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Peer Reviewed

© 2013 Plant Management Network.
Accepted for publication 8 July 2013. Published 22 September 2013.

Effect of a High Tunnel, Organic Cropping System on Lettuce Diseases in Western Washington

Marianne Powell, Department of Plant Pathology, Jeremy Cowan and Carol Miles, Vegetable Horticulture Program, and Debra Ann Inglis, Vegetable Pathology Program, Northwestern Washington Research and Extension Center (NWREC), Washington State University, 16650 State Route 536, Mount Vernon, WA 98273

Corresponding author: Debra Ann Inglis.

Powell, M., Cowan, J., Miles, C., and Inglis, D. A. 2013. Effect of a high tunnel, organic cropping system on lettuce diseases in western Washington. Online. Plant Health Progress doi:10.1094/PHP-2013-0922-01-RS.


Incidence of gray mold and lettuce drop, and yield of six cultivars representing market classes Boston/Crisphead, Leaf, and Romaine, were evaluated in open ended high tunnel and open field organic production systems near Mount Vernon, WA from 2010 to 2012. Each year seedlings were transplanted in April and heads harvested in June/July. In 2010, Romaine types had significantly (P < 0.0001) greater incidence of gray mold (caused by Botrytis cinerea) than other types. In 2011, incidence of gray mold was significantly (P = 0.004) greater in high tunnel than open field plots, and greatest in high tunnels when fog persisted. All cultivars were equally susceptible to lettuce drop (caused by Sclerotinia sclerotiorum), although in 2012, incidence was significantly (P < 0.0001) greater in high tunnel than open field plots. ‘Green Star’ (Leaf type) had reduced incidence of gray mold and lettuce drop in 2010 and 2011. Incidence of tipburn was significantly (P = 0.032 and P = 0.001, respectively) greater in the high tunnels in 2011 and 2012 compared to the open field. Total yield (kg) was greater in the open field in 2012, but not in 2011 and 2010.


Lettuce (Lactuca sativa L.) is a cool-season crop, and in western Washington it is usually sold on the fresh market, where quality is critical. Regional growers are challenged because production is limited by wet conditions, low number of growing degree days, and farmland availability. Production commonly occurs from late April to mid September. Lettuce grows optimally when air temperatures range from 15.5 to 18°C, which is about 3°C warmer than the average spring and fall temperatures in this region. Growers are interested in ways to increase yield and quality by modifying the environment (17).

High tunnels are protective structures constructed upon native soil. They consist of a frame, which is a series of hoops, covered with a single or double layer of 0.10 to 0.15 mm clear, UV stabilized polyethylene film allowing optimal light penetration (17). High tunnels trap solar energy, increasing air and soil temperature, and block rainfall, reducing leaf wetness and excess soil moisture. Passive ventilation occurs by rolling up sidewalls and opening endwalls if present. High tunnels can reduce the risk of foliar diseases by altering environmental conditions conducive for disease outbreaks (19). Although high tunnels increase costs of production due to labor and equipment, the value of lettuce crops in a region with high demand for locally grown foods may offset those costs (17).

Gray mold, caused by Botrytis cinerea Pers. ex Fr. and lettuce drop caused by Sclerotinia sclerotiorum (Lib.) de Bary are two common lettuce diseases in western Washington. During infection by B. cinerea, brown lesions lead to rot and masses of gray conidia form. Symptoms of lettuce drop include water-soaked lesions, rot, abundant white mycelial growth, and sclerotia are often produced in plant tissues. tipburn, a calcium deficiency, reduces lettuce quality and marketability when inner leaves (which cannot be trimmed or discarded) are affected (4). Few lettuce cultivars have exhibited tolerance to B. cinerea (5) while only physiological and partial resistance to S. sclerotiorum has been demonstrated (9,18). Continued breeding efforts are needed before lettuce cultivars resistant to these necrotrophic fungi are available commercially (18). In the meantime, improved cultural methods for the control of gray mold and lettuce drop remain important. Cultivar selection is used for the management of tipburn since incidence is affected by genotype and environment (10). Cultural methods for disease control, such as using high tunnels, are especially important for organic production where most fungicides are not permitted. There is limited information about advantages and disadvantages of high tunnels for lettuce production in cool climates. The objectives of this study were to: (i) compare naturally-occurring disease outbreaks on lettuce cultivars grown in organically managed high tunnel and open field production systems in western Washington; and (ii) evaluate disease incidence and yield as affected by cultivar in these systems. Lettuce yield and quality results for 2010 and 2011 were previously reported by Wallace et al. (17). However, this paper presents disease and disorder incidence, and additional yield data for 2012.

High Tunnel and Raised Bed Construction

Field trials were established each spring from 2010 to 2012 at the Washington State University Northwestern Washington Research and Extension Center (NWREC) near Mount Vernon, WA (48°43’24"N, 122°39’09"W, elevation 6 m). The study design was a two-by-six factorial with high tunnel and open field production as the main plots and six lettuce cultivars (Table 1) as subplots, arranged in a randomized complete block split-plot design. The high tunnels were assigned randomly to the field at study onset and erected at the same locations each year while the lettuce cultivars were re-assigned to subplots randomly each year. In 2012, a different cultivar set was selected in order to expand the evaluation. Cultivars were selected based on market trend, availability of certified organic seed, and potential performance in a coastal climate. The trial was managed organically all three years. Drip tape and black polyethylene mulch were laid on the raised beds as described by Wallace et al. (17). Seedlings were transplanted into open ended high tunnel (‘Solo’ model, Haygrove LTD, UK; 37 m long × 8 m wide × 3 m high) and open field subplots (0.9 m wide × 0.2 m high × 4 m long) per Table 2. Planting was in two staggered rows with spacing of 31 cm in-row and 31 cm between rows (Fig. 1). Irrigation and fertigation schedules were typical for commercial production in the area (Table 2). In 2010, lettuce was planted following four years of small-grain and legume cover crops that had been managed organically, and in 2011 and 2012, lettuce followed organic strawberry. Soilborne pathogen populations were presumed to be uniformly distributed in the soil prior to the experiment since crop rotation and tillage occurred uniformly throughout this field site and lettuce had never been planted.

Table 1. Lettuce cultivars evaluated.

Table 2. Yearly lettuce planting, high tunnel construction and fertigation schedules.


Fig. 1. Lettuce transplants grown on a raised bed covered with black plastic mulch, utilizing drip irrigation under an open ended high tunnel in western Washington.


Collection of Environmental Data

Each year environmental data were collected at 15-min intervals with a Hobo U-30 Weather Station (Onset Computer, Bourne, MA) placed within one plot replicate. Air temperature (30 cm from soil surface), leaf wetness, and relative humidity were recorded from transplanting to final harvest (22 April to 1 July in 2010, 30 April to 27 June in 2011, 26 April to 9 July in 2012), except in 2011 when monitoring started nine days after transplanting.

Disease and Yield Ratings at Harvest

Lettuce was harvested by hand when at least 75% of heads reached marketable size (30 cm diameter) or prior to the loss of marketability due to disease and disorders. Harvest dates were 15 June to 1 July in 2010, 10 June to 27 June in 2011, and 14 June to 9 July in 2012. Total head number and weight per subplot were recorded. Harvested heads were inspected visually for disease, noted by rot and/or sporulation, and quality on a subplot basis. Wrapper leaves and stems were trimmed to obtain total marketable head weight, and recorded on a subplot basis. In 2011 and 2012, heads with tipburn were counted and weighed separately.

Diseased heads were cut longitudinally to assess symptom development in inner leaves and vascular tissues. In 2012, the number and viability of sclerotia formed by S. sclerotiorum in lettuce tissues also were tracked based on preliminary observations in 2011. Sclerotia were removed from plants with forceps, rinsed under tap water, air dried for 24 h on a laboratory bench, and then stored in a sealed box. At five weeks, five randomly selected sclerotia were placed onto ½-strength potato dextrose agar with bromophenol blue (½ PDA + BPB) as reported by Steadman et al. (16) except that antibiotics were omitted. Pathogens were isolated from: (i) plant tissue dipped in 70% EtOH for 30 s, then 10% NaOCl for 30 s, and rinsed with sterile deionized water before placing onto various culture media or placing in a moist chamber; or (ii) sclerotia, dipped in 70% EtOH for 2 s and passed through a flame, before placing onto ½ PDA.

Statistical Analysis

Data for disease incidence at harvest were subjected to analysis of variance using the PROC MIXED procedure of SAS v. 9.2 (SAS Institute Inc., Cary, NC) with the Satterthwaite method for assessing degrees of freedom and least squared means to obtain mean separations. Where the data did not satisfy assumptions of normality and equal variance for analysis of variance, transformations were selected using the range method described by Kirk (11). When significant interactions between cultivar and production system were detected, data were analyzed separately by production system using PROC GLM and mean separations were determined by Fisher’s Least Significant Difference with alpha level of 0.05. Total lettuce yield and tipburn were subjected to analysis of variance using the PROC MIXED procedure as above except that the Kenward-Roger method was used to determine denominator degrees of freedom for F-tests in 2011 and 2012. Where transformed data failed to meet the assumptions of normality or homogeneity, the data were analyzed non-parametrically after transformation.

Incidence of Gray Mold

Gray mold was observed on harvested heads from high tunnel and open field plots each year (Fig. 2a). A severe gray mold outbreak occurred in 2010, four days after relative humidity reached 90% and when average air temperature was 14°C both inside and outside the high tunnels. In 2010, gray mold was significantly (P < 0.0001) greater on the Romaine cultivars Coastal Star and Jericho compared to other cultivars, but statistically similar (P = 0.06) for both production systems (Table 3). Jericho and Coastal Star accounted for over 50% and approximately 10%, respectively, of all heads infected with B. cinerea in 2010. In 2011, gray mold was significantly (P = 0.004) greater on heads harvested from high tunnel than from open field plots, and Coastal Star remained one of the most susceptible cultivars. Green Star was not affected by gray mold in 2010 or 2011. In 2012, a significant interaction was detected between production system and cultivar. Coastal Star had significantly (P = 0.0004) more gray mold in the high tunnels compared to the other cultivars. Red Sails had significantly (P < 0.0001) more gray mold in the open field compared to other cultivars, but appeared equally susceptible in both production systems.


Fig. 2. Symptoms of disease and disorders on lettuce:  (A) gray mold on heads (left) and crowns (right); (B) lettuce drop on heads (left) and crown tissues with sclerotial formation (right); and (C) tipburn observed in heads (left) and wrapper leaves (right).


Table 3. Disease incidence on lettuce cultivars at harvest, by production system and cultivar.

Incidence of Lettuce Drop

Lettuce drop was observed on harvested heads from both production systems each year (Fig. 2b), but was not significantly (P = 0.76 and P = 0.86, respectively) different between production systems in either 2010 or 2011 (Table 3). In 2012, lettuce drop was significantly (P < 0.0001) greater in heads from high tunnels than from the open field. Sclerotia were more numerous in Romaine cultivars Coastal Star and Valmaine and from heads harvested inside high tunnels (Table 4). All cultivars were similarly (P = 0.82, P = 0.68, and P = 0.06, respectively) affected by lettuce drop in 2010, 2011, and 2012.

Table 4. Number and viability of sclerotia of Sclerotinia sclerotiorum collected from lettuce heads harvested from high tunnels (HT) and open field plots (OF) in 2012.

Environmental Differences in High Tunnel and Open Fields

Across all three years, air temperature averaged 13 to 14°C in the high tunnels and 12 to 13°C in the open field (Fig. 3). On sunny days, average air temperature in the high tunnels exceeded the open field by 3°C. High tunnels reduced leaf wetness by 281, 64, and 133 h in 2010, 2011, and 2012, respectively, as compared to the open field (Fig. 4). In 2012, 451 accumulated hours of leaf wetness were recorded in the high tunnel, which exceeded 2010 and 2011 measurements by 212 and 157 h, respectively. For approximately three weeks after seedlings were transplanted, relative humidity was similar between production systems, ranging from 78 to 81% in the high tunnels and 79 to 81% in the open field (Fig. 5). In the high tunnels on warm and sunny days, relative humidity was reduced by a maximum of 10, 4, and 7% in 2010, 2011, and 2012, respectively. On average, relative humidity was 79% in the high tunnels and 81% in the open fields.


Fig. 3. Daily average air temperature recorded in a high tunnel and an open field plot.



Fig. 4. Leaf wetness recorded over three growing seasons between high tunnel and open field plots.



Fig. 5. Relative humidity averages recorded in a high tunnel and an open field plot.


Yield and Quality Analysis

As previously reported by Wallace et al. (17), total and marketable yields from 2010 and 2011 were not affected by production system, and in 2011 total yield was significantly (P < 0.0001) greater for Coastal Star, Jericho, and Green Star compared to other cultivars. With a different set of cultivars in 2012, total and marketable yields were significantly (P = 0.012 and P = 0.001, respectively) greater in the open field than in the high tunnels (Table 5), and total and marketable yields were significantly (P < 0.0001 and P < 0.0001, respectively) greater for Coastal Star compared to the other cultivars.

Table 5. Total and marketable lettuce yield between production system and cultivar.

Tipburn was assessed in 2011 and 2012; when inner leaves were affected, marketability was diminished (Fig. 2c). In 2011, tipburn was significantly (P = 0.03) greater in heads grown in high tunnels than in the open field, and Jericho had significantly (P < 0.0001) more tipburn compared to other cultivars (Table 6). In 2012, a significant interaction between production system and cultivar was detected for tipburn. New Red Fire had significantly (P = 0.01) greater tipburn than other cultivars in the open field, but in the high tunnels additional cultivars were significantly (P = 0.01) more affected by tipburn, including Red Sails and Valmaine.

Table 6. Number of lettuce heads affected by tipburn.

Implications for Growers

Growers of warm weather crops stand to benefit from high tunnels, especially in areas with a cool season climate (17). However, lettuce, a cool-season crop, may not receive a similar advantage in open ended high tunnels in western Washington, especially given springtime transplanting conditions. In this study, organically managed lettuce crops grown under high tunnels had greater incidence of gray mold and tipburn in 2011 and lettuce drop in 2012, while yields were not improved any year as compared to the open field. High tunnels provided additional heat units but did not affect relative humidity due to limited air flow and periods of persistent, penetrating fog through the open ends, and/or periods of relatively low air temperature. Zhao et al. (20) similarly reported only slight differences in relative humidity between open ended high tunnels and open fields in Kansas.

In all three years incidence of gray mold was affected by cultivar, whereas lettuce drop was not. However, incidence of both diseases was greatest in the high tunnels when favorable outside environmental conditions prevailed, such as fog that affected gray mold in 2011 and leaf wetness that affected lettuce drop in 2012. Even though leaf wetness and relative humidity were greater in the open fields than in the high tunnels, the elevated air temperature inside the high tunnels combined with leaf wetness and high relative humidity, and possibly also within the lettuce head itself, proved conducive for the growth of B. cinerea. Conidia of B. cinerea germinate over a wide range of temperatures when relative humidity reaches 90% (8) but disease can develop when humidity is relatively low (65%) on wounded crop tissues (15). For this reason and others, B. cinerea creates challenges for resistance breeding. The fungus has multiple modes of attack including the ability to detoxify various plant saponins (14) and produce multiple phytotoxins (6). Some cultivars may be more tolerant to gray mold due to production of lettucinen A, a phytoalexin of Lactucae species that limits the growth of B. cinerea (2).

High temperatures and leaf wetness in the high tunnels in 2012 provided a favorable environment for S. sclerotiorum as observed by greater incidence of lettuce drop and increased production of sclerotia. More than 100 sclerotia were recovered from a single infected head in 5 out of 14 plants (36%) in the high tunnels. However, the maximum number of sclerotia recovered in the open field was 50, from one out of five affected heads. Abawi and Grogan (1) showed that sclerotial formation is greater at 25°C than 10°C. Another study showed that sclerotial density was positively associated with percent lettuce drop incidence (3). Although differences in susceptibility to lettuce drop were not detected among cultivars in this study, sclerotia production was greatest for the Romaine cultivars Coastal Star and Valmaine compared to other lettuce types. Screening Lactuca species for the presence of oxalate oxidase and oxalate decarboxylase genes, which degrade oxalic acid secreted by S. sclerotiorum during infection, may provide new directions for resistance breeding programs (7).

Tipburn was more severe in the high tunnels in 2011 compared to the open field, and many cultivars in the high tunnels were severely affected in 2012. High relative humidity and air temperature within high tunnels in 2011 and 2012 likely caused tipburn by reducing evapotranspiration levels while promoting rapid growth. Calcium transport to rapidly growing plant tissue is reduced near maturity and under high relative humidity conditions (4).

Disease management in organic high tunnel production might be improved by the use of UV-absorbing films similar to ones used by Kritzek et al. (12) to reduce the sporulation of B. cinerea on plant tissue (13). Selecting cultivars that are harvested prior to sclerotial development may be an additional cultural method for disease control, particularly where high tunnels are stationary and lettuce is consecutively cropped. Growers who choose to offset limited farmland or high tunnel construction and material costs by cropping high value, fresh-market lettuce continuously, without sufficient crop rotation, risk build-up of soilborne pathogens like Botrytis and Sclerotinia. Utilizing disease resistant cultivars is another important tool for organic growers to control disease, but gray mold resistant cultivars are not available and only partial resistance to lettuce drop has been established (9,18). This study showed that Green Star is well-suited for western Washington high tunnel production because the cultivar has some resistance to gray mold, was not seriously affected by lettuce drop, and had relatively high yields. The results from the cultivar evaluations in 2012 also indicate that Nevada has potential for limiting outbreaks of gray mold and lettuce drop in the region.


This study was part of a USDA SCRI-SREP grant award (no. 2009-51181-05897). We thank Babette Gundersen and Jonathan Roozen for their field and laboratory contributions to this project, and Dr. Dennis Johnson and Dr. Tim Murray for their critical review of this manuscript.

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