Abstract: Background and Objective: Potato sclerotium rot disease causes a very high impact on the quality and quantity of potato production in Chiang Mai, Thailand. It has become a recurrent problem of potato production. The soil-borne pathogens of crop incited by Sclerotium rolfsii are very difficult to be managed because it produces sclerotia for the long-term survival in the soil. The objectives of this study were to identify pathogens and evaluate fungicides and wood vinegar for their efficacy against pathogens. Materials and Methods: The pathogens were identified based on morphology. The pathogenicity of fungal isolates was tested in the laboratory and compared with the non-treated control. Six fungicides and wood vinegar were tested by the poisoned medium technique, the analysis was arranged in a completely randomized design with 5 replicates of each treatment. Results: The fifteen fungal isolates were identified as S. rolfsii based on mycelial and sclerotial characters. A pathogenicity test was conducted in the laboratory, where the symptoms of rotten tissues and penetration of pathogens were observed. In vitro evaluation of fungicides and wood vinegar, found that the mycelial growth and sclerotial germination were completely inhibited by etridiazole+quintozene and mancozeb at recommended concentration and wood vinegar at 1% concentrations. Conclusion: The results revealed that S. rolfsii caused potato sclerotium rot disease in Northern, Thailand. The highly effective fungicide against S. rolfsii was etridiazole+quintozene and mancozeb. For the efficacy of wood vinegar, it can be decreased in vitro at 1% concentration onward.
INTRODUCTION
Potato (Solanum tuberosum L.) is an economically important crop worldwide. It is classified as the fourth main food crop in the world after maize, rice and wheat1. The Atlantic potato is a cultivar of S. tuberosum, which plays an important role in product processing in Thailand, it is thus a strategic crop2. The yearly demand for fresh potatoes of further processing is about 150,000-200,000 MT but they can produce approximately 100,000-120,000 MT3. As potato is frequently grown in rotated with other crops4, it is threatened by several soil-borne fungi. Among these fungal pathogens, the agent causing wilt and root and tuber rots are responsible for yield losses5.
Sclerotium rolfsii is widely distributed in tropics, subtropics and also in warmer parts of temperate zone such as Southern United States, West Indies, Southern Europe, Central and South America, countries bordering the Mediterranean, Africa, Hawaii, Japan, Philippines and India6,7. Sclerotium rolfsii is a destructive soil-borne plant pathogen causing sclerotium rot disease on more than 500 plant species worldwide5,8,9. The fungus is characterized by the white colony, fluffy, branched, septate mycelia and primary hyphae that have clamp connections at the septa10,11. The sclerotia are initially white, becoming darken to tan, brown or black when mature9,12 and tiny (1-2 mm in diameter) circular or globose shaped13,14. Sclerotium rolfsii is often found around the base and underground storage organs of the host plant9.
Handling fungicides to control plant diseases is a common practice. As this fungus was a soil-borne pathogen a wide hosts range. Management of crop rotation may not be of much help15 therefore, studies were undertaken to the evaluation of fungicides and wood vinegar to know their efficacy inhibition mycelial and sclerotial of Sclerotium rolfsii under in vitro condition for further utilization field to manage the disease.
MATERIALS AND METHODS
Fungal isolation: The sclerotia were collected from infected potato tuber showing symptoms suspected to be caused by Sclerotium rolfsii grown in the field from Chiang Dao and Chai Prakan District, Chiang Mai Province, Thailand in November 2019. The pure fungal isolation was made following a modified method of Akarapisan et al.7. The sclerotia were surface sterilized with 70% ethanol for 3 min, 1% sodium hypochlorite (NaOCl) solutions for 3 min, rinsed with distilled water 3 times for 5 min and air-dried on sterilized filter paper.
The sclerotia were placed on Potato Dextrose Agar (PDA) in a 9 cm petri dish and incubated at room temperature (28±2°C) for 3 days. The hyphal tips of mycelial growing from sclerotia were cut and transferred into a new PDA petri dish for obtaining pure culture for further studies.
Fungal identification: The pathogen was identified as Sclerotium rolfsii based on its mycelial and sclerotial characters viz., growth rate, colour, presence of hyphal clamp connection and nature of mycelium growth, number of the sclerotia/isolate, colour, shape, size and weight of 100 sclerotia/isolate were compared with the study of Banakar et al.6, Sarma et al.15 and Paul et al.16.
Pathogenicity test: Tubers were surface sterilized with 1% sodium hypochlorite solutions for 3 min, rinsed with distilled water 3 times for 5 min and air-dried. Inoculation was made following a modified method of Daami-Remadi et al.5. Tubers were wounded 6 mm in deep with the disinfected needle which was used to induce wound in the tuber. Tuber inoculation was accomplished by depositing a mycelial plug (5 mm in diameter) from 3 days old of S. rolfsii. Three tubers were used per isolates tested. Tubers that had been similarly wounded but not inoculated were used as controls. Inoculated tubes were placed in a moistened plastic box for 72 hrs, after that the mycelial plug removed on the tuber and incubated for 8 Days After Inoculation (DAI). After 8 DAI, inoculated tubers were cut longitudinally at each wound site in half and the cut surface was checked for rot severity. Maximal width (w) and depth (d) was recorded the pathogen penetration (P) and calculated according to the formula of Daami-Remadi et al.5 as follows:
In vitro fungicides assay: The poisoned medium technique15 was employed for evaluating the efficacy of different fungicides. Six fungicides viz., captan, mancozeb, etridiazole+quintozene, azoxystrobin, azoxystrobin+SHAM and difenoconazole were tested under in vitro condition. Each fungicide was tested at half-recommended and recommended concentration. For each treatment, a specified concentration of fungicide was added to the PDA at a lukewarm temperature and mixed thoroughly by shaking the flask. This medium was poured into a 9 cm petri dish. A mycelial plug of 5 mm in diameter from 3 days old was inoculated in the centre of each petri dish and then incubated at room temperature for 3 days. For the sclerotial germination test was used 10 sclerotial/plate (15 days old after formation) placed on a poisoned medium and incubated until all sclerotial germinate in the control plate. The petri dish containing PDA without any fungicide served (and PDA amended 100 μg/lSHAM for azoxystrobin+SHAM treatment) as control. Active ingredients (a.i.) and chemical groups of the fungicides are given in Table 1. There were five replicates of each treatment arranged in a CRD. Percentage of inhibition was calculated by the following equation:
where, D1 is the colony diameter of mycelial or sclerotial germinated in the control plates (mm) and D2 is the colony diameter of mycelial or sclerotial germinated in the plates containing fungicides (mm).
Finally, the variance and the treatment means were analyzed by the software IBM SPSS Statistics ver. 17 and compared using Duncan’s multiple range tests (DMRT) at a 5% level of probability (p<0.05).
In vitro wood vinegar assay: The antifungal assay of eucalyptus wood vinegar (Pyro Plus+®) was served to determine their ability to inhibit the mycelial growth and sclerotial germination of S. rolfsii using poisoned medium technique supplemented 1, 2 and 3% of wood vinegar in PDA. The petri dish was then centrally inoculated with a mycelial plug (5 mm in diameter) of each isolate. For the sclerotial germination test was used 10 sclerotial/plate (15 days old after formation). The petri dish containing PDA without wood vinegar were used as controls. Five replicates were used to all the concentrations and arranged in a CRD. The test dishes were incubated at room temperature. After incubation at room temperature for 3 days, fungi were measured colony diameter. The percentage of inhibition was calculated by the following equation similarly in vitro fungicides assay.
RESULTS
Morphological studies and pathogenicity test of fungal: The occurrence of disease in Atlantic potato tuber was observed in the field. The fungus infected the surface of the tuber resulting in the development of a lesion. When incubated tuber in humid condition, white mycelium in Fig. 1(a, b) and developing sclerotial spread over the surface of the tuber. Tuber rotten occurred within 3-4 DAI. For all 56 fungal isolates from 8 locations, there was a considerable variation of their growth on PDA in each isolate. The reaction of mycelial growth was divided into two groups including very fast and fast growth. Thirty-two fungal isolates rapidly grew over the petri dish within 3 DAI, considered as very fast growth isolates whereas, twenty-four fungal isolates grew within 4 DAI, considered as fast growth in Table 2. In each location, one isolate from very fast and fast growth was randomly selected for identification based on mycelial and sclerotial characters. There were 15 selected isolates including very fast growth by 8 isolates (MK1_8, MK4_3, MK5_9, MK7_1, MK9_5, CPK1_3, CPK3_6 and CPK13_3) and fast growth by 7 isolates (MK1_6, MK4_7, MK5_12, MK7_2, MK9_4, CPK1_4 and CPK3_3).
All 15 isolates were identified as Sclerotium rolfsii based on mycelial and sclerotial characters. There were differently both mycelial and sclerotial characters. In each isolate, their mycelial had a growth rate between 2.29-2.83 cm/day, several colours (dull-white, light white, white and extra white) and diverse appearance (suppressed thin strands, cottony puffy at edge dense at the margin and aggregated dense cottony) whereas, their sclerotial had number sclerotial/plate between 193-1125 sclerotial, brown or dark brown colours, spherical or irregular shape, several sizes between 0.6-1.8 mm, various weight between 15.0-55.0 mg 100–1 sclerotial bodies, periphery or all over in sclerotial arrangement, 3-7 DAI in initial formation and 5-10 DAI in maturity stage in Table 3. An example of S. rolfsii morphology was presented in Fig. 1. The mycelial of fungal had cotton-like white in Fig. 1c and it turned to dull white with radial spreading fan-like appearance.
| Table 1: Active ingredients and chemical groups of the fungicides | |||||
| Mobility in plant | Common names | Trade names | Active ingredients (%) | Recommended (ppm) | FRAC codes1/ |
| Contact | Mancozeb | Dithane M-45 | 80% WP | 2000 | M032/ |
| Captan | Orthocide | 50% WP | 750 | M04 | |
| Etridiazole+quintozene | Terraclor super X | 30% W/V | 540 | 14 | |
| Systemic | Azoxystrobin | Amista | 25% W/V | 50 | 11 |
| Difenoconazole | Gabina | 25% W/V | 112.5 | 3 | |
| 1/FRAC codes referenced from FRAC (2020) and 2/FRAC codes M03: Dithiocarbamates and relatives, M04: Phthalimides, 14: Aromatic hydrocarbons (AH), 11: Quinone outside inhibitors (QoI), 3: De-Methylation Inhibitors (DMI) | |||||
| Fig. 1(a-e): | Symptom and morphology of Sclerotium rolfsii (a) Symptom on tuber of potato cv. ‘Atlantic’ was collected from field, (b) Mycelial growth on tuber of potato cv. ‘Atlantic’ for 2 DAI in a moist chamber, (c) Mycelial growth 3 days on potato dextrose agar (PDA), (d) Sclerotial 15 DAI formed on PDA (scale bar = 100 μm) and (e) Clamp connection formed on the hyphal (scale bar = 50 μm) |
| Table 2: Growth rate of Sclerotium rolfsii causing potato sclerotium rot | |||||||||
| Growth rate (cm)1/ | Growth rate (cm)1/ | ||||||||
| Isolates | 24 hrs |
48 hrs |
72 hrs |
Reaction2/ |
Isolates |
24 hrs |
48 hrs |
72 hrs |
Reaction2/ |
| MK1_1 | 2.57 |
6.73 |
9.00 |
Very fast |
MK5_14 |
1.97 |
6.13 |
9.00 |
Very fast |
| MK1_2 | 1.42 |
4.82 |
9.00 |
Very fast |
MK5_15 |
1.20 |
4.47 |
9.00 |
Very fast |
| MK1_3 | 1.02 |
3.43 |
7.87 |
Fast |
MK5_16 |
2.65 |
6.80 |
9.00 |
Very fast |
| MK1_4 | 1.25 |
4.12 |
8.82 |
Fast |
MK7_1 |
1.53 |
5.23 |
9.00 |
Very fast |
| MK1_5 | 1.72 |
5.53 |
9.00 |
Very fast |
MK7_2 |
0.78 |
3.15 |
7.62 |
Fast |
| MK1_6 | 0.90 |
3.15 |
7.40 |
Fast |
MK7_3 |
0.95 |
3.42 |
8.08 |
Fast |
| MK1_7 | 1.18 |
4.72 |
9.00 |
Very fast |
MK7_4 |
0.82 |
3.32 |
7.65 |
Fast |
| MK1_8 | 1.12 |
4.28 |
9.00 |
Very fast |
MK7_5 |
1.15 |
3.93 |
8.42 |
Fast |
| MK4_1 | 0.88 |
3.55 |
7.93 |
Fast |
MK9_1 |
2.17 |
6.25 |
9.00 |
Very fast |
| MK4_2 | 1.12 |
4.10 |
8.45 |
Fast |
MK9_2 |
1.93 |
5.53 |
9.00 |
Very fast |
| MK4_3 | 1.32 |
4.48 |
9.00 |
Very fast |
MK9_3 |
1.90 |
5.98 |
9.00 |
Very fast |
| MK4_4 | 1.40 |
4.58 |
8.88 |
Fast |
MK9_4 |
1.33 |
4.53 |
8.75 |
Fast |
| MK4_5 | 1.67 |
5.23 |
9.00 |
Very fast |
MK9_5 |
1.57 |
5.05 |
9.00 |
Very fast |
| MK4_6 | 2.60 |
6.70 |
9.00 |
Very fast |
CPK1_1 |
1.25 |
4.65 |
9.00 |
Very fast |
| MK4_7 | 0.78 |
3.07 |
7.37 |
Fast |
CPK1_2 |
1.73 |
5.38 |
9.00 |
Very fast |
| MK5_1 | 0.98 |
3.82 |
8.38 |
Fast |
CPK1_3 |
1.40 |
5.32 |
9.00 |
Very fast |
| MK5_2 | 0.88 |
3.43 |
7.87 |
Fast |
CPK1_4 |
1.23 |
4.13 |
8.62 |
Fast |
| MK5_3 | 1.40 |
4.73 |
9.00 |
Very fast |
CPK1_5 |
1.50 |
4.73 |
9.00 |
Very fast |
| MK5_4 | 0.90 |
3.70 |
8.37 |
Fast |
CPK3_1 |
1.33 |
4.45 |
8.65 |
Fast |
| MK5_5 | 2.48 |
6.73 |
9.00 |
Very fast |
CPK3_2 |
1.82 |
5.25 |
9.00 |
Very fast |
| MK5_6 | 1.53 |
5.35 |
9.00 |
Very fast |
CPK3_3 |
1.00 |
3.63 |
7.70 |
Fast |
| MK5_7 | 0.77 |
2.87 |
7.08 |
Fast |
CPK3_4 |
1.27 |
4.58 |
8.78 |
Fast |
| MK5_8 | 1.05 |
3.80 |
8.12 |
Fast |
CPK3_5 |
0.98 |
4.07 |
8.80 |
Fast |
| MK5_9 | 1.15 |
4.40 |
9.00 |
Very fast |
CPK3_6 |
1.43 |
4.77 |
9.00 |
Very fast |
| MK5_10 | 1.07 |
4.00 |
8.25 |
Fast |
CPK13_1 |
2.12 |
6.08 |
9.00 |
Very fast |
| MK5_11 | 0.95 |
3.88 |
8.55 |
Fast |
CPK13_2 |
2.48 |
6.58 |
9.00 |
Very fast |
| MK5_12 | 0.90 |
3.45 |
7.75 |
Fast |
CPK13_3 |
2.60 |
6.68 |
9.00 |
Very fast |
| MK5_13 | 0.82 |
3.78 |
8.20 |
Fast |
CPK13_4 |
1.33 |
6.12 |
9.00 |
Very fast |
| 1/ Average 3 replicates and 2/Very fast is isolates recorded 9 cm growth within 3 DAI and Fast is isolates recorded 9 cm growth within 4 DAI | |||||||||
In the maturity stage of the mycelial, a small white knot was formed and it later turned to sclerotial. The sclerotial had a brown or dark brown colour, spherical or irregular shape in Fig.1d. Microscopic analysis of the mycelial found that it was hyaline and had a thin wall. Its primary hyphae showed clamp connection formed in septa in Fig. 1e. Arrangement of sclerotial on PDA divided into two groups including periphery and all over in petri dish in Fig. 2. Fifteen isolates had different characters same with the difference of characters. Mycelial grew on the host surface and had several colours (brown to dark brown, dark to reddish-brown and cinnamon to dresden brown), diverse shape (spherical and very few irregulars), 0.5-2.2 mm size and clamp connection synthesis in Table 4.
After inoculated the tubers by S. rolfsii, slight collapses of tuber tissue and water soak were observed whereas, non-inoculated tubers were symptomless. Tuber rot severity had no significant difference (p<0.05) in each isolate. The measurement of the lesion in 8 DAI at 24±2°C, penetration of the fungal was ranged from 6.33-10.00 mm in Fig. 3. The CPK13_3 and CPK3_3 isolates had the highest penetration of very fast and fast growth groups by 10.00 and 9.75 mm, respectively in Fig. 4.
Efficacy of fungicides and wood vinegar: Variable results were obtained from the in vitro evaluation of fungicides to inhibit the mycelial growth of S. rolfsii. As a result, three fungicides viz., mancozeb, etridiazole+quintozene and difenoconazole were completely inhibited the mycelial growth at both of half-recommended concentrations (1000, 270 and 56.25 ppm, respectively) and recommended concentration (2000, 540 and 112.5 ppm, respectively) by 100.00%. Also, captan effectively inhibited the mycelial growth by 64.78 and 75.22% at both halves-recommended concentration (375 ppm) and recommended concentration (750 ppm), respectively. Furthermore, azoxystrobin and azoxystrobin+SHAM could not inhibit mycelial growth and sclerotial germination at the recommended concentration. Mancozeb and etridiazole+quintozene were completely inhibited the sclerotial germination in comparison to control treatment. Captan inhibited the sclerotial germination by 87.00 and 95.00% at both of half-recommended and recommended concentration, respectively in Table 5. Moreover, the efficacy of wood vinegar completely inhibited the mycelial growth and sclerotial germination of S. rolfsii at a concentration of more than 1% in Fig. 5(a, b).
| Table 3: Morphological characteristics of fifteen representative isolates of Sclerotium rolfsii obtained from different locations | |||||||||||
| Mycelial characteristics | Sclerotial characteristics | ||||||||||
| Isolates | Growth rate (cm day–1)1 | Color | Appearance | No./plate1 | Color | Shape | Size2 (mm) | Weight3 (mg) | Arrangement | Initial (DAI) | Maturity (DAI) |
| MK1_6 | 2.30 | Dull white | Suppressed, thin strands | 471 | Dark brown | Spherical | 1.50 | 30.0 | All over | 3 | 6 |
| MK1_8 | 2.83 | Dull white | Suppressed, thin strands | 354 | Brown | Irregular | 1.40 | 25.0 | All over | 5 | 8 |
| MK4_3 | 2.83 | Light white | Suppressed, thin strands | 413 | Dark brown | Spherical | 1.55 | 47.5 | All over | 3 | 5 |
| MK4_7 | 2.29 | Dull white | Cottony, puffy at edge, dense at margins | 627 | Brown | Spherical | 0.60 | 15.0 | All over | 4 | 6 |
| MK5_9 | 2.83 | White | Cottony, puffy at edge, dense at margins | 293 | Dark brown | Irregular | 1.50 | 40.0 | All over | 7 | 10 |
| MK5_12 | 2.42 | White | Aggregated, dense cottony | 311 | Dark brown | Irregular | 0.70 | 22.5 | All over | 3 | 6 |
| MK7_1 | 2.83 | Dull white | Cottony, puffy at edge, dense at margins | 657 | Dark brown | Spherical | 1.55 | 32.5 | Periphery | 5 | 7 |
| MK7_2 | 2.37 | Extra white | Cottony, puffy at edge, dense at margins | 291 | Dark brown | Irregular | 1.40 | 30.0 | Periphery | 6 | 8 |
| MK9_4 | 2.75 | Light white | Suppressed, thin strands | 566 | Dark brown | Spherical | 0.85 | 27.5 | Periphery | 5 | 7 |
| MK9_5 | 2.83 | White | Aggregated, dense cottony | 274 | Dark brown | Spherical | 1.60 | 35.0 | All over | 3 | 5 |
| CPK1_3 | 2.83 | Extra white | Cottony, puffy at centre, dense at margins | 193 | Dark brown | Spherical | 1.60 | 37.5 | Periphery | 7 | 10 |
| CPK1_4 | 2.71 | White | Suppressed, thin strands | 604 | Dark brown | Spherical | 1.70 | 40.0 | Periphery | 3 | 5 |
| CPK3_3 | 2.40 | Extra white | Cottony, puffy at centre, dense at margins | 1,125 | Brown | Spherical | 0.70 | 22.5 | All over | 3 | 5 |
| CPK3_6 | 2.83 | White | Cottony, puffy at centre, dense at margins | 654 | Dark brown | Spherical | 1.40 | 32.5 | All over | 4 | 7 |
| CPK13_3 | 2.83 | Extra white | Aggregated, dense cottony | 278 | Dark brown | Spherical | 1.80 | 55.0 | Periphery | 4 | 6 |
| 1Average 4 replicates, 2Average of 50 sclerotial/plate, 3Average of 100 sclerotial/plate | |||||||||||
| Fig. 2: | Arrangement of Sclerotium rolfsii on PDA |
| Fig. 3: | Effect of pathogens on tuber rot severity incited by Sclerotium rolfsii and incubation for 8 days (24±2°C) Error bar represented in 95% confidence interval |
| Fig. 4: | Effect penetration of pathogens on tissue rot incited by Sclerotium rolfsii post-inoculation for 8 days VF: Very fast and F: Fast growth of mycelial |
| Fig. 5(a-b): | Efficacy of wood vinegar to control Sclerotium rolfsii causing potato sclerotium rot using the poisoned medium technique (a) Inhibition mycelial growth assay and (b) Inhibition sclerotial germinate assays |
| Table 4: Identification based on morphology of Sclerotium rolfsii causing potato sclerotium rot | ||||
| Characters | Banakar et al.6 | Sarma et al.15 | Paul et al.15 | Current study |
| Mycelial | Superficial on host | Superficial on host | Superficial on host | Superficial on host |
| Sclerotial (color) | Brown to dark brown | Dark to reddish brown | Cinnamon to dresden brown | Light brown to dark brown |
| Sclerotial (shape) | Spherical very few irregular | Spherical very few irregular | Spherical very few irregular | Spherical very few irregular |
| Sclerotial (size) | 1.5 mm | 1.0-2.2 mm | 0.5-2.0 mm | 0.6-1.8 mm |
| Clamp connection | Present | Not data | Present | Present |
| Table 5: Effect of fungicides to control Sclerotium rolfsii causing potato sclerotium rot for inhibited the mycelial growth and sclerotial germination by using poisoned medium technique | |||||
| Percent inhibition (%)1/ | |||||
| Mycelial growth | Sclerotial germination | ||||
| Fungicides3/ | Conc. (ppm) | CPK3_3 | CPK13_3 | CPK3_3 | CPK13_3 |
| Captanc | 375 | 64.78±1.45C2/ | 69.78±1.22C | 88.00±8.37B | 86.00±5.48B |
| 750 | 70.78±0.63B | 75.22±1.15B | 96.00±5.48A | 94.00±5.48A | |
| Mancozebc | 1000 | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A |
| 2000 | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A | |
| Etridiazole+qiuntozenec | 270 | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A |
| 540 | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A | 100.00±0.00A | |
| Azoxystrobins | 25 | 40.67±0.73F | 39.89±0.73G | 10.00±7.07E | 0.00±0.00D |
| 50 | 45.56±1.52E | 44.67±1.40F | 18.00±8.37D | 0.00±0.00D | |
| Azoxystrobins+SHAM | 25 | 57.22±0.88D | 56.56±0.73E | 4.00±5.48E | 6.00±8.94D |
| 50 | 64.33±1.86C | 57.89±1.44D | 28.00±8.37C | 22.00±8.37C | |
| Difenoconazoles | 56.25 | 100.00±0.00A | 100.00±0.00A | 6.00±5.48E | 18.00±8.37C |
| 112.5 | 100.00±0.00A | 100.00±0.00A | 20.00±7.07D | 22.00±8.37C | |
| CV (%) | 1.14 | 1.03 | 6.93 | 7.42 | |
| 1/Average 5 replicates, 2/Values with a column followed by the letter different are significantly (p<0.05) by DMRT. The experimental design was a completely randomized design (CRD), the data within the same column indicate standard deviation of mean, 3/Superscript of the letter are mobility of fungicides. c: Contact fungicide, s: Systemic fungicide | |||||
DISCUSSION
The potato sclerotium rot disease on ‘Atlantic’ potato caused major crop loss in some commercial field in Chiang Dao and Chai Prakan District, Chiang Mai Province, Thailand. The initial symptoms of potato tuber in the field were tissue collapsed and water-soaked lesion. The white mycelial will be produced on the surface of the wounded tuber. When the tuber was incubated in the humid condition, the sclerotia formed and the mycelia continued growing and covering the tuber, which resulted in the rot disease. Based on of morphological study, the fungus was identified as Sclerotium rolfsii.
Sclerotium rolfsii causes serious root and stem rots of a range of economically important fruit and vegetable crops with at least 500 species in 100 families. The most common hosts are legumes, crucifers and cucurbits9. Similar observations were made by Agrios17, who reported that the white mycelial were always present in and on infected tissues and they could infect the adjacent plants, starting the next-new infections. Invaded stem, tuber and fruit tissues are usually pale brown and soft but not watery. The fungus produced numerous small roundish sclerotia of uniform size that were white when immature, becoming dark brown to black at the maturity stage. Sarma et al.15 also described the characters of S. rolfsii that, the sclerotial initials were white and the matured ones are mostly dark to reddish-brown. They were spherical or ellipsoidal shaped and measured in the size of sclerotial by 1.0-2.2 mm. The character of fungus was found to resemble S. rolfsii.
There were several reports on the efficacy of fungicides that inhibit the growth of the pathogen in vitro condition. Akarapisan et al.7 has reported that etridiazole+quintozene shown a 100.00% inhibitory mycelial growth of S. rolfsii at a recommended concentration. Raghavendra and Srinivas18 reported that at 1000 ppm Dithane M-45 (mancozeb) reduced the mycelial growth of S. rolfsii by100% which contrasts with Patel et al.19 found that mancozeb at 1000 ppm reduced by 88.85%. Manu et al.14 reported that at 25 ppm difenoconazole reduced the mycelial growth by 100.00%. Bhagat and
Chakraborty20 reported that mycelial growth and sclerotial germination was completely inhibited by Thiodan and Calixin at a 0.0125% concentration. Reddi Kumar et al.12 found that tricyclazole+mancozeb, azoxystrobin and difenoconazole at 250 ppm concentration were low effective inhibition of S. rolfsii mycelial growth by 49.32, 36.14 and 56.89%, respectively. Based on the results obtained from this study, it was suggested that the application of mancozeb and etridiazole+quintozene could control S. rolfsii at the recommended concentration.
The properties of wood vinegar have been shown in different plant pathogens21. The wood vinegar decreased the growth of Sclerotinia sclerotiorum by 100 at 0.75% concentration and Rhizoctonia solani by 100 at 0.37% concentration22. Oramahi and Yoshimaru23 reported that the antifungal effects of wood vinegar may be due to phenolic and acetic acid whereas oramahi et al.24 identified the antifungal properties of wood vinegar by GC-MS analysis. They found that predominant compounds in wood vinegar contained 1-hydroxy-2-propanone, 3-hydroxy-2-butanone, acetic acid, propanoic acid and phenol. Céspedes et al.25 showed that wood vinegar replaces chemical pesticides or fungicides in organic farming. It had low mammalian toxicity, no neurotoxicity, low persistence in the environment and high biodegradability.
CONCLUSION
The highly effective fungicide against S. rolfsii is mancozeb and etridiazole+quintozene, which inhibited the mycelial growth and sclerotial germination in vitro by 100% at both of half-recommended and recommended concentration, respectively. Also, wood vinegar could decrease mycelial growth and sclerotial germination in vitro at a concentration of 1% onward. Therefore, the protection of potato against S. rolfsii, may be achieved by treating the tuber with fungicides or wood vinegar before planting or storage in the field.
SIGNIFICANCE STATEMENT
In this study, a disease of potato caused by Sclerotium rolfsii was identified. It was shown that etridiazole+ quintozene and mancozeb at 540 and 2000 ppm concentration (recommended dose), respectively were 100% inhibitory mycelial growth and sclerotial germination under in vitro condition. Moreover, wood vinegar at more than 1% concentration was 100% inhibitory same with the fungicides. Consequently, this study may be used for further utilization field of controlling the disease. This efficacy of wood vinegar can be recommended as potential biocontrol for reducing fungi plant pathogens of potato. It can temporarily use for replacing or reducing the fungicides and protecting the development in fungicide resistance of fungi.
ACKNOWLEDGMENT
Authors thank the Frito-Lay (Thailand) Co., Ltd. to support the “Atlantic” potato tubers, Division of Plant Pathology, Department of Entomology and Plant Pathology, Faculty of Agriculture, Chiang Mai University, for allowing them to do this research at laboratory rooms and Graduate School of Chiang Mai University. The authors are grateful to Dr. Wei Dong, for the language assistance and helpful comments on this manuscript.