Caladiums (
Currently, no root-knot nematode resistant caladium cultivars or post-plant nematicides are available for caladium growers. Biological products including live organisms such as different
Thermotherapy, as a non-chemical and environmentally friendly method, has been applied for plant disease management since the 1930s through hot water, air, and vapor (Kunkel, 1935, 1936). The first hot water treatment study on controlling root-knot nematodes in caladium tubers was conducted in 1961 (Rhoades, 1964). Based on the results, pre-planting hot water treatment at 50°C for 30 min has been adopted in the caladium tuber production industry as a standard (Kokalis-Burelle et al., 2010). The main objectives of this study are (1) to investigate the efficacy of different hot water immersion times, and (2) to evaluate if hot water treatment efficacy is affected by caladium variety and tuber size.
Greenhouse studies were conducted at the University of Florida Gulf Coast Research and Education Center (UF/GCREC), Wimauma, Florida between February and June 2020. Experiments were established in 16.5-cm diameter × 14.6-cm tall plastic pots with a soil holding capacity of 2 kg. The soil (95% sand, 2% silt, 3% clay with 0.9% organic matter, A & L Western Laboratories, Inc., CA) was heat sterilized at 185°F for 2 hr. During the study period, plants were maintained in a greenhouse with a temperature = 24 ± 3°C and humidity = 64 ± 12%. Caladium tubers were obtained from a commercial caladium field and were naturally infested with the peanut root-knot nematode,
Both experiments were terminated 90 d after the observation of the first shoot. Root-knot nematode eggs were extracted by shaking the whole root system of each plant in a 0.6% sodium hypochlorite solution for 2 min (Hussey and Barker, 1973). Second-stage juveniles (J2) were extracted from a well-mixed 200 cm3 subsample of soil from each pot using a modified Baermann funnel method with 2-d incubation at 25°C (Forge and Kimpinski, 2007). Plant heights were collected from each plant. Fresh shoots and roots from each pot were weighed separately, then dried at 54°C for a week and weighed again.
Data were analyzed using the Glimmix model in SAS University Edition (SAS Institute, Cary, NC). Data from both repetitions were combined if no statistically significant differences were found between experiments (
An ANOVA summary table. Effects of hot water exposure time, caladium cultivar, and tuber size on tuber infection and reproduction of root-knot nematode (
Factor | Gall indexa | Eggs per | Eggs per | Eggs per | Number of J2 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
(0–10) | fresh root systema | gram fresh roota | gram dry roota | per 200 cc soila | |||||||||||
DF | P-value | DF | DF | DF | DF | ||||||||||
Exposure time | 2 | 247.12 | <0.01 | 2 | 242.76 | <0.01 | 2 | 304.14 | <0.01 | 2 | 260.12 | <0.01 | 2 | 274.5 | <0.01 |
Cultivar | 1 | 0.43 | 0.51 | 1 | 11.07 | <0.01 | 1 | 13.10 | <0.01 | 1 | 13.67 | <0.01 | 1 | 4.20 | 0.04 |
Tuber size | 2 | 1.35 | 0.26 | 2 | 0.27 | 0.77 | 2 | 0.06 | 0.94 | 2 | 0.00 | 1.00 | 2 | 0.21 | 0.81 |
Exposure time × Cultivar | 2 | 3.07 | 0.05 | 2 | 8.81 | <0.01 | 2 | 6.83 | <0.01 | 2 | 7.99 | <0.01 | 2 | 5.61 | <0.01 |
Exposure time x Tuber size | 4 | 1.87 | 0.12 | 4 | 1.33 | 0.26 | 4 | 1.02 | 0.40 | 4 | 1.37 | 0.25 | 4 | 1.33 | 0.26 |
Cultivar × Tuber size | 2 | 4.33 | 0.01 | 2 | 0.59 | 0.55 | 2 | 0.58 | 0.56 | 2 | 0.67 | 0.52 | 2 | 0.13 | 0.88 |
Exposure time x Cultivar × Tuber size | 4 | 2.30 | 0.06 | 4 | 0.71 | 0.59 | 4 | 0.46 | 0.76 | 4 | 0.66 | 0.62 | 4 | 0.32 | 0.86 |
a Data were fit to Lognormal distribution in the Glimmix model in SAS University Edition.
The effects of different factor combinations on the reproduction of
Effects of factor combinations (hot water exposure time, caladium cultivar, and tuber size) on reproduction of root-knot nematode (
Cultivar | Exposure time @ 50°C (min) | Tuber size | Gall index (0–10) | Eggs per fresh root system‡ | Eggs per gram fresh root‡ | Eggs per gram dry root‡ | Number of J2 per 200 cc soil‡ |
---|---|---|---|---|---|---|---|
PJR | 45 | #3 | 0.0 c† | 0 e | 0 c | 0 f | 23 b |
#1 | 0.2 c | 30 de | 2 c | 37 ef | 23 b | ||
Jumbo | 0.2 c | 30 de | 2 c | 37 def | 0 b | ||
30 | #3 | 0.0 c | 30 de | 3 c | 96 def | 0 b | |
#1 | 0.0 c | 90 de | 6 c | 105 def | 0 b | ||
Jumbo | 0.0 c | 270 de | 16 c | 351 def | 35 b | ||
0 | #3 | 4.2 b | 218,325 ab | 18,008 a | 526,917 a | 28,500 a | |
#1 | 5.5 ab | 297,060 a | 15,950 a | 336,192 ab | 40,486 a | ||
Jumbo | 7.5 a | 441,465 a | 24,584 a | 465,989 a | 18,169 a | ||
SWT | 45 | #3 | 0.7 c | 47,265 de | 3,418 bc | 128,769 def | 1,563 b |
#1 | 0.0 c | 0 e | 0 c | 0 f | 0 b | ||
Jumbo | 0.0 c | 270 de | 13 c | 364 def | 0 b | ||
30 | #3 | 0.9 c | 78,855 cd | 6,418 bc | 201,141 cd | 2,218 b | |
#1 | 0.7 c | 37,320 bc | 1,977 b | 53,004 bc | 3,838 b | ||
Jumbo | 0.9 c | 75,840 cd | 2,725 bc | 49,099 cde | 7,010 b | ||
0 | #3 | 5.6 ab | 275,415 a | 22,800 a | 731,665 a | 11,940 a | |
#1 | 5.0 b | 245,490 ab | 19,077 a | 497,953 ab | 15,698 a | ||
Jumbo | 5.1 b | 343,875 a | 21,171 a | 517,797 a | 23,823 a |
†Within each column, LSMeans sharing the same letter are not significantly different (
LSMeans, least squares means; PJR, Postman Joyner; SWT, Florida Sweetheart PP 8526.
Hot water exposure time (
An ANOVA summary table. Effects of hot water exposure time, caladium cultivar, and tuber size on caladium growth parameters.
Factor | Fresh shoot weight† (g) | Dry shoot weight† (g) | Fresh root weight† (g) | Dry root weight† (g) | Plant height† (cm) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
DF | DF | DF | DF | DF | |||||||||||
Exposure time | 2 | 19.62 | <0.01 | 2 | 8.03 | <0.01 | 2 | 18.46 | <0.01 | 2 | 6.07 | <0.01 | 2 | 0.58 | 0.56 |
Cultivar | 1 | 9.75 | <0.01 | 1 | 29.47 | <0.01 | 1 | 0.03 | 0.87 | 1 | 5.37 | 0.02 | 1 | 389.94 | <0.01 |
Tuber size | 2 | 275.65 | <0.01 | 2 | 181.33 | <0.01 | 2 | 13.83 | <0.01 | 2 | 32.07 | <0.01 | 2 | 12.28 | <0.01 |
Exposure time × Cultivar | 2 | 1.86 | 0.16 | 2 | 0.94 | 0.39 | 2 | 6.08 | <0.01 | 2 | 6.98 | <0.01 | 2 | 6.50 | <0.01 |
Exposure time x Tuber size | 4 | 0.87 | 0.49 | 4 | 0.49 | 0.75 | 4 | 0.87 | 0.48 | 4 | 0.47 | 0.76 | 4 | 1.16 | 0.33 |
Cultivar × Tuber size | 2 | 4.63 | 0.01 | 2 | 4.57 | 0.01 | 2 | 2.12 | 0.12 | 2 | 3.34 | 0.04 | 2 | 1.06 | 0.35 |
Exposure time x Cultivar × Tuber size | 4 | 1.73 | 0.15 | 4 | 1.83 | 0.13 | 4 | 0.88 | 0.48 | 4 | 0.56 | 0.69 | 4 | 1.21 | 0.31 |
†Data were fit to Gaussian distribution in the Glimmix model in SAS University Edition.
Both 30 min and 45 min hot water exposure similarly improved caladium shoot and root growth; root weight was greater when caladium tubers received 45 min hot water treatment as compared to 30 min treatment. Caladium cultivar PJR had greater (
The effects of different factor combinations on caladium plant growth are presented in Table 4. Fresh shoot weight ranged from 67.8 g to 197.1 g, with #3 PJR exposed for 0 min and Jumbo PJR exposed for 30 min supporting the lowest and highest numbers; dry shoot weight ranged from 4.4 g to 13.1 g with #3 SWT exposed for 0 min and Jumbo PJR exposed for 30 min supporting the lowest and highest weights; fresh root weight ranged from 10.0 g to 24.0 g with #1 SWT exposed for 0 min and Jumbo SWT exposed for 45 min supporting the lowest and highest weights; dry root weight ranged from 0.33 g to 1.02 g with #3 SWT exposed for 0 min and Jumbo SWT exposed for 45 min supporting the lowest and highest numbers. Similar trends were observed for caladium shoot and root weights, with the greater shoot and root weights resulting from larger tubers for the same condition of hot water treatment, regardless of caladium cultivar. For both caladium cultivars, within the same tuber size, hot water treated caladium tubers had more shoots and roots when compared to non-treated caladium tubers (Fig. 1). Overall, cultivar PJR grew taller than cultivar SWT.
Effects of factor combinations (hot water exposure time, caladium cultivar, and tuber size) on caladium growth parameters.
Cultivar | Exposure time @ 50°C (min) | Tuber size | Fresh shoot weight (g) | Dry shoot weight (g) | Fresh root weight (g) | Dry root weight (g) | Plant height (cm) |
---|---|---|---|---|---|---|---|
PJR | 45 | #3 | 89.8 ghij† | 6.4 fghij | 15.5 bc | 0.59 abc | 34.4 b |
#1 | 121.0 efg | 8.0 defg | 20.2 ab | 0.93 ab | 34.3 b | ||
Jumbo | 184.2 ab | 11.7 ab | 20.9 ab | 0.90 ab | 36.3 ab | ||
30 | #3 | 74.7 ij | 5.4 hij | 13.85 bc | 0.5 bc | 33.8 b | |
#1 | 131.0 def | 9.3 bcde | 14.8 bc | 0.75 abc | 36.0 ab | ||
Jumbo | 197.1 a | 13.1 a | 17.1 abc | 0.85 ab | 39.8 a | ||
0 | #3 | 67.8 j | 5.1 ij | 13.1 bc | 0.52 bc | 34.2 b | |
#1 | 106.5 fghi | 8.1 defg | 18.3 abc | 0.93 ab | 37.7 ab | ||
Jumbo | 154.7 bcd | 11.3 abc | 15.3 bc | 0.88 ab | 37.4 ab | ||
SWT | 45 | #3 | 84.0 hij | 5.2 ij | 15.6 bc | 0.50 bc | 27.5 c |
#1 | 126.9 def | 7.9 efgh | 21.4 ab | 0.84 ab | 27.6 c | ||
Jumbo | 169.3 abc | 10.5 bcd | 24.0 a | 1.02 a | 28.4 c | ||
30 | #3 | 80.7 hij | 5.7 ghij | 15.4 bc | 0.54 bc | 25.1 c | |
#1 | 111.8 efgh | 7.6 efghi | 17.4 abc | 0.69 abc | 25.7 c | ||
Jumbo | 155.2 bcd | 10.0 bcde | 20.8 ab | 1.00 a | 27.4 c | ||
0 | #3 | 67.9 j | 4.4 j | 10.1 c | 0.33 c | 23.9 c | |
#1 | 99.3 fghij | 6.7 fghij | 10.0 c | 0.39 c | 24.3 c | ||
Jumbo | 143.5 cde | 8.9 cdef | 15.5 bc | 0.69 abc | 27.3 c |
†Within each column, LSMeans sharing the same letter are not significantly different (
LSMeans, least squares means; PJR, Postman Joyner; SWT, Florida Sweetheart PP 8526.
Root-knot nematodes are the most widely distributed plant-parasitic nematodes in the world. Caladium is very susceptible to five species of root-knot nematode that have been reported in Florida (Kokalis-Burelle et al., 2017). Hot water treatment has been used for more than 100 yr across the world as an environmentally friendly pre-planting disease management strategy, and included in the purposes for which it is applied is control of plant-parasitic nematodes on different crops, such as bananas, rice, citrus, vegetable tubers, and ornamentals (Bridge, 1975). While different nematode genera have different heat-tolerant thresholds (Khanal et al., 2020), limited heat threshold information is available for species within the same nematode genus (Rhoades, 1961; Brcka et al., 2000).
The results from our study confirm that hot water treatment significantly reduces root-knot nematode population density in caladium tubers, and can significantly reduce root-knot damage caused by planting infected tubers (Rhoades, 1970; Brcka
Although root-knot nematodes in caladium tubers were not eliminated by hot water treatments, the previous results confirm the benefits of hot water treatment for the management of root-knot in caladium. A longer exposure time (45 min instead of the industry standard of 30 min) may be needed, especially when tubers are more infected, and for cultivars such as SWT that have more scale-like tissue. Tuber size did not seem to affect hot water efficacy, indicating that the nematode contamination is near the surface of the tubers, and no differential treatments are needed to treat different tuber sizes. Elimination of root-knot nematodes from caladium tubers will require increasing hot water immersion temperature or exposure time. While hot water treatment at higher temperatures delayed caladium tuber germination (Rhoades, 1961), it is not known how this affects subsequent growth in the field, and whether or not longer exposure times can be a safe option. In conclusion, hot water treatments remain an essential tool for caladium growers to help manage nematode and disease problems.