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Impact of sulphur dioxide exposure on conidial germination of powdery mildew fungi
Environmental Pollution 70 (1991) 81-88
Impact of Sulphur Dioxide Exposure on Conidial Germination of Powdery Mildew Fungi M. Wajid K h a n & M a d h u Kulshrestha Agriculture Centre, Plant Pathology and Plant Nematology Laboratories, Department of Botany, Aligarh Muslim University, Aligarh-202002, India (Received 1 March 1990; revised version received 17 June 1990; accepted 8 October 1990)
ABSTRACT The impact of sulphur dioxide, in two different concentrations (286 ltg m-3 and 571 l~g m- 3) for various exposure periods, on conidial germination of some powdery mildew fungi was investigated in artificial treatment conditions. S02 in general was inhibitory for conidial germination of all the studied powdery mildew fungi and the species did not differ much from each other in their sensitivity to SO z. The per cent conidial germination was increasingly inhibited with an increase in the concentration of S02. The concentration of S02 and the exposure period were important determinants of the inhibitory effect.
INTRODUCTION Air pollutants affect spore germination of fungal plant pathogens and development of foliar plant diseases caused by fungi (Heagle, 1973, 1982). Some reports (Hibben & Walker, 1966; Heagle & Strickland, 1972; Hibben & Taylor, 1975) also suggest that air pollutants like SO2 and 0 3 inhibit powdery mildew on some plants and reduce the disease severity. Most powdery mildews are ectophytic and remain exposed to the ambient air. Any alteration in the ambient air, particularly in the close vicinity of the infected plants, is likely to affect various stages of their pathogenesis. The first phase in the pathogenesis of powdery mildew fungi like many others is germination of their conida, which are produced abundantly and are a means for initiating the primary disease cycle or of secondary spread of the disease. 81 Environ. Pollut. 0269-7491/91/$03'50 ~) 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
M. Wajid Khan, Madhu Kulshrestha
82
Any kind of influence on their germination may reflect in the progress of pathogenesis and severity of the disease. The impact of gaseous air pollutants on conidial germination of powdery mildew fungi has received some consideration in the studies related to air pollution-powdery mildew relationship (Hibben & Taylor, 1975; Krieg & Knosel, 1983). The ambient concentration of SO2 varies from place to place depending upon the amount and concentration of emission, type of coal, its burning temperature, distance from source, meteorological and topographic conditions, etc. SO 2 near point sources like power plants, smelters, etc., with no or little pollution control equipment may be as high as 28 571 pg m -3. In large urban areas SO 2 varies from 142.86-1142.86 pg m -3 (Heagle, 1973). Such ambient concentrations of SO2 may affect conidial germination and development of powdery mildew fungi on plants. In the present study, we have determined the impact of SO2, a common air pollutant originating from the burning of fossil fuels, particularly in thermal power plants, in India, on conidial germination of some powdery mildew fungi in artificial treatment conditions. MATERIALS A N D METHODS
Powdery mildew fungi Eight powdery mildew species were tested in the study listed below against the host from which they were collected. Powdery mildew
Sphaerotheca fuliginea (Schlecht.) Poll.
Erysiphe cichoracearum DC. Sphaerotheca cassiae Pandotra & Ganguly Erysiphe trifolii Grev. Erysiphe pisi DC. Erysiphe polygoni DC.
Microsphaera alphitoides Griff & Maubl. f.sp. zizyphi Phyllactinia dalbergiae
Host
Lagenaria siceraria (Molina) Stand.
Coccinia grandis Voight. Cassia occidental& L. Trigonella foenum-graecum L. Pisum stivum L. Chenopodium ambrosoides L. Zizyphus jujuba Lam. Dalbergia sisoo Roxb.
Piroz.
S02 exposure equipment The SO2 exposure chamber (Standard Appliances, Varanasi, India) was 90 x 90 × 120cm in size and made of transparent fibre glass. A micro-gas
Conidial germination of powdery mildewfungi
83
EXHAUST-~', DUCT
:
90 cm
ooo___R
* o o ,
o*
*
o
*
PERFORATED BOTTOM PLATE
t CONTROL PANEL
Fig. 1. Exposure chamber. exposure cabinet was also used in the study. It had an exhaust duct at the top and a double walled bottom. The upper side of the bottom was provided with openings and the lower was equipped with a blower assembly. A fumigation controller regulated the voltage supply to the blower and was displayed on a panel meter (Fig. 1). Sulphur dioxide was produced in a generator, by the reaction of sulphuric acid (10% H2SO4) on sodium sulphite (Na2SO3). Bottles of Na2SO 3 and HESO 4 were mounted over the SO2 generator. For calibration, the volume of solution discharged from the bottle was determined by collecting the solution dropping through capillary tube into a graduated cylinder and expressing the rate in ml m i n - 1. On the basis of flow rate or solution feeding rate, solutions of N a 2 S O 3 and H2SO 4 (10%) were prepared to produce the required amount of SO 2 gas/min. On complete reaction, 1M Na2SO 3 produces 1M SO2 or 126mg Na2SO3 produces 64mg SO2. Na2SO 3 -t- H2SO 4
~ SO 2 + NaESO 4 + H20
The concentration of SO2 in the chamber was also determined by sampling the air using a portable air sampler (Kimoto Electronics, Japan) and was analysed by spectrophotometry (Anon. 1986). The outlet of the gas generator was connected to the gas inlet nozzles provided in the blower housing of the exposure chamber.
84
M. Wajid Khan, Madhu Kuishrestha STERILE AIR
-~
3sc,...°
///
"
",
Fig. 2. Micro-gas exposure cabinet.
The micro-gas exposure cabinet (length 35 and width 25 cm) was also made of transparent fibre glass and had an inlet hole at one side which was connected with the air supply device by a plastic tube, which took the exposure chamber air containing SOz and after filtering, released it into the micro-exposure cabinet. The bottom of the cabinet had a small outlet perforation for release of the gases (Fig. 2).
Experimental procedure Conidia from fresh samples of leaves infected with powdery mildew species were dusted onto dry clean glass slides by gently tapping the leaves. The slides were exposed to 286 ( _ 68.57) and 571 ( __+26,0) #g m - 3 of SO2 for 3, 6 or 9 h in separate experiments. For exposures, the slides were kept in the micro-gas exposure cabinet and the whole system (micro-gas cabinet with the slides + sterile air supply device) was placed inside the larger exposure chamber. The outlet of the gas generator was connected to the gas inlet nozzles and the generator was started. Exposures were continued at requisite concentrations of SOz for the desired periods, according to the treatments. Air flow inside the chamber was 2"0-2.15 m s - 1. Control slides dusted with conidia kept in micro-gas exposure cabinet were exposed to ambient air inside the air pollutant exposure chamber. The SO2 concentration in the ambient air was 11.72/~g m -3. Each treatment, including the control, consisted of three replicate slides. At the termination of exposure, slides, including the controls, were placed on a glass triangle and kept in Petri dishes containing sterilized distilled water at the b o t t o m and with an upper lid lined with moistened cotton wool. The Petri dishes with the slides were incubated at 20°C for 36 h. The percent germinated conidia in each treatment was then determined by examining 100 conidia from each replicate slide selected at random under the microscope. Conidia showing germ tube initiation or a well developed germ tube were taken as germinated. The percent
Conidial germination of powdery mildewfungi
85
germination values were transformed into angular values and a two way analysis of variance (ANOVA) was used to determine the effects of SO2 concentration on conidial germination, contrasted against the controls, and variation between the powdery mildew fungi at both the concentrations. RESULTS At the concentrations used, SO2 inhibited conidial germination of all the powdery mildew fungi tested (Table 1). Conidial germination was significantly (P < 0-05 or P < 0.01) inhibited after exposure to SO 2 at 571 ~g m -3 and 286pg m -3 for 3, 6 and 9h, compared with controls kept in ambient air, and in general the inhibition was greater for the higher (571/~g m - 3) concentration. At both concentrations, an increase in the duration of exposure caused increased inhibition. Greatest inhibition occurred at the 571/~g m -3 concentration with an exposure period of 12h. These effects were consistent for all the powdery mildew fungi. The data showed that the concentration of SO2 and the exposure duration were important determinants of the inhibition. For example, when the conidia of S.fuliginea were exposed to 286 pg m - 3 SO2 for 6 h, the percent germination was 35-5, but with 571 pg m - 3 SO2 for 3 h exposure, the percent germination was 27.7. This variation was significant (P<0"05) which showed that the concentration of SO 2 was the more important determinant of the inhibitory effect. In contrast, in E. pisi, E. polygoni, M. alphitoides f. sp. zizyphi and P. dalbergiae significantly ( P < 0-05 or P<0"01) greater inhibition occurred with 571 pg m - 3 SO2 for 3 h exposure period than with 286 pg m -- 3 SO2 for 6 h exposure period. This indicated a greater importance of the duration of exposure to SO2 than the SO 2 concentration. In E. tr(/blii and S. cassiae the percent germination obtained either with 571/~g m -3 for 3h exposure period or with 286/~g m - 3 for 6 h exposure period did not differ significantly.
DISCUSSION The present study considered the impact of SO 2 exposure on conidial germination of eight powdery mildew fungi belonging to four genera Sphaerotheca, Erysiphe, Microsphaera and Phylactinia--commonly appearing on a number of host plants. Other aspects of their pathogenesis were not considered. All the studied powdery mildew fungi were substantially sensitive to SO2 but did not differ much from each other in their response to SO2 in relation to conidial germination. The good repeatability of inhibition shown by various powdery mildews is a confirmation of sensitivity of conidia to SO2.
M. Wajid Khan, Madhu Kulshrestha
86
~
0
~.
p..
x
~-.
,2.
~
~
,:.,
~
,:.,
~
© r~ ~
X
~
×
0 e., 0
tt% tt% c,q
~
> 0 e~o
X 0
0 x
tt%
~=
o
.o
Conidial germination of powdery mildewfungi
87
Within the range of S O 2 concentrations tested their tolerance potential to SO 2 is apparently alike regardless of genera or species. As conidia are the source inoculum for the primary and/or secondary disease cycles of the powdery mildews, if such an inhibitory effect of SO2 is occurring as a normal p h e n o m e n o n at ambient levels of SO2, disease severity in field conditions would be affected. Some observations (Kock, 1935; Hibben & Walker, 1966) made earlier in relation to the occurrence of powdery mildews in polluted areas confirm this is so. Some other studies using artificial SO2 exposure have shown similar effects (Hibben & Taylor, 1975). Ozone (O3), another important air pollutant, causes inhibition of Erysiphe graminis on barley (Heagle & Strickland, 1972). When conidial germination is affected adversely the inoculum potential of the pathogens will be greatly reduced and subsequently disease development will be poor. Ambient concentration of SO 2 around a thermal power plant at Kasimpur, 15 km away from the University Campus, was found to be 222.86 ( _ 54"28) pg m - 3 at 1 km and 520 (_+68.57)#g m - 3 at 2 k m distance from the stack in the windward direction (Khan, 1988). These concentrations of SO 2 in a polluted area around a thermal power plant are very close to the concentrations used in this study. The inhibitory effects of SO2 demonstrated would be significant under field conditions, if such high concentrations of SO 2 occur in any SO 2 polluted area. A m o n g the powdery mildew fungi studied, S.fuliginea is more important economically than the others. It attacks cucurbits, legumes, composites and several other plants which are mostly annuals. E. pisi has similar plants in its host index. Such plants are very sensitive to air pollutants, which affect their physiological and biochemical processes and result in a reduced productivity (Heck et al., 1982). Therefore, cultivation of such annuals around the point source of SO2 may not be desirable even if the disease is controlled partially or substantially. The tree hosts like Dalbergia sisoo and Zizyphus jujuba are unlikely to be as sensitive to ambient SO 2 pollution, particularly at lower ambient levels. If these hosts can remain free of powdery mildews in a polluted area, this would be advantageous for host growth. SO 2 inactivates the sulphydryl groups of enzymes and inhibits acid phosphatase activity (Saunders, 1966). Sulphur, as a dust or a wettable powder, is a well-established chemical to control powdery mildew fungi in general. SO2 might have inactivated the process of germination by affecting it at a biochemical level. SO2 also causes adverse effects on a number of plants including agricultural crops (Heck et al., 1982). Since powdery mildews are exposed to ambient air, SO 2 may affect them directly or indirectly through their hosts, causing poor growth. The direct effect of SO 2 on conidial germination has been demonstrated in this study, but the exact mechanism needs to be investigated.
88
M. Wajid Khan, Madhu Kulshrestha REFERENCES
Anon. (1986). Air Quality Monitoring. A Course Manual. National Environmental Engineering Research Institute, Nagpur, India. Heagle, A. S. (1973). Interaction between air pollutants and plant parasites. Annual Review of Phytopathology, 11, 365-88. Heagle, A. S. (1982). Interaction between air pollutants and parasitic plant diseases. In Effects of Gaseous Air Pollution in Agriculture, ed. H. M. Unsworth & D. P. Ormrod, Butterworth Scientific, London. Heagle, A. S. & Strickland, A. (1972). Reaction of Erysiphe graminis f. sp. hordei to low levels of 0 3. Phytopathology, 62, 1144-8. Heck, W. W., Taylor, D. C., Adams, R. M., Bingham, G. & Miller, J. E. (1982). Assessment of crop loss from ozone. Journal of Air Pollution Control Association, 32, 353-61. Hibben, C. R. & Taylor, M. P. (1975). Ozone and SO2 effects on the lilac powdery mildew fungus. Environmental Pollution (A), 9, 107-14. Hibben, C. R. & Walker, J. T. (1966). A leaf roll necrosis complex of lilacs in an urban environment. American Society of Horticultural Sciences, 89, 33942. Khan, M. R. (1988). Studies on root-knot nematodes in relation to environmental pollution on some vegetable crops. PhD thesis Aligarh Muslim University, Aligarh, India. p. 268. Kock, G. (1935). Eichen mehltau und Rauchg asschaden (Oat mildew and smoke gas injury) Zeitschrift fur Pflanzenkrankheitung, 65, 44-55 (Review of Applied Mycology, 14, 406-7). Krieg, W. & Knosel, D. (1983). Modellversuche zur Wirkung von Schwefeldioxid auf pflanzliche Mykosen. Angewandte Botanik, 57, 19-30. Saunders, P. J. W. (1966). The toxicity of SO 2 to Diplocarpon rosae Wolf causing black spot of roses. Annals of Applied Biology, 58, 103-14.
Impact of Sulphur Dioxide Exposure on Conidial Germination of Powdery Mildew Fungi M. Wajid K h a n & M a d h u Kulshrestha Agriculture Centre, Plant Pathology and Plant Nematology Laboratories, Department of Botany, Aligarh Muslim University, Aligarh-202002, India (Received 1 March 1990; revised version received 17 June 1990; accepted 8 October 1990)
ABSTRACT The impact of sulphur dioxide, in two different concentrations (286 ltg m-3 and 571 l~g m- 3) for various exposure periods, on conidial germination of some powdery mildew fungi was investigated in artificial treatment conditions. S02 in general was inhibitory for conidial germination of all the studied powdery mildew fungi and the species did not differ much from each other in their sensitivity to SO z. The per cent conidial germination was increasingly inhibited with an increase in the concentration of S02. The concentration of S02 and the exposure period were important determinants of the inhibitory effect.
INTRODUCTION Air pollutants affect spore germination of fungal plant pathogens and development of foliar plant diseases caused by fungi (Heagle, 1973, 1982). Some reports (Hibben & Walker, 1966; Heagle & Strickland, 1972; Hibben & Taylor, 1975) also suggest that air pollutants like SO2 and 0 3 inhibit powdery mildew on some plants and reduce the disease severity. Most powdery mildews are ectophytic and remain exposed to the ambient air. Any alteration in the ambient air, particularly in the close vicinity of the infected plants, is likely to affect various stages of their pathogenesis. The first phase in the pathogenesis of powdery mildew fungi like many others is germination of their conida, which are produced abundantly and are a means for initiating the primary disease cycle or of secondary spread of the disease. 81 Environ. Pollut. 0269-7491/91/$03'50 ~) 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
M. Wajid Khan, Madhu Kulshrestha
82
Any kind of influence on their germination may reflect in the progress of pathogenesis and severity of the disease. The impact of gaseous air pollutants on conidial germination of powdery mildew fungi has received some consideration in the studies related to air pollution-powdery mildew relationship (Hibben & Taylor, 1975; Krieg & Knosel, 1983). The ambient concentration of SO2 varies from place to place depending upon the amount and concentration of emission, type of coal, its burning temperature, distance from source, meteorological and topographic conditions, etc. SO 2 near point sources like power plants, smelters, etc., with no or little pollution control equipment may be as high as 28 571 pg m -3. In large urban areas SO 2 varies from 142.86-1142.86 pg m -3 (Heagle, 1973). Such ambient concentrations of SO2 may affect conidial germination and development of powdery mildew fungi on plants. In the present study, we have determined the impact of SO2, a common air pollutant originating from the burning of fossil fuels, particularly in thermal power plants, in India, on conidial germination of some powdery mildew fungi in artificial treatment conditions. MATERIALS A N D METHODS
Powdery mildew fungi Eight powdery mildew species were tested in the study listed below against the host from which they were collected. Powdery mildew
Sphaerotheca fuliginea (Schlecht.) Poll.
Erysiphe cichoracearum DC. Sphaerotheca cassiae Pandotra & Ganguly Erysiphe trifolii Grev. Erysiphe pisi DC. Erysiphe polygoni DC.
Microsphaera alphitoides Griff & Maubl. f.sp. zizyphi Phyllactinia dalbergiae
Host
Lagenaria siceraria (Molina) Stand.
Coccinia grandis Voight. Cassia occidental& L. Trigonella foenum-graecum L. Pisum stivum L. Chenopodium ambrosoides L. Zizyphus jujuba Lam. Dalbergia sisoo Roxb.
Piroz.
S02 exposure equipment The SO2 exposure chamber (Standard Appliances, Varanasi, India) was 90 x 90 × 120cm in size and made of transparent fibre glass. A micro-gas
Conidial germination of powdery mildewfungi
83
EXHAUST-~', DUCT
:
90 cm
ooo___R
* o o ,
o*
*
o
*
PERFORATED BOTTOM PLATE
t CONTROL PANEL
Fig. 1. Exposure chamber. exposure cabinet was also used in the study. It had an exhaust duct at the top and a double walled bottom. The upper side of the bottom was provided with openings and the lower was equipped with a blower assembly. A fumigation controller regulated the voltage supply to the blower and was displayed on a panel meter (Fig. 1). Sulphur dioxide was produced in a generator, by the reaction of sulphuric acid (10% H2SO4) on sodium sulphite (Na2SO3). Bottles of Na2SO 3 and HESO 4 were mounted over the SO2 generator. For calibration, the volume of solution discharged from the bottle was determined by collecting the solution dropping through capillary tube into a graduated cylinder and expressing the rate in ml m i n - 1. On the basis of flow rate or solution feeding rate, solutions of N a 2 S O 3 and H2SO 4 (10%) were prepared to produce the required amount of SO 2 gas/min. On complete reaction, 1M Na2SO 3 produces 1M SO2 or 126mg Na2SO3 produces 64mg SO2. Na2SO 3 -t- H2SO 4
~ SO 2 + NaESO 4 + H20
The concentration of SO2 in the chamber was also determined by sampling the air using a portable air sampler (Kimoto Electronics, Japan) and was analysed by spectrophotometry (Anon. 1986). The outlet of the gas generator was connected to the gas inlet nozzles provided in the blower housing of the exposure chamber.
84
M. Wajid Khan, Madhu Kuishrestha STERILE AIR
-~
3sc,...°
///
"
",
Fig. 2. Micro-gas exposure cabinet.
The micro-gas exposure cabinet (length 35 and width 25 cm) was also made of transparent fibre glass and had an inlet hole at one side which was connected with the air supply device by a plastic tube, which took the exposure chamber air containing SOz and after filtering, released it into the micro-exposure cabinet. The bottom of the cabinet had a small outlet perforation for release of the gases (Fig. 2).
Experimental procedure Conidia from fresh samples of leaves infected with powdery mildew species were dusted onto dry clean glass slides by gently tapping the leaves. The slides were exposed to 286 ( _ 68.57) and 571 ( __+26,0) #g m - 3 of SO2 for 3, 6 or 9 h in separate experiments. For exposures, the slides were kept in the micro-gas exposure cabinet and the whole system (micro-gas cabinet with the slides + sterile air supply device) was placed inside the larger exposure chamber. The outlet of the gas generator was connected to the gas inlet nozzles and the generator was started. Exposures were continued at requisite concentrations of SOz for the desired periods, according to the treatments. Air flow inside the chamber was 2"0-2.15 m s - 1. Control slides dusted with conidia kept in micro-gas exposure cabinet were exposed to ambient air inside the air pollutant exposure chamber. The SO2 concentration in the ambient air was 11.72/~g m -3. Each treatment, including the control, consisted of three replicate slides. At the termination of exposure, slides, including the controls, were placed on a glass triangle and kept in Petri dishes containing sterilized distilled water at the b o t t o m and with an upper lid lined with moistened cotton wool. The Petri dishes with the slides were incubated at 20°C for 36 h. The percent germinated conidia in each treatment was then determined by examining 100 conidia from each replicate slide selected at random under the microscope. Conidia showing germ tube initiation or a well developed germ tube were taken as germinated. The percent
Conidial germination of powdery mildewfungi
85
germination values were transformed into angular values and a two way analysis of variance (ANOVA) was used to determine the effects of SO2 concentration on conidial germination, contrasted against the controls, and variation between the powdery mildew fungi at both the concentrations. RESULTS At the concentrations used, SO2 inhibited conidial germination of all the powdery mildew fungi tested (Table 1). Conidial germination was significantly (P < 0-05 or P < 0.01) inhibited after exposure to SO 2 at 571 ~g m -3 and 286pg m -3 for 3, 6 and 9h, compared with controls kept in ambient air, and in general the inhibition was greater for the higher (571/~g m - 3) concentration. At both concentrations, an increase in the duration of exposure caused increased inhibition. Greatest inhibition occurred at the 571/~g m -3 concentration with an exposure period of 12h. These effects were consistent for all the powdery mildew fungi. The data showed that the concentration of SO2 and the exposure duration were important determinants of the inhibition. For example, when the conidia of S.fuliginea were exposed to 286 pg m - 3 SO2 for 6 h, the percent germination was 35-5, but with 571 pg m - 3 SO2 for 3 h exposure, the percent germination was 27.7. This variation was significant (P<0"05) which showed that the concentration of SO 2 was the more important determinant of the inhibitory effect. In contrast, in E. pisi, E. polygoni, M. alphitoides f. sp. zizyphi and P. dalbergiae significantly ( P < 0-05 or P<0"01) greater inhibition occurred with 571 pg m - 3 SO2 for 3 h exposure period than with 286 pg m -- 3 SO2 for 6 h exposure period. This indicated a greater importance of the duration of exposure to SO2 than the SO 2 concentration. In E. tr(/blii and S. cassiae the percent germination obtained either with 571/~g m -3 for 3h exposure period or with 286/~g m - 3 for 6 h exposure period did not differ significantly.
DISCUSSION The present study considered the impact of SO 2 exposure on conidial germination of eight powdery mildew fungi belonging to four genera Sphaerotheca, Erysiphe, Microsphaera and Phylactinia--commonly appearing on a number of host plants. Other aspects of their pathogenesis were not considered. All the studied powdery mildew fungi were substantially sensitive to SO2 but did not differ much from each other in their response to SO2 in relation to conidial germination. The good repeatability of inhibition shown by various powdery mildews is a confirmation of sensitivity of conidia to SO2.
M. Wajid Khan, Madhu Kulshrestha
86
~
0
~.
p..
x
~-.
,2.
~
~
,:.,
~
,:.,
~
© r~ ~
X
~
×
0 e., 0
tt% tt% c,q
~
> 0 e~o
X 0
0 x
tt%
~=
o
.o
Conidial germination of powdery mildewfungi
87
Within the range of S O 2 concentrations tested their tolerance potential to SO 2 is apparently alike regardless of genera or species. As conidia are the source inoculum for the primary and/or secondary disease cycles of the powdery mildews, if such an inhibitory effect of SO2 is occurring as a normal p h e n o m e n o n at ambient levels of SO2, disease severity in field conditions would be affected. Some observations (Kock, 1935; Hibben & Walker, 1966) made earlier in relation to the occurrence of powdery mildews in polluted areas confirm this is so. Some other studies using artificial SO2 exposure have shown similar effects (Hibben & Taylor, 1975). Ozone (O3), another important air pollutant, causes inhibition of Erysiphe graminis on barley (Heagle & Strickland, 1972). When conidial germination is affected adversely the inoculum potential of the pathogens will be greatly reduced and subsequently disease development will be poor. Ambient concentration of SO 2 around a thermal power plant at Kasimpur, 15 km away from the University Campus, was found to be 222.86 ( _ 54"28) pg m - 3 at 1 km and 520 (_+68.57)#g m - 3 at 2 k m distance from the stack in the windward direction (Khan, 1988). These concentrations of SO 2 in a polluted area around a thermal power plant are very close to the concentrations used in this study. The inhibitory effects of SO2 demonstrated would be significant under field conditions, if such high concentrations of SO 2 occur in any SO 2 polluted area. A m o n g the powdery mildew fungi studied, S.fuliginea is more important economically than the others. It attacks cucurbits, legumes, composites and several other plants which are mostly annuals. E. pisi has similar plants in its host index. Such plants are very sensitive to air pollutants, which affect their physiological and biochemical processes and result in a reduced productivity (Heck et al., 1982). Therefore, cultivation of such annuals around the point source of SO2 may not be desirable even if the disease is controlled partially or substantially. The tree hosts like Dalbergia sisoo and Zizyphus jujuba are unlikely to be as sensitive to ambient SO 2 pollution, particularly at lower ambient levels. If these hosts can remain free of powdery mildews in a polluted area, this would be advantageous for host growth. SO 2 inactivates the sulphydryl groups of enzymes and inhibits acid phosphatase activity (Saunders, 1966). Sulphur, as a dust or a wettable powder, is a well-established chemical to control powdery mildew fungi in general. SO2 might have inactivated the process of germination by affecting it at a biochemical level. SO2 also causes adverse effects on a number of plants including agricultural crops (Heck et al., 1982). Since powdery mildews are exposed to ambient air, SO 2 may affect them directly or indirectly through their hosts, causing poor growth. The direct effect of SO 2 on conidial germination has been demonstrated in this study, but the exact mechanism needs to be investigated.
88
M. Wajid Khan, Madhu Kulshrestha REFERENCES
Anon. (1986). Air Quality Monitoring. A Course Manual. National Environmental Engineering Research Institute, Nagpur, India. Heagle, A. S. (1973). Interaction between air pollutants and plant parasites. Annual Review of Phytopathology, 11, 365-88. Heagle, A. S. (1982). Interaction between air pollutants and parasitic plant diseases. In Effects of Gaseous Air Pollution in Agriculture, ed. H. M. Unsworth & D. P. Ormrod, Butterworth Scientific, London. Heagle, A. S. & Strickland, A. (1972). Reaction of Erysiphe graminis f. sp. hordei to low levels of 0 3. Phytopathology, 62, 1144-8. Heck, W. W., Taylor, D. C., Adams, R. M., Bingham, G. & Miller, J. E. (1982). Assessment of crop loss from ozone. Journal of Air Pollution Control Association, 32, 353-61. Hibben, C. R. & Taylor, M. P. (1975). Ozone and SO2 effects on the lilac powdery mildew fungus. Environmental Pollution (A), 9, 107-14. Hibben, C. R. & Walker, J. T. (1966). A leaf roll necrosis complex of lilacs in an urban environment. American Society of Horticultural Sciences, 89, 33942. Khan, M. R. (1988). Studies on root-knot nematodes in relation to environmental pollution on some vegetable crops. PhD thesis Aligarh Muslim University, Aligarh, India. p. 268. Kock, G. (1935). Eichen mehltau und Rauchg asschaden (Oat mildew and smoke gas injury) Zeitschrift fur Pflanzenkrankheitung, 65, 44-55 (Review of Applied Mycology, 14, 406-7). Krieg, W. & Knosel, D. (1983). Modellversuche zur Wirkung von Schwefeldioxid auf pflanzliche Mykosen. Angewandte Botanik, 57, 19-30. Saunders, P. J. W. (1966). The toxicity of SO 2 to Diplocarpon rosae Wolf causing black spot of roses. Annals of Applied Biology, 58, 103-14.