Introduction

In 2022, the total potato yield in China, the biggest potato (Solanum tuberosum L.) producer worldwide, was 95, 631, 395.81 tonnes (t) and the spanning area was 5, 726, 096 hectare (ha)1. Soilborne weeds and fungi, especially Spongospora subterranea f. sp. subterranea are exhibiting a very strong potential to decrease potato production and commercial quality (see Fig. 1). Spongospora subterranea f. sp. subterraneaidentified as the primary cause of potato powdery scab, has been documented globally in main potato producing countries including China2,3,4,5.

Fig. 1
figure 1

Potato tubers infected by potato powdery scab (right) compared with the health (left).

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It is quite a challenging and time-consuming study as the pathogen can only infect plant parts below the ground and cannot be grown in a laboratory6. The reason why there are no highly effective chemicals for controlling potato powdery scab is because the pathogen’s resting spores are very strong, the soilborne pathogen is not evenly distributed7, and toxic soil chemicals are being phased out over time8. Currently, there was no pesticides are registered for controlling potato powdery scab in China. A range of chemical treatments only had limited effects on potato powdery scab control worldwide. For example, conventional fungicides (mancozeb, propineb, dimethomorph, azoxystrobin, fluazinam or flusulfamide) did not control this disease or only provide partial efficacy6,7,9,10,11,12,13. Methyl bromide (MB) applied at 50 g m−2as one excellent soil fumigant was reported to provide good control on this soilborne inoculum14. However, MB, as one ozone-depleting compound, was completely banned except for critical use nominations in China since 201815. Several soil fumigants metham sodium11,12,13,14, and chloropicrin12,13,16 have been tested to reduce Spongospora subterranea f. sp. subterranea under some circumstance in potato production before planting worldwide. Scientists around the world have become increasingly interested in using reduced doses of fumigants in combination with other methods to improve their effectiveness against soilborne pests.

Binary soil fumigant combinations, 1,3-D + Pic17,18,19,20,21, 1,3-D + DMDS15,22,23 and DMDS + Pic20,21have been reported to control soilborne pests in various crops production, such as tomato and cucumber worldwide. To control soil nematode, DMDS or 1,3-D is worldwide proven to be equally effective to MB15,24,25,26. Though DMDS or 1,3-D does not work very well against soilborne fungi and oomycetes, it does have some effectiveness against weeds. Pic is a useful tool for managing soilborne fungi, but it does not have much impact on controlling weeds21. So, it would be beneficial to use Pic as an additional fumigant to enhance control over soilborne fungi and weeds. Therefore, it is theoretically possible that the novel combination of 1,3-D + DMDS + Pic could be used as a promising soil fumigant. The relevant patent of invention has been applied in China and made available to the public anywhere in the world27. However, very few information on the reports of 1,3-D + DMDS + Pic combinations are present to control potato powdery scab and weed.

Current study was mainly initiated to determine the potential value of 1,3-D + DMDS + Pic combination for controlling potato powdery scab and weed in potato under field conditions. Along with, DZ and Pic at standard doses as two currently widely used soil fumigants in ginger, strawberry, vegetable, and pseudo-ginseng in China were designed as well, for the determination of the efficacy for potato powdery scab and weed control at the same time.

Materials and methods

Field trials

In year 2019, two demonstration trials were conducted in the following locations: Dahetang village, Guyuan, Hebei Province, China (trial I); No.23 village, Guyuan, Hebei Province, China (trial II). The farms, which have been growing potatoes for over five years are currently dealing with significant issues caused by soilborne pests and weeds, particularly potato powdery scab.

95 1,3-dichloropropene EC (Dao Renji solvent chemical plant, Yunxi District, Yueyang, containing 1,3-D of 95%), 90 DMDS TC (Arkema (Shanghai) Chemical Co., Ltd., containing DMDS of 99%), 99.5 Pic TC (Dalian Lvfeng Chemical Co., Ltd., containing Pic of 99.5%) and 98 DZ MG (Zhejiang Haizheng Chemical Co., Ltd., containing DZ of 98%) were picked in the present study. The polyethylene film (PE) and totally impermeable film (TIF) used from Shouguang Yutian Plastic Co., Ltd., and had thickness of 0.04 mm.

A randomized blocks design was used to perform the experiment with three different chemical treatments. Each treatment was replicated four times (Table 1). In two field trials, treatments of DZ alone, Pic alone, 1,3-D + DMDS + Pic (1:3:2) combination and control group (untreated) were set up. DZ was practiced by soil mixture method at the normal rate of 50 g a.i. m−2. The Pic application was done by chisel injection at 40 g a.i. m−2 normal rate. 1,3-D + DMDS + Pic (1:3:2) combination was also practiced by chisel injection at a reduced 30 g a.i. m−2 rate. To reduce the emission of fumigants, soil mulch PE or TIF film was covered immediately after fumigant application. Because farm site and fumigation date are similar for trial I and trial II, soil temperatures (at 5 and 15 cm depth) in fumigation period were only observed in the plots of 1,3-D + DMDS + Pic in trial I using Soil Temperature Data logger (HZ-TJ1, Beijing Hezhong Bopu Technology Development Co., Ltd.). And the soil temperatures at 5 and 15 cm depth were 2.3–27.8℃ and 3.6–18.0℃, respectively. Table 2 provided the relevant details of the two field trials. The size in trials for individual plot was 31.5 m2 (5 m wide by 6.3 m long). In first trial the fumigants were practiced on April 16, 2019, and the traps were removed on April 30, 2019. Shepody and Favorita as two widely cultivated commercial potato cultivars in China have been both officially identified and approved by National Variety Approval Committee, China. Shepody was bred in Canada in 1980 and introduced from the USA in 1987; Favorita was introduced from The Netherlands in 1981 by the Ministry of Agriculture of PRC. Shepody was provided by Simplot China and Favorita was provided by Zhangjiakou Hongji Potato Breeding Center, China. The potato seeds known as cultivar “Shepody” were planted on May 9, 2019. The investigation into the potato powdery scab disease was completed on August 30, 2019. The fumigants were applied on April 15, 2019, in field trial II. The potato seeds, known as cultivar “Favorita” were planted on May 10, 2019. The investigation into the potato powdery scab disease was completed on August 29, 2019 (Table 2).

Table 1 Experimental field trials treatments.
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Table 2 Important trial dates and other experimental information.
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The effects of chemical treatments were evaluated on weeds by counting the different weed species and measuring the fresh weight of weed plants in each plot. For each plot, species, number, and weight of weeds in four randomly selected quadrates measuring 0.5 m x o.5 m were recorded. The potato germination rates at 6 weeks after sowing (WAS) by counting all the plants per plot were estimated. An investigation was conducted on the powdery scab disease of potato tubers at the end of trials. Each plot consisted of 20 plants. The severity of potato fruits (tuber) potato powdery scab disease was assessed using a scale ranging from 0 to 6. A rating of 0 indicated scabbing between 0.1% and 2%. Ratings 2,3,4,5, and 6 corresponded to scabbing percentages of 2.1–5%, 5.1–10%, 10.1–25%, 25.1–50%, and 50.1–100% respectively28. The potato fruit yields, and income of the growers were collected at the end of trials, during the potato harvest.

Statistical analyses

The effectiveness of weed control for plants or fresh weight was measured using following equation. 

$$Y=\frac{{{X_1} - {X_2}}}{{{X_1}}} \times 100,$$
(1)

where Y represents the effectiveness of weed control for plants or the fresh weight, X1 represents the number or fresh weight of weeds in the untreated group, and X2 represents the number or fresh weight of weeds in the treated fumigated plots.

The scores for powdery scab disease in potato fruits (tubers) recorded for each plot were converted into tuber disease (% TDI) using the formula described as below29:

$$\:\left(\text{\%}\right)TDI=\frac{\sum\:\left(fv\right)}{NX}$$
(2)

where ƒ represents the number of fruits in each class, v represents the class value, N is total number of observed fruits, and X represents the highest value of the evaluation scale.

The effectiveness of controlling potato fruits (tubers) powdery scab disease was measured by the equation following.

$$\:Y=\frac{{TDI}_{1}-{TDI}_{2}}{{TDI}_{1}}\times\:100$$
(3)

where Y represents the control efficacy for potato powdery scab disease, TDI1 is the tuber disease indices of potato powdery scab disease in control group (untreated), TDI2 is tuber disease indices of potato powdery scab disease in treated fumigated plots.

SPSS, statistical software version 22.0 was used to run analysis of variance (ANOVA) for the analysis of present study. Before analyzing, percentages were transformed using the arcsine transformation to make the variables more consistent. However, data is given as raw values. The Fisher’s LSD test with a significance level of 0.05 was used to determine significant differences among the means.

Results

Potato germination and weed control

Compared with control (untreated) group, rate of potato germination at 6 WAS were not significantly changed on all fumigation treatments in both field trials (Table 3).

Table 3 Effects of soil fumigation on potato germination and weed density, respectively.
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Chenopodium glaucum and Digitaria sanguinalis as two main species of weed were selected to assess the fumigation effects. The control (untreated) groups in trials I and II were heavily infested by Chenopodium glaucum with 21.5–60.5 plants m−2 (fresh weight of 3.78–5.81 g m−2) and Digitaria sanguinalis with 37.8–58.8 plants m−2 (fresh weight of 2.53–3.83 g m−2) at 6 WAS, respectively (Table 3). The significantly higher numbers and weight of Chenopodium glaucum and Digitaria sanguinalis weed plants were noticed in the control (untreated) group compared to all the fumigant treatments in both field trials (P = 0.05). The combination treatment of 1,3-D, DMDS and Pic (1:3:2) reduced the number of Chenopodium glaucum and Digitaria sanguinalis weed plants by 95.0–97.7% and 98.7%−96.7%, respectively, while the outcomes were not found statistically different from DZ or Pic treatment at both field trials. Similarly, the combination treatment of 1,3-D, DMDS and Pic (1:3:2) reduced the fresh weight of Chenopodium glaucum and Digitaria sanguinalis weed plants by 85.6–97.4% and 89.0–96.3%, respectively; and the results again were not found statistically different from DZ or Pic treatment in both treated fields (Table 3).

Potato powdery scab control

The untreated control plots in both trials I and II had high potato powdery scab disease incidences of 61.8% and 77.2%, which showed that the control (untreated) plots were heavily infested by potato powdery scab (Table 4). However, on comparing with control (untreated) group, potato powdery scab disease incidences were not found significantly affected in all treatments of fumigant except for combination treatment of 1,3-D, DMDS and Pic (1:3:2) at the end of the trials.

Table 4 Soil fumigation effects on potato powdery scab.
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When comparing the chemical treatments to the untreated control groups, it was found that all of the chemical treatments had a significant impact on the tuber disease index (P = 0.05) and provided good control efficacy of 64–86.9% on potato powdery scab (Table 4). The untreated control plots at two field trials provided the highest index for tuber disease, with percentages of 40.0% and 47.8%. During the trial, the combination treatment of 1,3-D, DMDS, and Pic (in a ratio of 1:3:2) showed the lowest disease index at 5.2%. This result was not statistically different from the disease index of Pic alone at 11.8% but it was significantly lower than the disease index of DZ alone at 14.4% (Fig. 2). During trial II, the combination treatment of 1,3-D, DMDS, and Pic (in a ratio of 1:3:2) showed the lowest root disease index at 6.4%. This result was not significantly different from the treatment with Pic alone at 12.8%. However, it was significantly lower than the treatment with DZ alone at 15.3% (P = 0.05) (Table 4).

Fig. 2
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Potato tubers in plots treated with (A) DZ 50 PE, (B) Pic 40 PE, (C) 1,3-D + DMDS + Pic (1:3:2) 30 TIF, and (D) Untreated control at the end of the trials, respectively (trial I).

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Potato yield, income and economic benefits

The yield of potatoes varied with the chemical treatment (P = 0.05) (Table 5). When comparing the chemical treatments, the potato yield in the control plots was the lowest. It was 1.66 kg m−2 at trial I and 1.60 kg m−2 at trial II. During trial I, combination treatment of 1,3-D, DMDS, and Pic (in a ratio of 1:3:2) to some plots. These plots had the highest production, with a yield of 4.87 kg m−2 which was a 193.4% increase compared to the control. The yield was not significantly different from the plots treated with Pic alone, which had a yield of 4.60 kg m−2 and a 177.1% increase. However, the yield was significantly higher than the plots treated with DZ alone, which had a yield of 3.59 kg m−2 and a 116.3% increase (Fig. 2). In the second trial, plots treated with Pic had the highest yield, producing 3.64 kg m−2, which was a 127.5% increase compared to the control. This yield was not significantly different from the combination treatment of 1,3-D, DMDS, and Pic in a ratio of 1:3:2, which had a yield of 2.94 kg m−2, an 83.8% increase. However, yield from the Pic treatment was significantly higher than the yield from DZ alone, which was 2.79 kg m−2, a 74.4% increase (P = 0.05) (Table 5).

Table 5 Effects of soil fumigation on potato yield, income and financial benefits.
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Similarly, the income earned by the grower through potato production also varied depending on the chemical treatments used (Table 5). The minimum gross income from untreated potatoes was obtained, which was 1.66 and 1.28 ¥ m−2, respectively. At trial I, plots treated with 1,3-D, DMDS and Pic combination (1:3:2) had the maximum gross incomes, with 8.28 ¥ m−2, which is a 388.2% increase. This result was not significantly different from the plots treated with Pic alone, which had a gross income of 7.82 ¥ m−2, a 358.8% increase. However, the combination treatment had significantly higher gross incomes compared to the plots treated with DZ alone, which had 6.10 8.28 ¥ m−2, a 258.8% increase. In the second trial, the plots treated with Pic had the highest gross incomes (5.46 ¥ m−2, a 326.6% increase). This was not significantly different from the combination treatment of 1,3-D, DMDS, and Pic (1:3:2) (4.41 ¥ m−2, a 244.5% increase), but it was significantly higher than the plots treated with DZ alone (4.19 ¥ m−2, a 277.3% increase) (P = 0.05) (Table 5).

Furthermore, the economic benefits of various soil fumigation treatments were evaluated (referred to as net income). The cost of fumigating different soil treatments was calculated by adding the cost of fumigants and the cost of the film. The treatment using a combination of 1,3-D, DMDS, and Pic (in a ratio of 1:3:2) had a relatively low cost for fumigation, which was lower than the cost of using DZ treatment or using Pic alone. The specific costs per square meter were 1.17 ¥ m−2 for the combination treatment, 1.95 ¥ m−2 for DZ treatment, and 1.65 ¥ m−2 for Pic alone (as shown in Table 5). The net income of growers was ultimately determined by subtracting the fumigation cost from the gross income (Table 5). During trial I, combination of 1,3-D, DMDS, and Pic (in a ratio of 1:3:2) was applied in certain plots. These plots showed the highest net income with a value of 7.12 ¥ m−2, representing a significant increase of 328.7%. The net incomes of these plots were not significantly different from the plots treated with Pic alone, which had a net income of 6.17 ¥ m−2 and a 272.0% increase. However, the net incomes of the combination treatment plots were significantly higher compared to the plots treated with DZ alone (4.15 ¥ m−2, 149.9% increase) and the untreated control plots. In trial II, the plots treated with Pic had the highest net income (3.81 ¥ m−2, a 197.7% increase). This was not significantly different from the combination treatment of 1,3-D, DMDS, and Pic (1:3:2) (3.24 ¥ m−2, a 153.1% increase), but it was significantly higher than the plots treated with DZ alone (2.24 ¥ m−2, a 75.0% increase) and the untreated control plots (P = 0.05) (Table 5).

Discussion

Serious damage of soilborne Chenopodium glaucum, Digitaria sanguinalis and Spongospora subterranea f. sp. subterranea on potato yield and quality were obviously found and confirmed in untreated plots in the current study (see Fig. 2), which has also been reported before8,11,30. Spongospora subterranea f. sp. subterraneafrom the soils, which could stay inactive for extended periods and are greatly resistant to chemicals and environment related stresses30, are the main and utmost vital factor for powdery scab31,32,33. So, adopting proper soil fumigants to minimize the initial levels of S. subterranea f. sp. subterranea in soil will achieve maximum results with little effort in controlling potato powdery scab. Based on our two filed trial results, the 1,3-D + DMDS + Pic (1:3:2) novel combined use at 30 g a.i. m−2 with 20–25% dose reduction, which provided similar outcomes as Pic regular treatment dose (40 g a.i. m−2) in reducing disease severity of potato powdery scab, but was more efficient than DZ normal treatment dose (50 g a.i. m−2), was confirmed as a promising soil fumigant for controlling potato powdery scab and two main weeds in potato. In current field studies, there was no soil nematode disease. However, the broad-spectrum advantage of the combination 1,3-D + DMDS + Pic can be put into full play when there was also serious soil nematode infestation, for example, Ditylenchus destructor34, Globodera rostochiensis35, in the future studies.

Soil fumigant Pic as one of the most promising MB alternatives were widely used in controlling various soilborne pathogens. Recently, there have been numerous studies on Pic fumigation for controlling potato powdery scab which could reduce disease severity and increase tuber yield12,36,37, and the results of present study trials also supported this finding. However, a failure result for the management of powdery scab, was ever obtained by Pic fumigation at rates of 7 to 20.2 g a.i. m−2 with no cover, which only reduced the levels of Spongospora subterranea f. sp. subterranea in the soil by 11–34%16. The failure could be attributed to the rapid escape of fumigant Pic without plastic cover or bed pressing12,36 and the relative low dose which was not enough for control soilborne pathogens compared with the current studies (40 g a.i. m−2) and previous studies (39.6–50 g a.i. m−2)12,37,38.

DZ is a soil fumigant which decomposes into methyl isothiocyanate in moist soil. Rigorous soil mixture, sufficient soil moisture and proper soil temperature are all vital for the successful application of DZ fumigation39,40. In our two filed trials, DZ treated plots provided good control efficacy in reducing disease severity of potato powdery scab (64% and 68%) which was similar with Pic treatments (70.6% and 73.2%) but was lower significantly than that in the combination 1,3-D + DMDS + Pic treated plots (86.9% and 86.5%). The occurrence of crop diseases and the effectiveness of pesticide application are closely related to environmental conditions (especially temperature and humidity) in the field41. As we all know, potatoes are mostly planted in cool/temperate regions, the soil temperature during pre-planting soil fumigation was relatively low which was not optimal for soil fumigation but was similar as the optimal range for Spongospora subterranea f. sp. Subterraneanzoospores8,42. In current field trials, pre-planting soil fumigation of potato was applied in North Hebei in April, and the soil temperatures at 5 and 15 cm depth were 2.3–27.8℃ and 3.6–18.0℃, respectively, which was similar with the temperature during pre-planting soil fumigation of ginger in Shandong in March38,40, but the current minimum temperature was lower than that in previous studies in Israel (minimum temperature: 18℃)12. In our field trials, all fumigant treatments provided good control efficacy in controlling disease index of potato powdery scab but had relatively poor performance in controlling disease incidence, which could be partly because the soil temperature is relatively low and there are high levels of soil infestation9,12. In all fumigant treatments, DZ treatment had the lowest yield and gross income but the highest fumigation cost, which will hinder its application especially in low economic value potato production compared with high economic value crop production, such as ginger, strawberry, and pseudo-ginseng.

This is the first report of soil fumigation for potato soilborne pests especially powdery scab control in China. In conclusion, the soil fumigant combination 1,3-D + DMDS + Pic (1:3:2) through chisel injection was applied at 30 g a.i. m−2 effectively controlled main weeds (Chenopodium glaucum and Digitaria sanguinalis), significantly reduced the disease incidence and disease index of potato powdery scab with good control efficacy while providing abundant potato yields and income for farmers. The potential of biological control agents to reduce root rot pathogens and improve crop production has been much reported earlier43. So, in order to reduce the possible side effects on soil beneficial organisms44, detailed research work on its use proper composites with potential biological control agents (e.g. Trichoderma harzianum, Trichoderma atroviride, Bacillus subtilisor others)15,45are necessary before the combination can be recommended in integrated pest management programs44(including appropriate planting date46, biological control43 and chemical control) of potato for controlling potato powdery scab and weeds in China.

Statement. The collection of field studies on plants related to the article “Application of the combination of 1,3-dichloropropene, dimethyl disulfide and chloropicrin for potato powdery scab and weed control in China” complies with the relevant laws of Mainland China.