pollution. He reported increased population of saprophagous and predaceous nematodes in the soil of pine trees severely damaged by SO2 and alkaline particulate material compared to the soil of those pines away from the source and exhibiting a mild injury of SO2. Weber et al. (1979) reported that intermittent exposures of soybean plants to 655 μg SO2 m-3 had adverse effects on Heterodera glycines and Paratrichodorus minor. Some more studies have also been conducted to evaluate the effect of coal smoke pollutants (primarily SO2) on the pathogenesis of nematodes (Khan and Khan, 2000). At a site, 2 km away from the stack of a coal fired thermal power plant having 233 (43-348) μg SO2 m-3 and 112 (29-174) μg NO2 m-3, disease intensity (galls) and reproduction (egg masses) of root-knot nematode, Meloidogyne incognita on tomato, were significantly low compared to plants grown at unpolluted site with same soil.
Under artificial treatment condition in close top exposure chambers the reproduction of M. incognita on tomato was inhibited at 571 μg m-3 for 3 h every third day for 75 days. Severity of root-knot (galling) caused by M. javanica was also low on pea plants exposed to 0.1 and 0.2 ppm SO2 exposures (Singh et al., 1995). Root galling and egg mass production of M. javanica were declined by 23 and 24% at 300 μg SO2 m-3 exposure, respectively. The fecundity was 16% lower at 300 μg SO2 m-3 (Khan et al., 1995).
Kausar et al. (2005) observed the interaction of SO2 (0.2 ppm) and seed gall nematode, Anguina tritici on five wheat varieties. LOK-1, HD-2329, Raj-3765, PPW-154, UP-2328 in artificial treatment condition. SO2 alone significantly reduced plant growth of all the varieties. Reduction in growth was more in nematode inoculated and SO2 exposed sets compared to nematode alone-inoculated sets. However, highest reduction in yield was observed in nematode inoculated sets. Effect of different concentrations of SO2 (0.05, 0.1 and 0.2 ppm) on the mortality of seed gall nematode, A. tritici at different time intervals (1, 2, 3 and 4th days) has also been studied under artificial treatment condition (Kausar and Khan, 2009). All concentrations of SO2 were found harmful to juveniles of A. tritici. Further, investigation of the interactive effect of different concentration of SO2 and inoculum levels (2,500, 5,000 and 10,000) of nematode on wheat, Triticum aestivum revealed that plant growth, yield, photosynthetic pigments, seed carbohydrate and seed protein were reduced significantly. Both the pathogens and SO2 were antagonistically interacted.
Stimulation of nematode pathogens: Considerable numbers of reports have also revealed stimulatory effect of SO2 on the activity of plant parasitic nematode (Table 3). Weber et al. (1979) reported