2022 Volume 91 Issue 4 Pages 501-507
In the replanting of Japanese pear, the growth of young trees after replanting is often suppressed due to soil sickness syndrome and white root rot. Pre-planting soil disinfection by hot water drip irrigation (HWD) treatment was developed as a control technique for white root rot. In addition, it is also expected to reduce the risk of soil sickness and promote tree growth. We therefore investigated the HWD treatment of pre-planting soil to promote the growth of young trees and obtain high yields at an early stage after replanting Japanese pear. HWD treatment of the pre-planting soil accelerated the growth of young trees in the first two years after planting and increased the yield in the third year compared to untreated soil. The HWD and water treatments of soil lowered the risk of soil sickness syndrome, suggesting that washing out growth-inhibiting substances with water promoted the growth of young trees. The HWD treatment flushed out nitrate-nitrogen from the soil and temporarily increased ammonia nitrogen due to decreased nitrifying activity one day after treatment. However, the change in nitrogen in the soil did not significantly affect growth, and the pH of the soil did not change significantly before and after the treatment. This suggests that the growth-promoting effect was not solely due to changes in soil chemistry, but rather due to the reduction in the risk of soil sickness. These results suggest that HWD treatment of pre-planting soil effectively reduces the risk of soil sickness and promotes the growth of young trees during the replanting of Japanese pear.
The Japanese pear is an important tree species in fruit farming in Ibaraki prefecture; however, many orchards are more than 30 years old, and there is concern about a decline in yield and quality. In fact, the ratio of young orchard areas is low (2%), and replanting has not progressed (Ibaraki prefecture, 2016) because the growth of young trees after replanting is often poor, and it is not easy to recover the yield quickly. One reason for the poor growth of young trees is pear white root rot, which is a soil disease that is difficult to control. It is caused by Rosellinia Prillieax and causes wilting of leaves and, in severe cases, death. A new disinfestation method using hot water drip irrigation (HWD) at 60–70°C has been developed in former pear fields (Eguchi et al., 2008). It is expected to be used as a disinfestation technique before planting in fields contaminated with white root rot.
In addition to its therapeutic effect, it has been reported that HWD treatment promotes the growth of pear trees, regardless of whether or not the soil is infected with pear white root rot (Shiota et al., 2013). Shiota et al. (2013) reported that pear trees grown for one year after treatment with 80°C hot water in soil that had never been cultivated with pears produced more fine roots than those grown under normal water treatment or no treatment. Hirai et al. (2019) also reported that HWD treatment at the extraction point of previous trees resulted in good growth of pear trees after planting, suggesting that changes in the amount of inorganic nitrogen in the soil before and after HWD treatment is a factor. These studies have shown only the effect on tree growth one or two years after treatment and did not investigate the effect on yield of young trees after HWD treatment.
One of the causes of poor growth of young trees after replanting is soil sickness syndrome (Toya et al., 2012). Soil sickness is defined as the induction of negative conditions in the soil by the plants themselves for their vegetative and reproductive needs (Bennett et al., 2012; Huang et al., 2010). Soil sickness syndrome is a complex, multifactorial phenomenon, and several hypotheses regarding its cause it have been proposed. Plant species inhibit growth of their own kind through the release of toxic chemicals into the environment. This phenomenon plays a significant role in orchards and is the major reason for the replant problem (Singh et al., 1999). However, it is unclear whether HWD treatment improves the soil for young trees by removing growth inhibitors.
In this study, three tests were conducted to clarify the effect of HWD treatment on orchard soil before planting on growth and yield (Test 1), soil chemicals (Test 2), and the risk of soil sickness (Test 3). Based on these results, the growth-promoting factors of HWD treatment are discussed.
We set up plots (humic Haplic Andosols) at the Horticultural research institute, Ibaraki agricultural center, Kasama city, Ibaraki prefecture, Japan. Restricted root zone cultivation of pears was set before the test. In the restricted root cultivation, the rows were 3.6 m apart, a root barrier sheet with a width of 2 m was laid on the ground, and an embankment 120 cm wide and 20 cm high was placed on it. In October 2013, the planted trees were pulled out, the root barrier sheets were removed, and the soil was thoroughly turned over. The trees in restricted-root-zone beds and those planted before installing restricted-root-zone cultivation were not affected by white root rot.
The test plots were set up along the rows of the former restricted root-zone beds. Two treatment plots were set up in the field, one with HWD + root bottom restricted treatment and a control. In the HWD + root bottom restricted treatment, a root barrier sheet (length:width = 2 m:2 m, Root wrap sheet, 30A; Hasegawa Kogyo Co., Miyazaki, Japan) was placed at a depth of 30 cm. This root barrier sheet is permeable, breathable, and inhibits vertical root elongation. In anticipation of its widespread use in white root rot-contaminated fields, the sheets were set up to protect young trees after planting by inhibiting vertical movement of fungus from the subsoil to the roots. This is because HWD is less effective in soil layers deeper than 30 cm, and the fungus remains (Eguchi et al., 2008). In the control, we performed only tillage, and no HWD or root bottom restricted treatments were applied.
In the HWD + root bottom restricted treatment, the soil was treated according to a method described in the “Manual for hot-water treatment of white root rot, revised and supplemented in 2013” (NARO, 2015). After preparing the surface layer of the soil, drip tubes (Uniram RC17, 20 cm pitch; Netafim Japan Co., Ltd., Tokyo, Japan) were arranged in a ladder-like configuration with a spacing of 15 cm (length:width = 1 m:1 m). The water heater temperature was set at 80°C to obtain a high water temperature in the dripping area. Soil temperature at a depth of 30 cm was measured at three locations during the treatment. The treatment was terminated when the temperature exceeded 55°C for 25 min or 45°C for 125 min at all locations (total: 5–5.5 h, approximately 400–600 L·m−2). The test cultivar was ‘Kosui’ (three trees per stock and one main branch training), and the first-year seedlings were planted in November 2013. Three stocks were planted in each plot (n = 4). The planting interval was 3.6 m × 3.6 m. The three trees per stock and one main branch training method included three trees per site, planted in an equilateral triangle of approximately 30 cm at each site, and one main branch grew per tree to expand the canopy quickly (Kagawa et al., 2022). Subsequent cultivation management was carried out according to prefectural fruit tree cultivation standards (Ibaraki agricultural center, 2016). Fruiting started in the third year after planting, and the fruit set was one fruit per three fruit pairs. No watering was applied.
Shoot length, number of shoots, yield, and fruit quality (fruit weight, sugar content, and firmness) were investigated. The yield was calculated by harvesting and weighing the whole fruit during the harvest season (August 10–24, 2016, August 16–29, 2017, and August 6–20, 2018) based on a color chart value for a ground color of 2.5–3.0. Nine to 10 fruits were randomly selected from each plant. The sugar content (Brix%) was measured using a refractometer (pocket refractometer PAL-1; ATAGO Co., Ltd., Tokyo, Japan), and firmness was measured using an MT type fruit firmness tester (FUJIWARA SCIENTIFIC Co., Ltd., Tokyo, Japan). Shoot length and number of shoots were measured after defoliation (November–December) for branches with a length of 10 cm or more. For statistical processing, a t-test was performed for each item.
Effect of HWD + root bottom restricted treatment on soil chemistry (Test 2)This study was conducted in a pear field (humic Haplic Andosols) in Ishioka city, Ibaraki prefecture, Japan. The plots were prepared in a space of 3.6 m × 3.6 m. The HWD + root bottom restricted treatment and control plots were set up at the sites where plants died due to white root rot in contrast to Test 1. In the HWD + root bottom restricted treatment plots, root-barrier sheets were buried, and HWD treatment was applied in October 2014 before planting, using the same method as in Test 1. The test cultivar was ‘Kosui’, and six trees in the HWD + root bottom restricted treatment plot and three trees in the control were planted in October 2014. One tree was planted at the same spot where the old tree had been planted, unlike the planting method used in Test 1.
Soil samples were taken using a hand auger from a depth of 0–20 cm at one point within 1 m from each tree before, one day after, and 30 days after the HWD treatment. The pH, EC, and inorganic nitrogen content (ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen) were measured. Nitrogen content was measured using the Kjeldahl method (Bremner, 1996). No fertilization was applied during this period.
In addition, the number and average length of shoots in the first year after planting were measured in the same manner as in Test 1. For statistical processing, a t-test was performed for each item. The presence or absence of white root rot in young trees during the first year after planting was diagnosed in July 2015, using the branch insertion method (Eguchi et al., 2009). In the branch insertion method, pear branches of 1–2 cm in diameter were cut to 30 cm in length. Six branches per tree were then inserted at a depth of 25 cm approximately 10 cm from the edge of the tree. The branches were then removed three weeks after branch insertion and checked for attachment of white root rot mycelium.
Effect of HWD and water treatment on the risk of soil sickness syndrome (Test 3)Two test plots, one with the HWD treatment and the other with the water treatment, were set up. In September 2021, 10-year-old ‘Kosui’ trees grown in root zone restricted beds (2 m wide, 2 m between plants, 20 cm deep) in a steel-frame greenhouse at the Ibaraki prefectural institute of horticulture were removed, and the soil was thoroughly stirred and collected. The soil was transferred into a non-woven fabric pot (“J-master K-20”; GUNZE Limited, Osaka, Japan) which had a diameter of 20 cm, depth of 19 cm, and capacity of 6 L. The treatment was administered on October 11, 2021. Each plot contained five pots (five replicates each).
Pots were placed on the ground and drip tubes (Uniram RC17, 20 cm pitch; Netafim Japan) were placed on pots to leave one sprinkler hole in the center of each pot. In the HWD treatment plot, hot water was dripped at 70°C using a hot water treatment machine for five hours. The treatment was terminated when the temperature exceeded 45°C for 125 min at the bottom of the pot (23.7 L per pot, approximately 750 L·m−2). In the water treatment plot, the same amount of normal temperature water was dripped as in the HWD treatment plot.
The inhibition rate of the soil was calculated using the rhizosphere soil assay method (Toya et al., 2020), and the risk of soil sickness was evaluated. The rhizosphere soil assay method was developed to measure the soil sickness syndrome of asparagus (Motoki et al., 2006) and velvet beans (Fujii, 1994), while Toya et al. (2020) applied it to the soil sickness syndrome in pears. Lettuce seeds (cultivar: ‘Legacy’) were sown on agar containing the sample soil and kept under dark conditions at 25°C for three days to measure the lettuce root length. The inhibition rate was calculated as the degree to which the root length of the sample soil was shortened, based on the root length of the blank wells. For statistical processing, a t-test was performed after arcsine transformation of the inhibition rate before and after treatment for each plot.
In Test 1, we investigated the effect of HWD treatment on the growth and yield of pear trees during the first five years after replanting at the former pear site to clarify the effect on early yield. The total length of shoots was 1.2 m/stock in the first year, 2.2 m/stock in the second year, and 9.3 m/stock in the fifth year in the HWD + root bottom restricted treatment plots (Fig. 1). They were significantly larger than 0.4 m/stock in the first year after planting, 1.1 m/stock in the second, and 6.1 m/stock in the fifth year in the control plots. The number of shoots was significantly higher in the HWD + root-bottom restricted treatment plots than in the control plots in the first, second, and fifth years after planting (Fig. 2).
Effect of HWD + root bottom restricted treatment before planting on total shoot length of young pear trees from the first year to five years after planting (Test 1). Vertical bars represent standard errors (n = 4). * indicates significant difference at P < 0.05 by t-test (n = 4).
Effect of HWD + root bottom restricted treatment before planting on the number of shoots of young pear trees from the first year to five years after planting (Test 1). Vertical bars represent standard errors (n = 4). ** and * indicate significant difference at P < 0.01 and P < 0.05, respectively by t-test (n = 4).
The total number of lateral branches was significantly higher in the HWD + root bottom restricted treatment than in the control in the third and fourth years after planting (Fig. 3). The total number of lateral branches in the HWD + root bottom restricted treatment was almost constant at 25 branches at three to five years after planting. In contrast, the number of lateral branches in the control increased three to five years after planting. There was no significant difference between the HWD + root bottom restricted plots and the HWD + root bottom restricted treatment plots five years after planting.
Effect of HWD + root bottom restricted treatment on the number of lateral branches of young pear trees from the third year to five years after planting (Test 1). Vertical bars represent standard errors (n = 4). * indicates significant difference at P < 0.05 by t-test (n = 4).
Yield in the HWD + root bottom restricted treatment was 8.7 kg/stock in the third year, significantly higher than 3.0 kg/stock in the control plots (Fig. 4). Four to five years after planting, there were no significant differences in yield among the treatments.
Effect of HWD treatment + root bottom restricted treatment of soil prior to planting on yield of young pear trees from the third year to five years after planting (Test 1). Vertical bars represent standard errors (n = 4). ** indicates significant difference at P < 0.01 by t-test (n = 4).
The number of harvested fruits was significantly higher in the HWD + root bottom restricted treatment three years after planting (Table 1). Fruit weight, firmness, and sugar content were not significantly different among the treatments in any year.
Effect of HWD + root bottom restricted treatment on pear site soil on the quality of harvested fruits from the third year to five years after planting (Test 1).
In contrast to those of Test 1, the fields in Test 2 were infested by white root rot. No significant differences were observed in the total shoot length or number among the treatments; however, the mean shoot length was significantly longer in the HWD treatment (Table 2).
Effect of HWD + root bottom restricted treatment on the growth of shoots from the first year of panting in former pear soil (Test 2).
To clarify the effect of the HWD treatment on soil chemicals, we examined soil chemicals immediately and after one month of HWD treatment and investigated the growth of pear trees in the first year after replanting. The pH of the soil did not change significantly after treatment (Fig. 5). The EC was significantly lower in the HWD treatment plot than in the control after one day of treatment. The ammonium nitrogen content was significantly higher in the HWD treatment group than in the control one day after treatment. The total content of nitrate-nitrogen and nitrite-nitrogen was significantly higher in the control than in the HWD treatment one day after treatment. The total amount of inorganic nitrogen in the soil was significantly higher in the control than in the HWD treatment one day after treatment, and higher in the HWD treatment than in the control 30 days after treatment.
Effect of HWD treatment on nitrogen, pH, and EC in soil (Test 2) (A: pH, B: EC, C: NH4-N, D: NO2+NO3-N, E: Total inorganic nitrogen). Vertical bars represent standard errors (n = 3–6). ** and * indicate significant difference at P < 0.01 and P < 0.05, respectively by t-test (n = 3–6).
The branch insertion method was performed eight months after replanting, and white root rot was not detected in any trees (data not shown).
Effect of potted pear soil HWD treatment on the risk of soil sickness syndrome (Test 3)A pot test was conducted to determine the effect of the HWD and water treatments on the risk of soil sickness syndrome. As described above, the risk of soil sickness syndrome was reduced by water treatment, and the HWD treatment could promote tree growth by decreasing the risk of soil sickness syndrome. Soil inhibition rates in the rhizosphere soil method were significantly lower after the HWD and water treatments than before the treatments (Fig. 6). Before the treatment, the soil inhibition rate was as high as 87.0% in the HWD treatment and 90.1% in the water treatment. After treatment, the inhibition rate of soil was 18.9% in the HWD treatment and 18.5% in the water treatment.
Effect of HWD treatment on the inhibition rate of soil in a pot experiment (Test 3). *** and ** indicate significant difference at P < 0.001 and P < 0.01, respectively by t-test after arcsine transformation (n = 5).
The HWD treatment has been shown to promote the growth of young pear trees one to two years after replanting (Hirai et al., 2019; Shiota et al., 2013), and in this study, the growth for five years after planting and initial yield were investigated. In Test 1, there was a significant difference in the total length and number of shoots of the trees in the first and second years after planting, which was supported by these results. Furthermore, the yield in the third year after planting in the HWD treatment was significantly higher than that in the control, indicating that the HWD treatment increased the initial yield after replanting. In the HWD + root bottom restricted treatment, the number of lateral branches remained constant at approximately 25 branches per stock from the third year after planting. In terms of the number of trees planted, this was 1,875 branches per 10 acres, which was higher than the target number of lateral branches in the mature orchard (1,715 branches per 10 acres; Ibaraki agricultural center, 2016). Therefore, the canopy expansion of the HWD treatment + root bottom restriction was considered complete in the second year after planting, while that of the control was completed in the fifth year. The difference in canopy expansion may have contributed to the difference in the initial yield between the HWD treatment and the control. A root barrier sheet was not placed in the control plots, and it was impossible to accurately assess whether the HWD or root bottom-restricted treatment had a stronger effect on tree growth. However, the main root zone in humic Haplic Andosols is 10–100 cm deep (Yoshioka et al., 1975), and preventing vertical root growth is a negative factor for tree growth. Therefore, the significant growth of the HWD + root bottom-restricted treatment in the first two years after planting could be attributed to the effect of the HWD treatment. Similar effects of the HWD treatment on growth after replanting in Test 1 were also observed in Test 2, which was examined in a field infested by white root rot. In Test 2, trees in neither the HWD treatment nor the control were infected with white root rot, indicating that the promotion of growth in the HWD treatment resulted from factors other than its therapeutic effect.
Regarding changes in soil chemistry due to the HWD treatment, Hirai et al. (2019) found that the amount of NH4 and total inorganic nitrogen tended to be higher in the soil one to two days after the HWD treatment, suggesting that changes in inorganic nitrogen may contribute to the increased growth of pears. In the present study, there was an increase in ammonia nitrogen in the HWD + root bottom-restricted plot one day after treatment, which corresponds with the reports by Hirai et al. (2019). However, there was no difference in ammonia nitrogen after 30 days between the HWD treatment and the control, or in the total amount of inorganic nitrogen between before and 30 days after the HWD treatment. This suggests that the HWD treatment promotes growth by factors other than the change in inorganic nitrogen in the soil.
The substance that causes soil sickness in pears has not yet been identified; however, it has been suggested that it is water soluble (Toya et al., 2021). Toya et al. (2020) found that at 30% soil inhibition, the dry matter weight of the planted trees was 88.6% compared to that of the new soil, and that at 40% inhibition, it was 65.9% compared to that of the new soil. In the present study, the inhibition rate of the soil decreased from approximately 80% to less than 30% in both the HWD and water treatments, suggesting that the substance causing the soil sickness syndrome was washed out in both treatments, thereby reducing the soil risk to the same extent as planting in new soil. Toya et al. (2021) also found that the amount of water required to reduce the soil inhibition rate significantly was 100 L/10 L of soil. However, in Test 3, the amount of water that flowed into one pot converted to 10 L of soil was 39.5 L, which was less than that reported by Toya et al. (2021). This suggests that the amount of water applied in Test 3 was sufficient to flush out the substance that causes soil sickness.
Therefore, we conclude that HWD treatment on soil prior to planting may increase the initial yield of young trees by reducing the risk of soil sickness in the replanting of former pear sites. Our results suggest that HWD treatment, which has been developed for disinfection of former white root rot sites, reduces the risk of soil sickness. Shiota et al. (2013) reported better growth with HWD treatment than with ambient water treatment; however, the amount of water used was 240–260 L·m−2, which was less than that in this study. This implies that high temperature water may reduce the risk of soil sickness syndrome, even with smaller amounts of water. From the results of this study, it can be considered that treatment with 750 L·m−2 water for five to six hours continuously reduces the risk to the soil, regardless of the increase in soil temperature, to obtain growth-promoting effects. However, more detailed studies are required to determine the water volume, temperature, and soil conditions that effectively reduce the risk of soil sickness syndrome.
