SOIL SOLARIZATION TO ELIMINATE DISEASES FROM GREENHOUSES

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1 SOIL SOLARIZATION TO ELIMINATE DISEASES FROM GREENHOUSES W.L. Kline 1, W.J. Roberts 2, S.T. Kania 3, and S.A. Johnston 4 Abstract: Greenhouse solarization was evaluated as a method to control soil borne diseases. A heated greenhouse with infrared 6 mil plastic on the soil surface or clear 4 mil plastic on the soil surface was compared to a unheated greenhouse covered with infrared 6 mil plastic plus clear 4 mil plastic on the soil surface or black 6 mil plastic on the soil surface and a clear 4 mil plastic tent stretched above the black plastic. Maximum soil temperatures reached in the heated greenhouse were F ( C) for both plastic treatments at the 1 in (2.54 cm) and 4 in (10 cm) depths. At 12 in (30 cm) temperatures were F ( C). Soil temperatures in the unheated greenhouse varied between F ( C) at 1 in (2.54 cm), F ( C) at 4 in (10 cm), and F (39-41 C) at 12 in (30 cm). Soil samples were collected at each depth and the soil tested for Rhizoctonia solani. Rhizoctonia solani colonized beet seeds 11-85% in the control houses followed by the unheated clear 2-10%, unheated black plus clear 1-6%, heated clear 0-0.5% and heated infrared 0%. Keywords: Solarization, Rhizoctonia solani, soil temperature, greenhouse. introduction Disease control in greenhouses is a continual need for growers who plant directly in the soil. This is a common practice in Southern New Jersey for growers who raise their own transplants. Greenhouses are fumigated with metam-sodium in the fall for disease control 1. The greenhouses are then left uncovered until late winter when one year plastic is applied and greenhouses seeded. Solarization may offer an alternative to chemical control if it reduces deleterious fungi, nematodes and weeds below damaging levels. Soil solarization has been reported to control Rhizoctonia solani in onions, beans, okra and cotton 2,4 ' 5 ' 6. The thermal time and temperature has been calculated for many organisms including Rhizoctonia solani. Pullman et al 7 determined that temperatures of 108 and 130 F (39 and 50 C) killed the organism on agar in 14 days and 10 minutes, respectively. These temperatures have been achieved through soil solarization at another site in New Jersey to a depth of 12 in (30 cm) 3. Southern New Jersey is warmer than the central or northern parts of the state which should make solarization more advantageous for those farmers. This study was design to determine if the organism could be controlled through solarization under commercial conditions. 1 County Agricultural Agent, Rutgers Cooperative Extension of Cumberland County, 291 Morton Ave., Millville, New Jersey Professor and Director, Center for Controlled Environment Agriculture, Cook College, Rutgers University, New Brunswick, New Jersey Horticultural Engineer, Center for Controlled Environment Agriculture, Cook College, Rutgers University, New Jersey Professor and Extension Specialist in Plant Pathology, Rutgers Agricultural Research & Extension Center, 121 Northville Rd., Bridgeton, New Jersey

2 Methods and Materials The greenhouses used in the solarization study were located at a commercial farm in Vineland, New Jersey. The hoop greenhouses were constructed with galvanized 1/2 inch (13 mm) pipe. The two treatment houses were 95 ft (29 m) long, 14 ft (4 m) wide and 10 ft (3 m) high. Two small control houses were 36 ft (11 m) long, 14 ft. (4 m) wide and 8 ft. (2.5 m) high. All the greenhouses had a mixture of sand and peat moss for floors. The heated treatment house had galvanized 1 1/4 in (32 mm) pipe placed 12 in (30 cm) apart at a 8 in (20 cm) depth in the floor. Water temperature through floor heat system was maintained at 160 F (65 C). The unheated house was covered with a single layer 6 mil IR polyethylene glazing. All four greenhouses had a row of concrete blocks around the base to a height of 10 in (25 cm). Each treatment greenhouse was divided into two equal sections. A set of three copper-constantan thermocouples were placed in the soil in the center of each section at 1 in (2.5 cm), 4 in (10 cm) and 12 in (30 cm) from the soil surface. A reference thermocouple was placed outside midway between the two greenhouses at 40 in (101 cm) from the soil surface. Shielded psychrometers were placed in the center of each treatment greenhouse near the top attached to a support galvanized pipe. An additional psychrometer was placed outside and between the greenhouses. All psychrometers measured dry bulb temperatures. An additional thermocouple was suspended under the clear (non IR) tent to determine temperature gradient. When the thermocouples were in place the soil was saturated with water and the different polyethylene coverings applied to each half of a greenhouse. Clear (non IR) 4 mil and 6 mil IR polyethylene was applied directly to the soil surface in the heated greenhouse. Care was taken to minimize air pockets that reduce the soil heating process. Black 6 mil and clear (non IR) 4 mil polyethylene was applied to the soil surface in the unheated greenhouse. A tent effect was created over the black 6 mil polyethylene by placing concrete blocks on the plastic and running a galvanized 1 1/4 inch (32 mm) pipe on top of them. Clear 4 mil polyethylene was stretched over the pipe creating a tent 18 in (45 cm) high. An additional thermocouple was suspended under the tent to determine the temperature gradient. A Campbell 21x micrologger was installed to monitor all the sensors and to save averaged data in a storage module. All sensors were scanned at one minute intervals, and each fifteen consecutive values were averaged and stored in the storage module. The storage module was downloaded regularly to a personal computer. Soil samples were collected at the end of the solarization period to determine Rhizoctonia solani control. A section of 3 in (7.5 cm) irrigation pipe was inserted into the soil to 1 in (2.5 cm), 1-4 in ( cm), 4-8 in (10-20 cm) and 8-12 in (20-30 cm) to remove a soil core. Four separate soil cores were taken from each treatment and the two control greenhouses and air dried. A 15 gm soil subsample was placed in a petri dish and 1.35 gm of autoclaved beet seeds placed on top. An equal amount of soil was placed on top of the beet seed and moistened. After two days, the seed was washed, dried and 10 seeds under sterile conditions placed on each of 5 petri dishes which contained 1.5% water agar amended with 0.25 gm chloramphenical per liter of agar. The dishes were left at room temperature for 24 hours and counted for Rhizoctonia solani. The organism was suspected if 25% or more of the beet seed was infected. 274

3 Results and Discussion Soil Temperatures Temperature recording started July 26 (Julian day 208) and concluded September 24 (Julian day 268). No data was recorded for days 213, 214, 262, 263 and 264. A student T-tests at the 99% confidence level was calculated for the remaining maximum daily temperature values within each greenhouse. Figures 1 shows the maximum daily temperatures for the season for all recording depths and polyethylene covers in the heated greenhouse. The maximum outside air temperature recorded was 96.7 F (33.4 C) and the maximum soil temperature at 40 in (lm) was 88.2 F (29.1 C). The temperature above the polyethylene film was approximately 4 F (1.6 C) warmer than the outside surrounding temperature. This is probably caused by reflection from the polyethylene surface. There were no significant differences among the different recording depths in the heated greenhouse except at the 1 vs 12 inch depth when the 1 inch measurement was warmer (table 1). When the same depths between the two polyethylene covers were compared there were no significant differences. Maximum temperatures at the 1 in (2.54 cm) depth ranged between and F (64.4 and 64.5 C), at the 4 in (10.2 cm) depth ranged between and F (63.3 and 63.6 C) and at the 12 in (30 cm) depth ranged between and F (57.3 and 58 C). These temperature were reached before the heat was turned off on day 229. Table 1. Comparisons between maximum temperatures at recording depths and under polyethylene covers in the heated greenhouse. Vineland, New Jersey Treatment Comparison T-value 1 Significant 2 Clear 1" vs 4" 1.04 ns Clear 4" vs 12" 1.36 ns Clear l"vs 12" 2.52 sig IR 1" vs 4" 0.81 ns IR 4" vs 12" 1.60 ns IR 1" vs 12" 2.46 sig Clear vs IR 1" vs 1" 0.17 ns Clear vs IR 4" vs 4" 0.10 ns Clear vs IR 12" vs 12" 0.07 ns 1 Critical value for all tests was All t-tests used critical value for 100 df, 99% confidence. If bottom heat is available in this type of greenhouse either clear or IR film will provide sufficient solarization to raise soil temperatures. The problem with the clear film without IR protection is that it 275

4 may crack before the solarization is complete. This did not happen in 1996, but there were many cloudy days. Subsequent research in 1997 showed that the clear plastic does breakdown. Figure 2 shows the maximum daily temperatures for the unheated greenhouse at all recording depths.. The maximum air temperature in the greenhouse was F (56.5 C) and between the black film and the clear tent F (74.1 C). Temperature in the greenhouse was 46.2 F (25.6 C) warmer than the outside air. This resulted from the 6 mil IR polyethylene glazing over the house. Maximum soil temperatures at the 1 in (2.54 cm) depth ranged between and F (55.8 and 57.4 C), at the 4 in (10.2 cm) depth ranged between 130 and F (50 and 52.5 C) and at the 12 in (30 cm) depth ranged between and F (39.2 and 41.1 C). Soil temperatures in the unheated greenhouse were significantly different from each other at the different recording depths for both polyethylene covers. This was not true when the same depths were compared between polyethylene films except at the 12 in (30 cm) depth where the clear/black cover was 4 F (2.2 C) warmer. The clear tent over the black film is hard to maintain since the clear film has a tendency to stretch as the greenhouse warms. Since there is little different between the two treatments based on this one year's data using the clear film directly on the soil surface is more practical. Table 2. Comparisons between maximum temperatures at recording depths and under polyethylene covers in the unheated greenhouse. Vineland, New Jersey Treatment Comparison T-value 1 Significant 2 Clear/black (C/B) 1" vs 4" 3.25 sig Clear/black (C/B) 4" vs 12" 9.90 sig Clear 1' vs 4" 4.16 sig Clear 4" vs 12" 9.47 sig C/B vs Clear 1" vs 1" 0.87 ns C/B vs Clear 4" vs 4" 1.98 ns C/B vs Clear 12" vs 12" 3.41 sig 1 Critical value for all tests was " vs 12" not tested since first two significant 2 All t-tests used critical value for 100 df, 99% confidence. Rhizoctonia solani control High temperatures are needed in soil solarization to control disease organisms without chemicals. It has been reported that temperatures in the range of 120 to 130 F (48.9 to 54.4 C) control Rhizoctonia solani 36. The maximum temperatures in the heated greenhouse ranged between and F (57.3 and 64.5 C) and the unheated greenhouse and F (39.2 and 57.4 C). Except for the 12 in (30 cm) depth in the unheated greenhouse, temperatures should have been sufficient to control the organism. The data in table 3 shows there was Rhizoctonia solani present in the two control greenhouses. Beet seed incubated from the soil surface samples were colonized 72 to 85 %. The percent colonization declined as samples were taken from deeper in the soil. Most organisms follow this same pattern. 276

5 Table 3. Percent beet seed colonized by Rhizoctonia solani. Vineland, New Jersey. 1996'. Depth in Control 1 Control 2 Heated IR Heated Clear Unheated Clear Unheated B/C Inches Mean value from four soil subsamples Both treatments in the heated greenhouse provided approximately 100% control of the organism. One soil surface subsample had 2% colonization, but maximum temperature was also reached at this depth. Control was not as complete in the unheated greenhouse. There were more colonized beet seeds in the top four inches (10-12%) than in the heated greenhouse. Whether this percentage would affect the succeeding crop was not determined. Some of the discrepancy between the temperatures when Rhizoctonia solani should be controlled and the control in the unheated greenhouse may be due to sampling method. This experiment has been repeated in 1997 and the data is being analyzed. Conclusions Soil solarization in greenhouses will raise the soil temperature to control Rhizoctonia solani. The most reliable non-chemical system with growers who grow transplants directly in the soil is to use heat through underground pipes. Soil temperatures must be at least 130 to F (50 to 52.5 C) for complete control. Without underground heat control is not complete, but may be adequate depending on the ambient air temperatures and length of solarization. Literature Cited 1 Garrison, S.A. (ed) Commercial vegetable production recommendations. NJAES Publ. EOOIM. 146pp. 2 Kahn, V.A., C. Stevens, J.Y. Lu, D.J. Collins, M.A. Wilson, J.E. Brown, M.K. Kabwe, and O. Adeyeye Response of Okra transplants on soil solarizing plots during the active period of soil solarization. American Society of Plasticulture. 24: Kania, S. and W.J. Roberts Solarization study of soil in plastic greenhouses. American Society of Plasticulture. 26: Osman, A.R. and A.F. Saheb Control of Rhizoctonia solani by soil solarization. Acta horticulturae 152: Pullman, G.S., J.E. De Vay, C.L. Elmore, and W.H. Hart Soil solarization a nonchemical method for controlling diseases and pests. UC Division of Agric. & Natural Resources. Leaflet Pullman, G.S., J.E. De Vay, and R.H. Garber Soil solarization and thermal death: A logarithmic relationship between time and terperature for four soilbome pathogens. Phytopathology 71: Pullman, G.S., J.E. De Vay, R.H. Garber, and A.R. Weinhold Soil solarization: Effects on Verticillim wilt of cotton and soilborne populations of Verticillium dahliae, Phythium spp., Rhizoctonia solani, and Thielaviopsis basicola. Phytopathology 71:

6 Julian Time (July26-Sept24,1996) Figure 1. Maximum daily temperature for two polyethylene covers at three soil depths in a heated greenhouse. Vineland, New Jersey ro Q. E Julian Time (July26-Sept24,1996) Figure 2. Maximum daily temperature for two polyethylene covers at three soil depths in an unheated greenhouse. Vineland, New Jersey