EFFECT OF TUBER-BORNE INOCULUM OF RHIZOCTONIA SOLANI ON THE DEVELOPMENT OF STEM CANKER AND BLACK SCURF OF POTATO
M. Rahman, M. A. Ali, M. U. Ahmad and T. K. Dey. 2013. Effect of tuber-borne inoculum of Rhizoctonia solani on the development of stem canker and black scurf of potato. Bangladesh J. Plant Pathol.29 (1&2):29-32.
An experiment was conducted to determine the effect of level of Rhizoctonia solani infected seed tubers on germination, plant growth and development of stem canker and black scurf diseases of potato. The levels of infected seed tubers tested in the experiment were 0, 5, 10, 20, 50, and 100%. Germination, stem number per hill, plant height, disease incidence and percent disease incidence (PDI) under control (0% infected seed tuber) were 98.33%, 5.1, 60.58 cm, 18.33% and 7.29, respectively. At 5-100% infected seed tubers, the three growth parameters decreased to 86.33-94.08%, 4.43-4.70/hill, 54.33-59.55 cm, whereas DSI and PDI increased to 27.08-51.24% and 9.16-21.25, respectively. Decrease in growth parameters and increase in DSI as well as PDI were corroborated with the levels of infected seed tubers. Planting 5-100% infected seed tubers increased production of diseased tubers by number as well as by weight compared 100% healthy seed tubers (control). At 100% healthy seed, number of russet, deformed and sclerotia bearing tubers were 8.25, 5.50 and 4.00/plot. Use of potato seeds mixed with 5-100% R. solani infected tubers reduced the three parameters within the range of 12.75-20.00, 9.75-17.00 and 90.50/plot, respectively. The results revealed that increase in inoculum level caused increased in infection on progeny tubers. Number of russet, deformed and sclerotia bearing tubers were higher when 100% infected tubers were sown. The other levels of tuber-borne R. solani incoculum showed low of infection.
Potato is the major vegetable crop in Bangladesh. It also plays an important role for food security in the country (Hashem 1990). The area under potato cultivation is increasing year after year and farmers are adopting it as a cash crop. During 2008-2009, about 6.89 million tons of potato was produced from 0.46 million hectares of land with an average yield of 14.86 t/ha (Uddin et al. 2010). The national average yield of potato is much lower as compared to other potato growing countries like the Netherlands, where the average potato yield is 41.3 t/ha (Chadha 1995, Swaminathan 2000).
The major constraint of potato production in Bangladesh is prevalence of epidemic diseases and shortage of quality seed tuber. A total of 39 diseases (both biotic and abiotic) of potato have been recorded in the country (Ali and Khan 1990). The major soil and tuber-borne diseases of potato are black scurf and stem canker, bacterial wilt and common scab. Among them stem canker and black scurf caused by Rhizoctonia solani (Kuhn) is the most common and widespread disease throughout Bangladesh (Ali and Dey 1994).
Rhizoctonia solani infects the underground stem and produces necrosis called stem canker, whereas tuber infection produces symptoms on skin in the form of black sclerotia called black scurf. The pathogen is also involved in the early dying syndrome of potato plants (Kotcon et al. 1985). Tuber-borne inocula act as the main source for introducing the disease into the new areas (Wicks et al. 1996). The highest level of inoculum causes the highest infection resulting in maximum reduction in plant growth and tuber yield (Rahman et al. 1996a). So, it is essential to know the effect of tuber-borne inoculum on disease development. The present study was undertaken to determine the effect of inoculum levels of seed tuber infected with R. solani on the development of stem canker and black scurf of potato.
MATERIALS AND METHODS
Healthy as well as R. solani infected seed tubers of a susceptible variety, Diamant were collected from the Breeder Seed Production Centre (BSPC) Debiganj under Panchagarh district. Both healthy and infected potato seed tubers were mixed before planting at 0, 5, 10, 20, 50 and 100% infected seed tubers. The experiment was conducted at the Tuber Crops Research Sub-Centre (TCRSC) of Bangladesh Agricultural Research Institute (BARI), at Chalopara, Bogra district during 2007-2008 potato growing season. The experiment was laid out in a randomized complete block design with four replications. The unit plot size was 3 m × 3 m. Block to block and plot to plot distances were 100 cm and 50 cm, respectively. Row to row and tuber to tuber distances were 60 cm and 25 cm, respectively.
Recommended doses of fertilizers and manure were applied as suggested by Tuber Crops Research Centre, BARI, Gazipur. Cowdung was incorporated in the soil during land preparation at the rate of 10 t/ha. Urea, Triple super phosphate (TSP), Muriate of potash (MOP), Gypsum, Zinc sulphate and Boric acid were applied at the rate of 360, 220, 250, 120, 14 and 6 kg per hectare, respectively. Half of urea and the entire amounts of TSP, MP, Gypsum, Zinc sulphate and Boron were applied at the time of final land preparation. Tubers were planted on 30 November, 2007. Rest half of urea was applied at 30 days after planting. Weeding was done at 25 and 50 days after planting. Earthing up was done at 30 days after planting. Irrigation was applied at 25 and 45 days after planting. Dursban 50WP (Chloropyrifos) @ 0.5% and Admire (Imidacloprid) @ 0.1% were applied to control cutworm and aphid, respectively. Secure (Pyriminamine) @ 0.1% was sprayed at 10 days intervals as preventive measure against late blight of potato. The crop was harvested on 27th February, 2008.
Data on germination, number of stems per hill, plant height, disease incidence, percent disease index (PDI), tuber infection and yield were recorded. Data on disease incidence were recorded at 70 days after planting. To collect data on disease incidence, 20 plants were uprooted carefully from each plot, washed with tap water and checked for infection. Number of infected and healthy plants was counted in each plot and incidence percentage was calculated based on total number of plants checked.
Disease severity was indexed on a 0-6 scale (Dey 2010), where 0=No symptom on stolon, 1=Minute brown lesion on stolon or root, 2=Moderately brown lesion on stolon and curling tendency on central leaf, 3=Stolon symptom discolored accompanied by brown discoloration on roots, 4=Brown to black discoloration on underground parts, tissue discoloration and tissue squeezed/curling of growing leaves, 5=Profuse emergence of auxiliary leaves and leaf size reduced markedly with pale green
margin, and 6=Production of aerial tubers with green colour. The plants were checked individually and categorized into different group according to the indexing scale. The number of stem canker infected plants under each grade was recorded and percent disease index (PDI) was calculated using the following formula:
Incidence of black scurf was recorded at harvest. The black scurf infected tubers were separated into three groups such as russet, deformed and sclerotia (Chand and Logan 1982). The number of tubers under each group was counted and the respective weight was recorded. Number and weight of healthy tubers of each plot was also recorded.
Data on different parameters were analyzed using computer program MSTAT-C. Differences among the treatment means were compared following Duncan’s Multiple Range Test (DMRT).
RESULTS AND DISCUSSION
Germination, stem number per hill, plant height, disease incidence (DSI) and percent disease incidence (PDI) under control (0% infected seed tubers) were 98.33%, 5.1, 60.58 cm, 18.33% and 7.29, respectively. At 5-100% infected seed tubers, the three growth parameters decreased to 86.33-94.08%, 4.43-4.70/hill, 54.33-59.55 cm, whereas the two disease related parameters (DSI and PDI) increased to 27.08-51.24% and 9.16-21.25, respectively. Decrease in growth parameters and increase in DSI as well as PDI were corroborated with the levels of infected seed tubers. However, decrease in germination and increase in DSI as well as PDI were significant compared to control but decrease in stem number per hill and plant height were statistically similar at 5-100% infected seed tubers (Table 1).
Planting 5-100% infected seed tubers significantly increased production of diseased tubers by number as well as by weight compared 100% healthy seed tubers (control). At 100% healthy seed, number of russet, deformed and sclerotia bearing tubers were 8.25, 5.50 and 4.00/plot. Use of potato seeds mixed with 5-100% R. solani infected tubers reduced the three parameters within the range of 12.75-20.00, 9.75-17.00 and 90.50/plot, respectively. Weight of russet, deformed and sclerotia bearing tubers were 600.0, 515.0 and 200.0 g/plot under control. All parameters decreased the three parameters to 600.0-1370.0, 515.0-1200.0 and 200.0-67.00 g/plot. The increase in number and weight of deformed and sclerotia bearing tubers were significant compared to control (100% healthy seed). The highest number and weight of diseased tubers were harvested from plots planted with 100% infected seeds followed by 50 and 20% infected seed tubers. The differences in production of diseased tubers by number as well as weight at three higher levels of infected seed tubers were significant with the exception of weight of russet tuber at 100 and 50% infected seed tubers. The minimum increase in diseased tubers having russet and deformed symptoms and bearing sclerotia were harvested from plots planted with 5% infected seed tubers followed by 10% (Table 2).
The highest yield of 21.50 t/ha was harvested from plots planted with 100% healthy seeds which was statistically similar to the yield of the plots planted with 5, 10 and 20% infected seed tubers. The lowest yield of 17.50 t/ha was harvested from plots planted with 50% infected seed tubers which was statistically similar to the yield at 100% infected seed tubers deformed and sclerotia bearing tubers. The yield at two higher levels of infected seed tubers was significantly lower compared to control (Table 2). It was found that the tuber yield decreased gradually with the increase in levels of infected seed tubers. Their relationship was negative and linear (Fig. 1).
In the present investigation, the highest germination, plant growth and lowest disease incidence and percent disease incidence were recorded from control (100% healthy seed tubers) followed by infected seed tuber levels of 5, and 10%. On the other hand, the lowest germination, plant growth and highest disease incidence as well as severity were recorded at 100% infected seed tubers (Tables 1 and 2).
The findings of the present investigation were found to be similar to the findings of many other researchers (Chand and Logan 1982, Read et al. 1989, Rahman et al. 1996a, 1996b, Naz et al. 2008). They found that severity of stem canker and stolon infection by R. solani increased with increasing levels of R. solani inoculum. Inoculum levels showed significant negative impact on germination of seed tuber and plant height. The treatment having higher level of tuber inocula significantly differed from other treatments in the degree of tuber infection. Read et al. (1989) reported that high inoculum levels caused severe infection with delay in shoot growth and decrease in stem height and foliage weight. Based on findings of the present study it may be concluded that higher level of seed tuber-borne inoculum of R. solani leads to increase in stem canker and black scurf infection in the subsequent crop resulting in decrease of tuber yield and quality of potato.
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