TREATMENT OF ONION BULBS WITH SURROUND® TO

REDUCE TEMPERATURE AND BULB SUNSCALD

Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR, 2004

Introduction

Onion prices generally decrease starting in September when harvest intensifies. Harvesting earlier from overwintered, transplanted, or normally planted full season onions could increase profits, but mechanized early harvest runs the risk of increased losses to sunscald. Sunscald occurs when the side of the bulb exposed to afternoon sun becomes excessively hot. Sunscald results in a flattened and shrunken area on the bulb surface. The 59-year-average maximum air temperature at the Malheur Experiment Station is 91, 90, and 80°F for July, August, and September, respectively. Maximum air temperatures in July and August often exceed 100°F, which can result in very high unprotected bulb temperatures and sunscald. Surround® (Engelhard Corp., Iselin, NJ) is a product made from kaolinite clay and works by forming a white coating on surfaces, thus reflecting solar radiation. Surround is a wettable powder that is labeled for reduction of sunscald in fruits and vegetables. Application of Surround after onions are lifted could reduce sunscald and make early mechanized harvests more feasible.

Methods

Trials were conducted in two fields in 2004.

Procedures for Growing Onions in Field 1

The onions were grown with subsurface drip irrigation at the Malheur Experiment Station, Ontario, Oregon on an Owyhee silt loam previously planted to wheat. Onion (cv. 'Vaquero', Nunhems, Parma, ID) was planted on March 17, 2004. The procedures can be found in "Effect of Short-duration Water Stress on Onion Single Centeredness and Translucent Scale" by Shock et al. in this report.

Procedures for Growing Onions in Field 2

The onions were grown with furrow irrigation on an Owyhee silt loam previously planted to wheat. Onion seed ('Vaquero') was planted on March 19, 2004. The procedures can be found in "2004 Onion Variety Trials" by Shock et al. in this report.

Procedures for Surround Treatments

Onions in each field were lifted on August 9. The lifted onions were divided into plots 23 ft long. The experimental designs were randomized complete blocks with four replicates in each field. There were seven treatments: treatment 1 was untreated; 2 received one Surround application after lifting; 3 received one Surround application after lifting and one after windrowing; and 4 was treated after windrowing (Table 1). Treatments 2-4 had an application rate of 25 lb Surround/acre. Treatments 5-7 were the same as treatments 2-4, except that the application rate was 50 lb Surround/acre. The Surround was applied after lifting on August 9 with a ground sprayer and a boom with 9 nozzles spaced 10 inches apart. The Surround was applied in 102 gal water/acre with 8004 nozzles at 40 psi.

Prior to the Surround application, temperature probes were installed in bulbs at 0.5-cm depth. The temperature probes in the monitored bulbs were placed so that they faced to the south-southeast in a position receiving direct sun. Three replicates in the drip-irrigated field and two replicates in the furrow-irrigated field each had one bulb monitored for temperature. The temperature probes were read hourly by a datalogger (Hobo datalogger, Onset Computer Corp., Bourne, MA).

On August 12 the temperature probes and probed onions were removed and the onions were topped and windrowed by hand. After windrowing the temperature probes were reinserted in different onions as before. The onion windrow was sprayed with Surround using a ground sprayer with 3 nozzles spaced 10 inches apart. Application rates and specifications were the same as the initial Surround application. Since only the windrow was sprayed (one-third of the field), the application rates were reduced to 8.3 lb Surround/acre for treatments 2-4 and to 17 lb Surround/acre for treatments 5-7.

The onions were bagged on August 16 and hauled to a shed. On August 19 the onions were graded. Bulbs were separated according to quality: bulbs without blemishes (No. 1s), bulbs with sunscald damage, double bulbs (No. 2s), neck rot (bulbs infected with the fungus Botrytis allii in the neck or side), plate rot (bulbs infected with the fungus Fusarium oxysporum), and black mold (bulbs infected with the fungus Aspergillus niger). The No. 1 bulbs were graded according to diameter: small (<2.25 inches), medium (2.25-3 inches), jumbo (3-4 inches), colossal (4-4.25 inches), and supercolossal (>4.25 inches). Bulb counts per 50 lb of supercolossal onions were determined for each plot by weighing and counting all supercolossal bulbs during grading.

To reduce the influence on the statistical analysis of the variability in onion yield and size between plots, the data for each field were normalized in relation to the average total yield for that field. Normalized data were subjected to analysis of variance.

Results and Discussion

The highest air temperature reached after lifting of the onions and before topping and windrowing was 100°F on August 11 (Table 2). The highest average bulb temperature reached after onions were lifted and before they were topped and windrowed was 129.5°F. Following the application of Surround after lifting on August 9, average maximum bulb temperatures were reduced 6-7°F compared to the untreated bulbs, but the temperature differences were statistically significant only on August 9 (Table 2, Fig. 1). Bulb temperatures for the 50-lb/acre Surround rate were slightly lower than for the 25 lb/acre rate, but the differences were not statistically significant.

The highest air temperature reached after topping and windrowing on August 12 was 99°F on August 13 (Table 3). The highest average bulb temperature reached after topping and windrowing was 129.2°F. For the onions treated with Surround after topping and windrowing, average maximum bulb temperatures were reduced by 5-6°F compared to the untreated check (Table 3, Fig. 1). After topping and windrowing, the bulb temperature differences between treatments were statistically significant for all days measured (August 12-15). After topping and windrowing, bulb temperatures for the 50-lb/acre Surround rate were slightly lower than for the 25-lb/acre rate and the differences were statistically significant on August 13, August 15, and on average.

The furrow-irrigated field (field 2) had lower marketable yield and higher yield of onions with sunscald than the drip-irrigated field (field 1, Table 4). There were no significant differences in onion yield or grade between treatments. However, in Field 2 there was a small but significant reduction in rot with increasing total amount of Surround applied (Fig. 2). In 2003, application of Surround resulted in statistically significant reductions in bulb sunscald and in increases in marketable yield (Shock et al. 2004). In 2003, the highest bulb temperature reached after lifting was 123°F and the highest bulb temperature after topping and windrowing was 121°F. These bulb temperatures were 6.5 and 8.2°F lower than the highest bulb temperatures in 2004. The higher bulb temperatures in 2004 could be related to the higher average bulb sunscald in 2004 (152 cwt/acre) compared to the average sunscald in 2003 (37 cwt/acre). The higher bulb temperatures in 2004 might also be related to the higher air temperatures reached during the 2004 trial (average of 97°F) than during the 2003 trial (average of 94°F).

References

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 2004. Treatment of onion bulbs with "Surround" to reduce temperature and bulb sunscald. Malheur Experiment Station Annual Report, Oregon State University Agricultural Experiment Station Special Report 1055:75-79.

Table 1. Treatments applied to onions to evaluate two application rates of Surround®, Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.

Treatment Surround rate lb/acre Post lifting Surround application Post topping and windrowing Surround application
1 none No No
2 25 Yes No
3 25 Yes Yes
4 25 No Yes
5 50 Yes No
6 50 Yes Yes
7 50 No Yes

Table 2. Maximum daily air temperature and maximum bulb temperature (°F) at 0.5-cm depth for onions treated with two rates of Surround® after lifting, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

Date Maximum air temperature Solar radiation Surround rate lb/acre LSD (0.05)
none 25 50

°F 24-hr total watt hr/m2 -----------------°F -----------------
9 Aug 92 7,531 123.6 116.2 114.2 5.4
10 Aug 97 7,340 127.4 122.6 120.7 NS
11 Aug 100 7,204 129.5 124.2 123.7 NS
Average

126.8 121.0 119.5 NS

Table 3. Maximum daily air temperature, solar radiation, and maximum bulb temperature (°F) at 0.5-cm depth for onions treated with two rates of Surround® after topping and windrowing, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

Date Maximum air temperature Solar radiation Surround rate lb/acre LSD (0.05)
none 25 50

°F 24-hr total watt hr/m2 -----------------°F -----------------
12 Aug 98 7,081 128.4 123.0 121.5 3.4
13 Aug 99 6,830 126.5 121.2 119.7 1.4
14 Aug 97 4,769 125.8 120.5 119.5 1.5
15 Aug 96 6,656 129.2 125.5 123.6 1.4
Average

127.5 122.5 121.1 1.2

Surround



Non-marketable yield
rate 1st applic. 2nd applic. Marketable yield
Small Doubles Scald Rot
lb/acre

cwt/acre % of total
---------------- cwt/acre ----------------
Field 1








none No No 674.0 79.2
7.9 0.4 165.3 3.0
25 Yes No 697.8 82.0
8.1 2.8 140.2 1.6
25 Yes Yes 718.7 84.5
6.7 0.8 121.8 2.6
25 No Yes 703.5 82.7
8.8 1.7 132.5 4.2
50 Yes No 667.2 78.4
9.4 0.0 172.2 1.8
50 Yes Yes 712.7 83.8
8.0 0.4 126.2 3.2
50 No Yes 712.9 83.8
9.5 0.6 125.2 2.4
average

698.1 82.1
8.3 1.0 140.5 2.7
Field 2








none No No 641.3 76.4
3.8 0.7 189.1 4.6
25 Yes No 664.7 79.2
4.4 0.7 168.6 1.2
25 Yes Yes 672.5 80.1
5.1 0.8 159.1 2.1
25 No Yes 676.0 80.5
3.8 2.2 155.5 2.1
50 Yes No 673.3 80.2
2.7 1.5 161.1 1.1
50 Yes Yes 679.9 81.0
4.6 2.3 150.9 1.8
50 No Yes 651.5 77.6
3.2 3.0 179.7 2.2
average

665.6 79.3
3.9 1.6 166.3 2.1

Fields 1 and 2 average








none No No 657.7 77.8
5.9 0.6 177.2 3.8
25 Yes No 681.3 80.6
6.2 1.8 154.4 1.4
25 Yes Yes 695.6 82.3
5.9 0.8 140.4 2.3
25 No Yes 689.7 81.6
6.3 1.9 144.0 3.1
50 Yes No 670.2 79.3
6.0 0.7 166.7 1.4
50 Yes Yes 696.3 82.4
6.3 1.4 138.6 2.5
50 No Yes 682.2 80.7
6.4 1.8 152.4 2.3
LSD (0.05) Trt NS NS NS
NS NS NS NS
LSD (0.05) Field NS NS 2.7
NS NS 22.6 NS
LSD (0.05) Trt X Fld NS NS NS
NS NS NS NS

Figure 2. Effect of total amount of Surround® applied on onion decomposition in a furrow-irrigated field, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.