Extended-duration row covers to suppress bacterial wilt on muskmelon: optimizing a new management strategy for organic growers

Final Report for GNC09-112

Project Type: Graduate Student
Funds awarded in 2009: $9,976.00
Projected End Date: 12/31/2012
Grant Recipient: Iowa State University
Region: North Central
State: Iowa
Graduate Student:
Faculty Advisor:
Dr. Mark Gleason
Iowa State University
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Project Information

Summary:

Field experiments using different row cover removal strategies on organically-grown muskmelon and butternut squash were carried out in 2010 and 2011 in Iowa. Little or no bacterial wilt was observed both years. In 2010, in the absence of bacterial wilt, row cover treatments reduced the need for fungicide sprays, increased yield, and enhanced earliness of muskmelon. This effect was not observed in 2011. On squash, season-long row covers and no row covers obtained the highest yield. In addition, six on-farm trials were carried out to demonstrate the use of delayed row cover for bacterial wilt management.

Introduction:

Cucurbits are among the most important vegetable crops in the U.S. In 2007, cucumbers, melons (including muskmelon), pumpkins, and squash were produced in approximately 50,500 ha in the U.S. (FAO, 2008), with a farm gate value exceeding $400 million a year. Striped (Acalymma vittatum) and spotted (Diabrotica undecimpunctata) cucumber beetles are the most important insect pests of melons and cucumber in the eastern half of the U.S. Cucumber beetles not only inflict direct feeding damage, but also vector the bacterium Erwinia tracheiphila, the causal agent of bacterial wilt. Bacterial wilt causes up to 80% crop loss in muskmelon (Latin, 1993) if the beetles are not controlled effectively. The bacteria are harbored in the beetle’s digestive system (Garcia-Salazar et al., 2000) and transmitted through direct contact of the mouth parts and frass (feces) to feeding wounds on stems and leaves.

Once in the plant’s vascular system, E. tracheiphila multiplies and blocks the water flow, causing wilting and death. No resistant varieties of muskmelon have been developed. Therefore, growers need to prevent the beetles from feeding and inoculating plants. Both conventional and organic growers struggle to contain bacterial wilt when beetles are numerous. The situation is especially challenging for organic growers, since organic insecticides are often ineffective deterrents of cucumber beetles. Many organic vegetable growers in the Upper Midwest avoid growing muskmelon after discouraging experiences with bacterial wilt. Clearly, there is a pressing need for alternative strategies that can deliver effective control of this devastating insect-disease complex.

Deploying row covers to provide a sheltered environment from transplanting until anthesis (start of flowering) has been a standard practice for many Midwest muskmelon growers for decades. Field experiments at ISU showed that this practice could delay the onset of bacterial wilt (Mueller et al., 2006), but did not provide acceptable season-long suppression. Recently, ISU researchers borrowed an idea that had been pioneered in Canada and Africa (Gaye et al., 1991; Vassiere and Froissart, 1996): extending the length of time that the muskmelons remain under row covers. By delaying row cover removal until 10 days after anthesis, the ISU team discovered that bacterial wilt could be suppressed throughout the remainder of the growing season (Jesse et al., 2007) without using any chemical insecticides. Pollination during the extended row cover period was accomplished by either opening the ends of the row covers or inserting a bumble-bee box under the cover. When transplanting was delayed several weeks due to wet weather, however, removing row covers at anthesis controlled wilt as effectively after 10 days (Saalau et al., 2008). These results suggest that timing of transplanting may be critical in gaining the benefits of the delayed row cover removal strategy, probably due to timing of vectoring by the cucumber-beetle vectors. This project will perform the additional experiments necessary to develop grower guidelines for consistent success.

Project Objectives:

The aim of this project was to provide a feasible strategy to control bacterial wilt and other insect pests on muskmelon and butternut squash in an organic management system. The first objective compared the use of row covers with different times of removal to prevent the incidence of bacterial wilt on both crops. The second objective was to involve organic growers in on-farm trials and demonstrate the use of extended duration row covers on cucurbits. Trial results were communicated to growers by presentations at a regional grower meeting, a field day, creation of a website, and newsletter publications.

Cooperators

Click linked name(s) to expand
  • Dr. Mark Gleason
  • Erika Saalau Rojas

Research

Materials and methods:
Muskmelon field trials at Iowa State University – Gilbert Horticultural Research Station (Gilbert, IA)

In 2010 and 2011, transitioning organic land was used for the multi-factorial experimental plots at the Iowa State University Gilbert horticultural research station. On May 27, 2010 and May 17, 2011, two to three-wk-old organic transplants of ‘Strike’ muskmelon were planted 2 ft apart in black plastic mulch with drip irrigation and 8 ft centers. Subplots consisted of 30 ft rows of 15 plants. Spunbond polypropylene row covers (Agribon® AG-30) were installed on wire hoops immediately after transplanting. Three row cover treatments were compared in 2010 (treatments 1-3) and a fourth row cover treatment was included in 2011 (treatment 4). Treatments were compared as follows:

1) No row covers (control)
2) Row covers applied at transplanting, with the ends opened at anthesis and row covers removed 10 days later
3) Row covers applied at transplanting and removed at anthesis (start of bloom)
4) Row covers applied at transplanting and removed 10 days after anthesis

Butternut squash field trials at Iowa State University – Gilbert Horticultural Research Station (Gilbert, IA)

In 2010 and 2011, transitioning organic land was used for the multi-factorial experimental plots at the Iowa State University Gilbert horticultural research station. On June 10, 2010 and June 8, 2011, 10 day-old organic transplants of ‘Betternut 401’ winter squash were planted 2 ft apart in black plastic mulch with drip irrigation and 9-ft row centers. 10 day-old organic transplants of ‘Betternut 401’ winter squash were planted 2 ft apart in black plastic mulch with drip irrigation and 9-ft row centers. Spunbond polypropylene row covers (Agribon® AG-30) were installed on wire hoops immediately after transplanting. Row cover treatments were compared as follows:

1) No row covers (control)
2) Row covers applied at transplanting, with the ends opened at anthesis and row covers removed 10 days later
3) Row covers applied at transplanting and removed at anthesis (start of bloom)
4) Row covers applied at transplanting; bumble-bee box placed under row covers at anthesis; row covers removed at first harvest

In 2011, treatment 4 consisted of row covers applied at transplant; removed at flowering (July 22) and replaced 18 days later (Aug 8); row covers removed at harvest.

Pest and weed management

OMRI-registered insecticides and fungicides were applied on a rescue basis only, triggered by results of weekly monitoring. On muskmelon, Pyganic® was applied to control picnic beetle damage on ripening fruit. On butternut squash, Entrust® (spinosad) was applied for squash vine borer and Microthiol® (sulfur) was applied to control powdery mildew. Champ 50WG® (copper hydroxide) was used to control anthracnose on both crops. Weed management was achieved with 6 inches of chopped corn stalk mulch between rows and composted bark was placed around the opening in the plastic around each seedling before row cover placement.

Data collection

Populations of insect pests were monitored weekly from transplant through the beginning of harvest using weekly visual counts on 5 randomly chosen plants. Disease incidence was monitored weekly. Melons were harvested twice weekly to optimize fruit quality for four weeks beginning. The number and weight of marketable and cull melons and squashes harvested from each subplot was recorded. Cull butternut squash fruit with a physiological disorder, in which the vine attaches to the underside of the fruit, were also noted.

On-farm cooperator trials

In 2010 and 2011 on-farm trials were carried out at three organic cooperator farms. At Turtle Farm (Grainger, IA), ‘Betternut 401’ winter squash was transplanted every two feet (2 seeds per hill) in 150-foot long segments. At Growing Harmony Farm (Nevada, IA) and ZJ Farm (Solon, IA), ‘Strike’ and ‘Athena’ muskmelon, respectively, were transplanted into black plastic mulch. At each farm, single-row treatments using polymer row covers (Agribon AG-30) on wire hoops, with edges buried in soil were compared as follows:

A) No row covers (control)
B) Rows covers removed at anthesis
C) Row covers removed 10 days after anthesis. At anthesis, both ends of row covers were opened to allow pollination

At each farm, striped and spotted cucumber beetle numbers were monitored weekly from transplant through the end of harvest, using yellow sticky cards. Beginning after row cover removal, the number of healthy, wilted, or dead plants in each row was assessed weekly. The umber and weight of squash and muskmelon harvested from each row were also recorded.

Research results and discussion:
Muskmelon field trials at Iowa State University – Gilbert Horticultural Research Station (Gilbert, IA)

In 2010, bacterial wilt was not detected in our trials. Absence of bacterial wilt from our muskmelon and butternut trials may be linked to the exceptionally low winter temperatures (-30° F) in central Iowa during January 2009. These extreme temperatures may have led to die-off of overwintering cucumber beetle adults, which spend the winter in litter at the soil surface or in the top inch of topsoil. In the absence of bacterial wilt, the trials were used to assess the effect of row cover treatments on fungal diseases, fruit quality and yield.

Average number of muskmelon fruit per subplot and marketable weight was lower in the no cover treatment (Table 1). The number of culls due to small size (possibly associated with poor pollination) did not differ between the row cover removal at anthesis versus the row cover removal 10 days after anthesis (Table 1); this suggests that pollination was not adversely affected by row cover removal time. Earliness (which often increases the market value of melons) was also enhanced in the subplots with row covers (Fig. 1). In 2011, low frequency of bacterial wilt was detected (11 of 240 plants) fairly late in the growing season (July 21) (Fig. 2) and did not affect yield (P>0.05). However, row cover treatments 3 and 4, which delayed removal until ten days after anthesis had lower numbers of wilt later in the season (Fig. 2). Row covers did not enhance earliness and yield this season (P>0.05) (Table 2). In addition to the lack of bacterial wilt, this may have been related to the absence of severe weather early in the growing season where row covers can offer protection to the young transplants. Since first harvest dates for treatment 3 were about one week earlier than for treatment 4, it is likely that pollinators were accessing the flowers under the row covers through the open ends.

In conclusion, even in the absence of bacterial wilt, row cover treatments reduced the need for fungicide sprays, increased yield, and enhanced earliness of muskmelon. Opening the row cover ends appears to have allowed for timely pollination.

  • 958465Tables 1 & 2 Muskmelon tables
    Tables 1 & 2 Muskmelon tables
Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Management of cucurbit bacterial wilt is often challenging to organic growers in the Midwest. Organic strategies to control cucumber beetles transmission of bacterial wilt are often inefficient and costly to implement. In previous trials we have proven that row covers may be an efficient strategy in muskmelon production, however, the purpose of this project was to optimize this strategy under an organic production system. Our field results are encouraging, although there was no bacterial wilt present in some of our trials, we have provided useful and practical information about the added benefits of row covers on two cucurbit crops important in the Midwest. We have also demonstrated the use of extended-duration row covers at two field days with an attendance of over 100 growers (ISU Horticulture Research Station, Gilbert, IA). We have presented the results at a regional grower meeting (IFVGA Annual Meeting, Jan 30 2010, Des Moines, IA) and demonstrated the use of extended-duration row covers at 6 on-farm trials. The creation of a website focused on organic cucurbit production, has also provided access to information about our trials, results, and ongoing research (http://organiccucurbit.plp.iastate.edu/).

Project Outcomes

Project outcomes:
Butternut squash field trials at Iowa State University – Gilbert Horticultural Research Station (Gilbert, IA)

In 2010, no bacterial wilt was observed in our trials. The average number of marketable butternut squash fruit per subplot was highest for the season-long-row-cover treatment, indicating good pollination and plant health (Table 3). Marketable weight did not differ among row cover treatments except for the removal-10 days after anthesis treatment, which suggests lower pollination rates in the latter treatment. The no-rowcover treatment had the highest marketable yield and also had half the percentage of culls due to vine attachment (Table 3) than the other three treatments with row covers. In 2011, no bacterial wilt was observed; however, the most serious pest threat was the squash bug in which egg masses were first seen in late July and within one month over 50 nymphs per plant were observed. A September killing frost accelerated the harvest and reduced the storage life of the squash. Plants under the season-long-row-cover treatment were not damaged by the cold.

As in 2010, the no-row-cover and season-long-row cover treatments in 2011 had the highest marketable yield and the fewest of culls due to vine attachment (Table 4). In conclusion, several factors must be considered when growers adopt row covers. The season-long-row-cover treatments protected the fruit from a damaging frost, saved two fungicide and three insecticide sprays, and had similar yields to the no-row-cover treatment. Removal of the row cover for 18 days during flowering allowed for adequate pollination. Poor performance of the 10-day-after-anthesis row cover removal treatment suggests problems with pollinator access, as was observed in 2010. The increase in vine attachment under row covers was unexpected and the mechanism for this physiological disorder is not understood.

On-farm cooperator trials

In 2010, extended-duration row covers provided an effective control against bacterial wilt at Growing Harmony Farm, (Table 5). The added protection from row covers increased yield when compared to the uncovered control. No bacterial wilt was observed at Turtle Farm or ZJ Farm. In 2011, little or no bacterial wilt was observed in any of the trials (data not shown). The absence of bacterial wilt may be related to the low cucumber beetle numbers and their appearance relatively late in the growing season.

In 2010, earliness and increase in harvest associated with row covers was not observed in two of the three trials. However, in 2011, row covers removed at anthesis provided slightly earlier muskmelon harvests and higher yields than other treatments (Fig. 3) for both Growing Harmony and ZJ Farms. Row covers under extremes weather conditions may enhance earliness but this effect may be negated when weather conditions are favorable for plant development. Row covers might promote vegetative growth, and delay pollination and fruit development.

  • Table 3 & 4 Squash Tables
    Table 3 & 4 Squash Tables
Recommendations:

Areas needing additional study

Due to the sporadic nature of bacterial wilt in the Midwest, further research focusing on the epidemiology of this disease is critical. A better understanding of the pathogen, Erwinia tracheiphila, and pathogen-vector interactions is necessary to develop much needed risk assessment and predictions tools available to growers. In addition, due to the labor intensity required to implement row covers, future research should focus on developing methods to mechanize the deployment and removal of row covers.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.