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Effects of Temperature, Humidity and Different Rice Growth Stages on Vertical Migration of Aphelenchoides besseyi
Rice Science, 2009, 16(4): 301–306 Copyright © 2009, China National Rice Research Institute. Published by Elsevier BV. All rights reserved DOI: 10.1016/S1672-6308(08)60094-3
Effects of Temperature, Humidity and Different Rice Growth Stages on Vertical Migration of Aphelenchoides besseyi SUN Min-jie1, LIU Wei-hong2, LIN Mao-song1 (1Plant Protection College, Nanjing Agricultural University, Nanjing 210095, China; 2Jiangdu Agricultural Technology Extension and Service Centre, Jiangsu 225200, China)
Abstract: The vertical migration of Aphelenchoides besseyi under different temperatures and humidities and at different rice growth stages was investigated. It was found that the optimum temperature for the development and reproduction of A. besseyi was 25–30°C. At the same temperature, the rate of vertical migration increased with rising relative humidity. Artificial inoculation tests showed that at the elongation stage, nematodes survived mainly on the upper and middle parts of rice culms, and the number of nematodes decreased by 50% at 20 days after inoculation compared with that at 5 days after inoculation. Whereas at the booting stage, nematodes accumulated in young panicles and reproduced quickly, and the average number of nematodes at 20 days after inoculation increased to 164.5, three times of that at 5 days after inoculation. Key words: Aphelenchoides besseyi; rice; growth stage; temperature; humidity; vertical migration; distribution
Aphelenchoides besseyi is an important seed-borne ectoparasite of rice and is disseminated primarily through infected seeds. The characteristic symptoms are whitening of the leaf tips, which later become necrotic, and a crinkling and distortion of the flag leaf enclosing the panicle [1-2]. In the middle and northern Jiangsu Province and some regions of southern Jiangsu Province, China, the occurrence of abnormal rice with small grains and erect panicles (SGP) caused by A. besseyi was serious in recent years [3]. The rice plants infected by A. besseyi show stunting of plant and late maturity, several centimeters of whitened leaf tips, necrosis, distortion and crinkling of flag leaf, reduction in panicle development and a brown stripe at the boundary of diseased and healthy tissues. Simultaneously, yield and quality of rice are strongly influenced. It generally causes yield loss ranging from 10% to 30%, even as high as 50% in heavily infected regions [4-6]. It was reported that the occurrence of SGP caused by A. besseyi in rice occupied 3.3×105 hm2 during 2001–2003 in Jiangsu Province, China, which resulted in at least 500 million kilogram yield loss [7]. A. besseyi can infect rice under all rice production
environments [8]. At present, the effective measures to control SGP caused by A. besseyi are applications of cartap and 16% prochloraz cartap nematicides. However, Prot and Gergon [9] found that these nematicides are ineffective when applied to flood water around plants. In order to prevent the recurrence of SGP in Jiangsu Province, China, to identify the role and the pathogenesis of A. besseyi, and to establish a more effective management for SGP, a study was conducted to determine the effects of different temperatures and humidities on the vertical migration and reproduction of A. besseyi at different rice growth stages.
MATERIALS AND METHODS Isolation of A. besseyi A. besseyi was isolated from rice with SGP symptom by the Baermann funnel technique [2, 5, 10]. The isolates were identified, sterilized with 3% H2O2 for 10 min, washed with double-distilled water for three times, inoculated onto the fungal (Botrytis cinerea) plates and then cultured at 25°C for 30 days. After that, it was stored in a refrigerator at 4°C [11-13]. Rice growth in greenhouse
Received: 24 April 2009; Accepted: 3 July 2009 Corresponding author: SUN Min-jie ([email protected]) This is an English version of the paper published in Chinese in Chinese Journal of Rice Science, Vol. 23, No. 3, 2009, Pages 304–308.
Healthy seeds of rice variety Wuyunjing 3 were used for inoculation experiments. One hundred rice
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seeds were selected at random and subjected to nematode extraction (replicated three times) and no nematodes were found. Rice seeds were sterilized by soaking in 0.1% mercuric chloride solution for 5 min. Fifteen-day-old rice seedlings were transplanted singly into pots containing steam-sterilized sandy loam soil. Effects of different temperatures, humidities on the vertical migration of A. besseyi Rice plants with 5-mm-diameter culm were placed in an upright position in a Syracuse dish filled with melted paraffin wax [14]. Nematode suspension with 200 mixed nematodes in one milliliter of sterile distilled water was dropped around each culm. The vertical migration of A. besseyi was investigated at 10°C, 20°C, 25°C, 30°C and 40°C (±1.0°C) in a growth chamber with 100% relative humidity (RH). Also, the vertical migration of A. besseyi was studied at different humidities of 50%, 80% and 100% (±5%) in a growth chamber at 25°C. Each treatment was replicated five times. RH was measured with a wet and dry bulb thermometer. After treated for 20 h, the culm above the suspension was cut for determining the number of nematodes. Vertical and horizontal migrations of A. besseyi
Laboratory inoculation for horizontal migration assay Rice culms about 60 mm long sampled at the booting stage were wrapped with moist gauze and horizontally placed in a growth chamber at 100% RH and 25°C. The middle parts of rice culms were inoculated with nematodes at a concentration of 400 nematodes per culm. The inoculation part was referred as 1 cm left and 1 cm right of the inoculation point, and the other parts were considered as the two ends. Rice culms were sampled at random to determine the number of nematodes every 5 days after inoculation with three replications. Measurement for number of nematodes Rice samples were cut into small pieces, immersed in water on a Baermann funnel apparatus at 25°C and examined for A. besseyi after 24 h, counting the number of nematodes under a dissecting microscope [15]. Statistical analysis DPS statistical software was used for data processing, computing mean and standard deviation and for comparing the significance of differences.
RESULTS
Greenhouse inoculation for vertical migration assay Rice plants at the elongation and booting stages were selected. The middle parts of the rice culms (at a height of 15 cm for the plants at the elongation stage, and at a height of 25 cm for those at the booting stage) were cut open. The cultured A. besseyi at a concentration of 400 nematodes per culm was directly injected into the small cut with a pipette. Three inoculation treatments of rice culms were used: 1) at the elongation stage; 2) at the booting stage; 3) at the booting stage with panicles removed. Each treatment included 50 rice seedlings with three replications. After inoculated, ten plants were sampled every 5 days at random to determine the number of nematodes present. To assess the vertical migration of A. besseyi, the inoculated central pieces of each rice culm were divided into three parts, namely (i) middle part, from 2.5 cm below the inoculation point to 2.5 cm above this point; (ii) upper part, the culm above the middle part and (iii) lower part, the culm below the middle part.
Effects of different temperatures on vertical migration of A. besseyi In the case of 100% RH, the numbers of A. besseyi migrated were 10.8 and 9.0 at 25°C and 30°C, respectively, showing no significant difference. At the range of 10–25°C, the migration of nematodes increased with rising temperature. When the temperature exceeded 30°C, the number of migrated nematodes declined quickly. When the temperature was lower than 15°C or higher than 40°C, nematodes stopped migrating (Fig. 1). Too high or too low temperature can inhibit the nematode activity and reduce its migration. It is shown that 25–30°C is the optimum temperature for the vertical migration of A. besseyi at 100% RH. Effects of different relative humidities on vertical migration of A. besseyi At 25°C, the numbers of migrated nematodes were
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1.4, 4.6 and 11.4 at 50%, 80% and 100% RH, respectively (Fig. 2). In the case of 80% RH, the migration of nematodes was only 40% of that at 100% RH. The migration of nematodes at 50% RH showed a significant difference from those at 80% and 100% RHs. This result shows that the migration of nematodes is sensitive to humidity. At the optimum temperature, the rate of vertical migration increases with the rising relative humidity. Reproduction and distribution of A. besseyi in plants at different rice growth stages
Fig. 1. Migration of A. besseyi under different temperatures. The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
Reproduction of A. besseyi in the plants at different rice growth stages The reproduction of A.besseyi at different rice growth stages is shown in Fig. 3. At the elongation stage, the average number of nematodes extracted from the rice samples was 40.2 at 5 days after inoculation, which was significantly fewer than the number when inoculated (400 nematodes per culm). With the passage of time, the average number of nematodes decreased, and it was only 15.4 at 20 days after inoculation. Whereas at the booting stage, the average number of living nematodes was 51.0 at 5 days after inoculation, significantly higher than that at the elongation stage (40.2). The number of nematodes increased by 37.3% at 10 days after inoculation, and it increased to 164.5 at 20 days after inoculation, three times higher than that at 5 days after inoculation. Moreover, a large proportion of the nematode population was juveniles. No difference was found between the number of nematodes at the booting stage with panicles removed and at the elongation stage. Distribution of A. besseyi in the plants at different rice growth stages As shown in Fig. 4-A, A. besseyi survived in the upper, middle and lower parts of the rice culms at 5 daysafter inoculation for the plants at the elongation stage. Most nematodes were present in the middle parts with an average of 18.4, followed by the upper parts with an average of 16.6 and the lower parts with an average of 5.2. The numbers of nematodes on upper and middle parts showed no significant difference from each other, whereas that on lower
Fig. 2. Migration of A. besseyi under different humidities. The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
Fig. 3. Reproduction of A. besseyi at differdent rice growth stages. The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
parts was significantly different from the other two parts. At 5 days after inoculation for the plants at the booting stage (Fig. 4-B), most nematodes were found in the panicles (41.5), and a few were distributed in the leaf sheaths (3.25). The average number of nematodes in the upper parts (44.75) was significantly higher than those in the other two parts. The average number of nematodes in the middle parts was only 6 and no
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at the elongation stage. The number of nematodes was the highest in the middle parts, followed by the upper and lower parts (Fig. 4-C). The results indicated that the reproduction and distribution of A. besseyi in the plants at the elongation stage and at the booting stage with panicles removed showed no significant differences. The number of nematodes decreased as rice grew and fewer juveniles were found. Most nematodes survived on the upper and middle parts of culms. This suggests that the migration of nematodes influences their distribution. However, nematodes reproduced quickly and mainly gathered in panicles at the rice booting stage when panicles were not removed. A large proportion of the nematode population was juveniles, accounted for 70% of the total. Therefore, the distribution of A. besseyi in the plants at the booting stage is not only related to the migration of nematodes, but also to their reproduction. Horizontal migration of A. besseyi on rice culms
Fig. 4. Distribution of A. besseyi in plants after inoculation at the elongation stage (A), booting stage (B) and booting stage with panicle removed (C). The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
nematodes were found in the lower parts. At 20 days after inoculation for the plants at the booting stage, the number of nematodes in the upper parts increased to 164.5, being 3.6 times more than that at 5 days after inoculation. Few nematodes were found in the leaf sheaths (2.25), and no nematodes were in the leaves. The distribution of nematodes in the plants at the booting stage with panicle removed was similar to that
As shown in Table 1, at 5 days after inoculation, the average number of living nematodes was 82.4 in the culms at the booting stage. Additionally, the average numbers of nematodes in the inoculation part, right end of culms (next to panicle) and left end of culms were 16.2, 62.4 and 3.2, respectively.At the same time, a total of 70.4 nematodes were found on the rice culms with panicle removed, among them, 42.4 nematodes were near the inoculation part and 13.2 and 14.8 nematodes were on the two ends, respectively (Table 1). The horizontal migration of A. besseyi on the rice culms with panicle removed indicates that A. besseyi is most abundant in the inoculation part, and there was no significant difference between the two sides of culms.
DISCUSSION According to the experiment, temperature and humidity are two important factors that affect the
Table 1. Horizontal migration of A. besseyi on rice culms at the booting stage at 5 days after inoculation. Treatment Culm with panicle Culm with panicle removed
Number of nematodes on the left end of culm 3.2±1.1 c 13.2±1.6 b
Number of nematodes on the inoculation part
Number of nematodes on the right end of culm (next to panicle)
Total number of nematodes
16.2±2.8 b 42.4±3.4 a
62.4±3.4 a 14.8±1.9 b
82.4±1.6 70.4±1.8
Within a row, data followed by the common lowercase letters represent no significant difference at P=0.05 levels (n=5).
SUN Ming-jie, et al. Effects of Temperature, Humidity and Different Rice Growth Stages on Vertical Migration of A. besseyi
vertical migration of A. besseyi. Higher and lower temperature, as well as low relative humidity can restrain the vertical migration of A. besseyi. This result is in line with the related reports in which the most suitable temperature is 25°C and RH is more than 70% for the migration of A. besseyi, and the saturated humidity will promote its activity [16-18]. Besides, Jagdale et al [12] reported that when RH reached 100%, the livability and activity of A. fragariae were higher than those at 90% RH. Rainy summers with high temperature in Jiangsu Province, China facilitate outbreaks of SGP abnormality. In the experiments of inoculating nematodes to the plants at the elongation stage and at the booting stage with panicle removed, the nematodes prefer to move upward rather than downward. This might be associated with the behavior of A. besseyi. The horizontal migration of A. besseyi on rice culms further proved that migration upward was a behavioral feature. The numbers of nematodes in the two above treatments were different from that at the booting stage. It might be related to the reproduction of nematodes in rice panicles. The growth and reproduction of nematodes need nutrition from panicles. The nutrition transfers to panicles during rice booting period, and more and more soluble sugar concentrates on panicles, which provides abundant nutrition for the reproduction of nematodes. However, what are the main factors that induce nematodes to migrate to panicles? Further study is necessary to find out whether the migration of A. besseyi is caused by exclusive nutrition, or is related to the stimulation by plant hormones during the rice booting stage. A. besseyi is transmitted primarily through infested seed, so seed disinfestation is the most effective way to prevent A. besseyi from spreading. The infestation of A. besseyi is widely distributed and influences various rice varieties in Jiangsu Province, China at present. It could be controlled by applications of 90% cartap, 16% prochloraz cartap, 40% ethyl acetate, isofenphos-methyl nematicides and so on [3-4, 6-7]. However, Prot et al [9] found that the chemical treatment is only partially effective when applied to flood water. Ou [19] believed that irrigation water containing nematodes or application of infested rice hulls on seedbed would make healthy seed be infected by A. besseyi. Good
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control effectiveness can be achieved by seed treatment in association with spraying nematicides during the rice booting stage. In conclusion, strengthening the research on the behavior of A. besseyi has great significance for understanding the pathogenesis of A. besseyi and for exploring new ways for the prevention and treatment of A. besseyi.
ACKNOWLEDGEMENT This work was supported by the National High Technology Research and Development Program of China (Grant No. 2001AA249021).
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Effects of Temperature, Humidity and Different Rice Growth Stages on Vertical Migration of Aphelenchoides besseyi SUN Min-jie1, LIU Wei-hong2, LIN Mao-song1 (1Plant Protection College, Nanjing Agricultural University, Nanjing 210095, China; 2Jiangdu Agricultural Technology Extension and Service Centre, Jiangsu 225200, China)
Abstract: The vertical migration of Aphelenchoides besseyi under different temperatures and humidities and at different rice growth stages was investigated. It was found that the optimum temperature for the development and reproduction of A. besseyi was 25–30°C. At the same temperature, the rate of vertical migration increased with rising relative humidity. Artificial inoculation tests showed that at the elongation stage, nematodes survived mainly on the upper and middle parts of rice culms, and the number of nematodes decreased by 50% at 20 days after inoculation compared with that at 5 days after inoculation. Whereas at the booting stage, nematodes accumulated in young panicles and reproduced quickly, and the average number of nematodes at 20 days after inoculation increased to 164.5, three times of that at 5 days after inoculation. Key words: Aphelenchoides besseyi; rice; growth stage; temperature; humidity; vertical migration; distribution
Aphelenchoides besseyi is an important seed-borne ectoparasite of rice and is disseminated primarily through infected seeds. The characteristic symptoms are whitening of the leaf tips, which later become necrotic, and a crinkling and distortion of the flag leaf enclosing the panicle [1-2]. In the middle and northern Jiangsu Province and some regions of southern Jiangsu Province, China, the occurrence of abnormal rice with small grains and erect panicles (SGP) caused by A. besseyi was serious in recent years [3]. The rice plants infected by A. besseyi show stunting of plant and late maturity, several centimeters of whitened leaf tips, necrosis, distortion and crinkling of flag leaf, reduction in panicle development and a brown stripe at the boundary of diseased and healthy tissues. Simultaneously, yield and quality of rice are strongly influenced. It generally causes yield loss ranging from 10% to 30%, even as high as 50% in heavily infected regions [4-6]. It was reported that the occurrence of SGP caused by A. besseyi in rice occupied 3.3×105 hm2 during 2001–2003 in Jiangsu Province, China, which resulted in at least 500 million kilogram yield loss [7]. A. besseyi can infect rice under all rice production
environments [8]. At present, the effective measures to control SGP caused by A. besseyi are applications of cartap and 16% prochloraz cartap nematicides. However, Prot and Gergon [9] found that these nematicides are ineffective when applied to flood water around plants. In order to prevent the recurrence of SGP in Jiangsu Province, China, to identify the role and the pathogenesis of A. besseyi, and to establish a more effective management for SGP, a study was conducted to determine the effects of different temperatures and humidities on the vertical migration and reproduction of A. besseyi at different rice growth stages.
MATERIALS AND METHODS Isolation of A. besseyi A. besseyi was isolated from rice with SGP symptom by the Baermann funnel technique [2, 5, 10]. The isolates were identified, sterilized with 3% H2O2 for 10 min, washed with double-distilled water for three times, inoculated onto the fungal (Botrytis cinerea) plates and then cultured at 25°C for 30 days. After that, it was stored in a refrigerator at 4°C [11-13]. Rice growth in greenhouse
Received: 24 April 2009; Accepted: 3 July 2009 Corresponding author: SUN Min-jie ([email protected]) This is an English version of the paper published in Chinese in Chinese Journal of Rice Science, Vol. 23, No. 3, 2009, Pages 304–308.
Healthy seeds of rice variety Wuyunjing 3 were used for inoculation experiments. One hundred rice
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seeds were selected at random and subjected to nematode extraction (replicated three times) and no nematodes were found. Rice seeds were sterilized by soaking in 0.1% mercuric chloride solution for 5 min. Fifteen-day-old rice seedlings were transplanted singly into pots containing steam-sterilized sandy loam soil. Effects of different temperatures, humidities on the vertical migration of A. besseyi Rice plants with 5-mm-diameter culm were placed in an upright position in a Syracuse dish filled with melted paraffin wax [14]. Nematode suspension with 200 mixed nematodes in one milliliter of sterile distilled water was dropped around each culm. The vertical migration of A. besseyi was investigated at 10°C, 20°C, 25°C, 30°C and 40°C (±1.0°C) in a growth chamber with 100% relative humidity (RH). Also, the vertical migration of A. besseyi was studied at different humidities of 50%, 80% and 100% (±5%) in a growth chamber at 25°C. Each treatment was replicated five times. RH was measured with a wet and dry bulb thermometer. After treated for 20 h, the culm above the suspension was cut for determining the number of nematodes. Vertical and horizontal migrations of A. besseyi
Laboratory inoculation for horizontal migration assay Rice culms about 60 mm long sampled at the booting stage were wrapped with moist gauze and horizontally placed in a growth chamber at 100% RH and 25°C. The middle parts of rice culms were inoculated with nematodes at a concentration of 400 nematodes per culm. The inoculation part was referred as 1 cm left and 1 cm right of the inoculation point, and the other parts were considered as the two ends. Rice culms were sampled at random to determine the number of nematodes every 5 days after inoculation with three replications. Measurement for number of nematodes Rice samples were cut into small pieces, immersed in water on a Baermann funnel apparatus at 25°C and examined for A. besseyi after 24 h, counting the number of nematodes under a dissecting microscope [15]. Statistical analysis DPS statistical software was used for data processing, computing mean and standard deviation and for comparing the significance of differences.
RESULTS
Greenhouse inoculation for vertical migration assay Rice plants at the elongation and booting stages were selected. The middle parts of the rice culms (at a height of 15 cm for the plants at the elongation stage, and at a height of 25 cm for those at the booting stage) were cut open. The cultured A. besseyi at a concentration of 400 nematodes per culm was directly injected into the small cut with a pipette. Three inoculation treatments of rice culms were used: 1) at the elongation stage; 2) at the booting stage; 3) at the booting stage with panicles removed. Each treatment included 50 rice seedlings with three replications. After inoculated, ten plants were sampled every 5 days at random to determine the number of nematodes present. To assess the vertical migration of A. besseyi, the inoculated central pieces of each rice culm were divided into three parts, namely (i) middle part, from 2.5 cm below the inoculation point to 2.5 cm above this point; (ii) upper part, the culm above the middle part and (iii) lower part, the culm below the middle part.
Effects of different temperatures on vertical migration of A. besseyi In the case of 100% RH, the numbers of A. besseyi migrated were 10.8 and 9.0 at 25°C and 30°C, respectively, showing no significant difference. At the range of 10–25°C, the migration of nematodes increased with rising temperature. When the temperature exceeded 30°C, the number of migrated nematodes declined quickly. When the temperature was lower than 15°C or higher than 40°C, nematodes stopped migrating (Fig. 1). Too high or too low temperature can inhibit the nematode activity and reduce its migration. It is shown that 25–30°C is the optimum temperature for the vertical migration of A. besseyi at 100% RH. Effects of different relative humidities on vertical migration of A. besseyi At 25°C, the numbers of migrated nematodes were
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1.4, 4.6 and 11.4 at 50%, 80% and 100% RH, respectively (Fig. 2). In the case of 80% RH, the migration of nematodes was only 40% of that at 100% RH. The migration of nematodes at 50% RH showed a significant difference from those at 80% and 100% RHs. This result shows that the migration of nematodes is sensitive to humidity. At the optimum temperature, the rate of vertical migration increases with the rising relative humidity. Reproduction and distribution of A. besseyi in plants at different rice growth stages
Fig. 1. Migration of A. besseyi under different temperatures. The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
Reproduction of A. besseyi in the plants at different rice growth stages The reproduction of A.besseyi at different rice growth stages is shown in Fig. 3. At the elongation stage, the average number of nematodes extracted from the rice samples was 40.2 at 5 days after inoculation, which was significantly fewer than the number when inoculated (400 nematodes per culm). With the passage of time, the average number of nematodes decreased, and it was only 15.4 at 20 days after inoculation. Whereas at the booting stage, the average number of living nematodes was 51.0 at 5 days after inoculation, significantly higher than that at the elongation stage (40.2). The number of nematodes increased by 37.3% at 10 days after inoculation, and it increased to 164.5 at 20 days after inoculation, three times higher than that at 5 days after inoculation. Moreover, a large proportion of the nematode population was juveniles. No difference was found between the number of nematodes at the booting stage with panicles removed and at the elongation stage. Distribution of A. besseyi in the plants at different rice growth stages As shown in Fig. 4-A, A. besseyi survived in the upper, middle and lower parts of the rice culms at 5 daysafter inoculation for the plants at the elongation stage. Most nematodes were present in the middle parts with an average of 18.4, followed by the upper parts with an average of 16.6 and the lower parts with an average of 5.2. The numbers of nematodes on upper and middle parts showed no significant difference from each other, whereas that on lower
Fig. 2. Migration of A. besseyi under different humidities. The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
Fig. 3. Reproduction of A. besseyi at differdent rice growth stages. The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
parts was significantly different from the other two parts. At 5 days after inoculation for the plants at the booting stage (Fig. 4-B), most nematodes were found in the panicles (41.5), and a few were distributed in the leaf sheaths (3.25). The average number of nematodes in the upper parts (44.75) was significantly higher than those in the other two parts. The average number of nematodes in the middle parts was only 6 and no
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at the elongation stage. The number of nematodes was the highest in the middle parts, followed by the upper and lower parts (Fig. 4-C). The results indicated that the reproduction and distribution of A. besseyi in the plants at the elongation stage and at the booting stage with panicles removed showed no significant differences. The number of nematodes decreased as rice grew and fewer juveniles were found. Most nematodes survived on the upper and middle parts of culms. This suggests that the migration of nematodes influences their distribution. However, nematodes reproduced quickly and mainly gathered in panicles at the rice booting stage when panicles were not removed. A large proportion of the nematode population was juveniles, accounted for 70% of the total. Therefore, the distribution of A. besseyi in the plants at the booting stage is not only related to the migration of nematodes, but also to their reproduction. Horizontal migration of A. besseyi on rice culms
Fig. 4. Distribution of A. besseyi in plants after inoculation at the elongation stage (A), booting stage (B) and booting stage with panicle removed (C). The common lowercase letters above the bars represent no significant difference at P=0.05 level (n=5).
nematodes were found in the lower parts. At 20 days after inoculation for the plants at the booting stage, the number of nematodes in the upper parts increased to 164.5, being 3.6 times more than that at 5 days after inoculation. Few nematodes were found in the leaf sheaths (2.25), and no nematodes were in the leaves. The distribution of nematodes in the plants at the booting stage with panicle removed was similar to that
As shown in Table 1, at 5 days after inoculation, the average number of living nematodes was 82.4 in the culms at the booting stage. Additionally, the average numbers of nematodes in the inoculation part, right end of culms (next to panicle) and left end of culms were 16.2, 62.4 and 3.2, respectively.At the same time, a total of 70.4 nematodes were found on the rice culms with panicle removed, among them, 42.4 nematodes were near the inoculation part and 13.2 and 14.8 nematodes were on the two ends, respectively (Table 1). The horizontal migration of A. besseyi on the rice culms with panicle removed indicates that A. besseyi is most abundant in the inoculation part, and there was no significant difference between the two sides of culms.
DISCUSSION According to the experiment, temperature and humidity are two important factors that affect the
Table 1. Horizontal migration of A. besseyi on rice culms at the booting stage at 5 days after inoculation. Treatment Culm with panicle Culm with panicle removed
Number of nematodes on the left end of culm 3.2±1.1 c 13.2±1.6 b
Number of nematodes on the inoculation part
Number of nematodes on the right end of culm (next to panicle)
Total number of nematodes
16.2±2.8 b 42.4±3.4 a
62.4±3.4 a 14.8±1.9 b
82.4±1.6 70.4±1.8
Within a row, data followed by the common lowercase letters represent no significant difference at P=0.05 levels (n=5).
SUN Ming-jie, et al. Effects of Temperature, Humidity and Different Rice Growth Stages on Vertical Migration of A. besseyi
vertical migration of A. besseyi. Higher and lower temperature, as well as low relative humidity can restrain the vertical migration of A. besseyi. This result is in line with the related reports in which the most suitable temperature is 25°C and RH is more than 70% for the migration of A. besseyi, and the saturated humidity will promote its activity [16-18]. Besides, Jagdale et al [12] reported that when RH reached 100%, the livability and activity of A. fragariae were higher than those at 90% RH. Rainy summers with high temperature in Jiangsu Province, China facilitate outbreaks of SGP abnormality. In the experiments of inoculating nematodes to the plants at the elongation stage and at the booting stage with panicle removed, the nematodes prefer to move upward rather than downward. This might be associated with the behavior of A. besseyi. The horizontal migration of A. besseyi on rice culms further proved that migration upward was a behavioral feature. The numbers of nematodes in the two above treatments were different from that at the booting stage. It might be related to the reproduction of nematodes in rice panicles. The growth and reproduction of nematodes need nutrition from panicles. The nutrition transfers to panicles during rice booting period, and more and more soluble sugar concentrates on panicles, which provides abundant nutrition for the reproduction of nematodes. However, what are the main factors that induce nematodes to migrate to panicles? Further study is necessary to find out whether the migration of A. besseyi is caused by exclusive nutrition, or is related to the stimulation by plant hormones during the rice booting stage. A. besseyi is transmitted primarily through infested seed, so seed disinfestation is the most effective way to prevent A. besseyi from spreading. The infestation of A. besseyi is widely distributed and influences various rice varieties in Jiangsu Province, China at present. It could be controlled by applications of 90% cartap, 16% prochloraz cartap, 40% ethyl acetate, isofenphos-methyl nematicides and so on [3-4, 6-7]. However, Prot et al [9] found that the chemical treatment is only partially effective when applied to flood water. Ou [19] believed that irrigation water containing nematodes or application of infested rice hulls on seedbed would make healthy seed be infected by A. besseyi. Good
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control effectiveness can be achieved by seed treatment in association with spraying nematicides during the rice booting stage. In conclusion, strengthening the research on the behavior of A. besseyi has great significance for understanding the pathogenesis of A. besseyi and for exploring new ways for the prevention and treatment of A. besseyi.
ACKNOWLEDGEMENT This work was supported by the National High Technology Research and Development Program of China (Grant No. 2001AA249021).
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