Sustainable Management Options for Whitefly-transmitted Viruses in Cucurbit Production

Progress report for LS18-299

Project Type: Research and Education
Funds awarded in 2018: $290,000.00
Projected End Date: 09/30/2021
Grant Recipient: University of Georgia
Region: Southern
State: Georgia
Principal Investigator:
Rajagopalbabu Srinivasan
University of Georgia
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Project Information

Abstract:

More than 1/3rd of major cucurbits in the United States (squash, watermelon, pumpkin, cucumber and cantaloupe) are grown in Florida and Georgia. More than 40 percent of that production occurs in the fall season. Recently, growing fall cucurbits in these states has become a challenge; the main limiting factors being whiteflies and a complex of whitefly-transmitted viruses. At least three whitefly-transmitted viruses, Cucurbit leaf crumple virus (CuLCrV), Cucurbit yellow stunting disorder virus (CYSDV), and Squash vein yellowing virus (SqVYV) affect cucurbits. Squash, pumpkin and canteloupe are extremely susceptible to CuLCrV and CYSDV, while SqVYV causes vine decline — a debilitating disease in watermelon. Mixed viral infections are also common. The viruses are established in Florida, and are increasingly common in southern Georgia. In Fall 2016 and 2017, whitefly-transmiited viruses in Florid and Georgia affected hundreds of fields spanning more than 10,000 acres. Due to virus-induced foliar symptoms and severe stunting, many fields of squash were plowed under causing serious monetary losses. Estimates suggest that in 2017 at least 50 percent of yellow squash and 25 percent of zucchini squash production was lost to these viruses. CuLCrV also infects snap beans, and 90 percent of fall bean production was negatively impacted in 2017. These losses amount to $50 million in farm gate receipts in Georgia alone. Coupled with the increase in pesticide applications the losses could be much higher.

If current trends continue, fall production of cucurbits will become unsustainable. The cause of alarm being that there are no effective management options. Due to lack of other effective management options, growers rely exclusively on insecticides for whiteflies and virus management. Such over reliance on insecticides has exacerbated the situation and produced undesirable outcomes such as development of insecticide resistance and whitefly control failures.

The goal of this proposal is to develop a holistic management strategy that is readily adopted by growers in the region. Growing virus-resistant cultivars is an obvious choice. However, there is no known commercial varieties with resistance to CuLCrV, CYSDV, or SqVYV. Our objective is to develop a whitefly and virus management package in which multiple management tactics will act synergistically. We intend to screen squash cultivars for tolerance to viruses, evaluate seedling protection in the greenhouse, assess the impact of mulch options, row covers, and examine the role of insecticides including biorationals in suppressing whiteflies and limited virus spread.

Project Objectives:
  1. Develop a pre-planting integrated management package using cultural and chemical tactics.
  2. Develop a post-planting integrated management package by examining cultivar tolerance, and using cultural and chemical tactics.
  3. Formulate a Risk Reduction Index, and showcase its usefulness by performing demonstration trials.
  4. Conduct econometric analyses including obtaining cost-benefit ratios.

Cooperators

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Research

Materials and methods:

Objective 1. Pre-planting integrated management

Seedlings in the greenhouse are vulnerable to whiteflies and viruses. The goal here is to evaluate protection measures on transplant seedlings in the greenhouse. Experiments for objective one were conducted at the UGA Tifton Greenhouse facility (Plant Pathology Department). Seedling protection options such as whitefly-proof screen (ProtekNet0.01 x 0.01 mesh), tray drenching with systemic insecticide cyantraniliprole (Verimark), biorational insecticide (Requiem), and plant defense modulator (salicylic acid analogue Actigard) were evaluated. Treatments were replicated in a split plot design. The presence of the insect proof screen and lack thereof were considered as main plot effects. The insecticide, biorational insecticide, and plant defense modulator were considered as sub-treatments. The main plot treatments were replicated three times in the open greenhouse. The main treatments effects were applied in alternating bench rows. The sub-plot treatments were replicated as five seedling flats for each. Whitefly infestations and virus infection percentages in each main and sub-treatment were assessed two weeks after seeding, and treatment efficacy were statistically evaluated.

Adult whiteflies on two leaves in each seedling flat were counted by turning leaves. In addition, two leaves were removed from each seedling flat and taken to the vector biology laboratory in Griffin GA where eggs and immature whiteflies were counted at 20X magnification using a dissection microscope. All flats were visually screened for virus symptoms. Plant samples were randomly sampled from each seedling flat and DNA extractions were conducted using specific primer sets and already established protocols. DNA samples were used to test for CuLCrV, Polymerase chain reaction (PCR) was performed to confirm CuLCrV infection in the seedlings using protocols already optimized in the molecular diagnostic laboratory in the Department of Plant Pathology at UGA, Tifton GA

Data analyses

Random count data of eggs, nymphs, and adults were analyzed using generalized linear mixed models by PROC GLMMIX in SAS (SAS Enterprise 4.2, Cary, NC) with suitable (Logistic) transformation at a 95% confidence level. Main plot and sub-plot treatments were considered as fixed effects and replications were considered random effects. Treatment means were separated using the LS means option in SAS at alpha =0.05. Percentages of infection data were treated as binomial responses (infected vs non-infected). Proportions of plant samples testing positive in each treatment/sub-treatment were analyzed by logistic regression using PROC GENMOD in SAS. Treatment means were separated using paired contrasts at alpha =0.05.

Objective 2

Post-planting integrated management

a. Screening for tolerance to whiteflies and/or viruses in squash cultivars.

Commercially available yellow and zucchini squash cultivars (up to 20, including new) were screened for susceptibility/tolerance to viruses. This trial was conducted both in Georgia and Florida.  Squash cultivars were transplanted as seedlings after germinating seeds in seedling flats in the greenhouse. Transplants were planted in raised beds covered with white plastic mulch. The raised beds were fumigated using Chlorpicrin at least two weeks prior to transplanting. Beds were on 6-ft centers with 1-ft plant spacing within rows. Single row plots were utilized for screening cultivars. Plots were15-ft long with 10-ft unplanted borders between plot ends. The test was conducted as a randomized complete block design with four replications. Plots were drip irrigated and fertilized weekly. All inputs fertilizers, fungicides, and insecticides were administered as per grower standards and as prescribed by UGA/UFL cooperative extension guidelines.

Insect counts, virus incidence, symptom severity counts were obtained weekly. Adult whiteflies on one leaf located in the middle of the plant on five plants in each plot were counted. In addition, one leaf was removed from each of the five-sampled plants and taken to the laboratory where eggs and immature whiteflies were counted at 20X magnification using a dissection microscope in one square inch. Immatures were divided into small nymphs (first and second instar) and large nymphs (third and fourth instar). The percentage of plants exhibiting virus symptoms such as severe stunting, leaf curling, yellowing, and stunting at least four times from planting to harvest. Severity rating of virus symptoms were assessed on a 1 to 5 scale with 1 representing mild or no symptoms and 5 representing very severe symptoms or death. Plant vigor were rated on a 1-9 scale with 1=death and 9=extremely vigorous. Leaf samples were collected from symptomatic and non-symptomatic genotypes and tested by polymerase chain reaction (PCR) to confirm virus infection. Yields were obtained by hand picking and weighing fruits from ten plants in each plot.

Random count data of eggs, nymphs, and adults were analyzed using generalized linear mixed models by PROC GLMMIX in SAS with suitable transformation at a 95% confidence level. Percentages of infection data were treated as a binomial response (infected vs non-infected). Proportions of plant samples testing positive in each treatment were analyzed by logistic regression using PROC GENMOD in SAS. Treatment means were separated using paired contrasts at alpha =0.05. The symptom severity and yield data will also be analyzed using PROC GLIMMIX assuming a Gaussian distribution.

b. Reflective mulch and companion crop and effects on whiteflies and viruses

The second trial evaluated the effects of reflective and live mulch; white mulch was treated as the standard control. The live mulch chosen in this trial was buckwheat (Fagopyrum esculentum). Three-row plots were used for this trial. The plots were at least 20 ft in length with 10 feet alleyways between adjacent plots within a row. Seedlings were transplanted in raised beds with the white and reflective mulch. For the live mulch plot, the crop was planted in white plastic, and the inter-row space on either side of the middle row was seeded with buckwheat seeds using a modified grain seed drill. Buckwheat seeds were planted at least two weeks prior to planting the squash crop. A randomized complete block design was used, and each treatment was replicated four times. Buck wheat plants were also irrigated through drip lines.

Whitefly adults and immatures counts Adult whiteflies on five leaves in each plot were counted by turning leaves. In addition, five leaves were removed from each seedling flat and taken to the Vector biology laboratory in Griffin GA (Srinivasan) where eggs and immature whiteflies were counted at 20X magnification using a dissection microscope, virus incidence, virus severity, and yields were assessed as described in trial 1. Random count data of eggs, nymphs, and adults were analyzed using generalized linear mixed models by PROC GLMMIX in SAS as described above. Percentages of infection data were treated as a binomial response (infected vs non-infected). Proportions of infected plants in each treatment were analyzed by logistic regression using PROC GENMOD in SAS as described above. The symptom severity and yield data will also be analyzed using PROC GLIMMIX assuming a Gaussian distribution.

c. Insecticides, biorationals, and row cover: effects on whiteflies and virus incidence.

Three conventional insecticides (Imidacloprid (Admire Pro) cyantraniliprole (Exirel) and Fluorpyradifurone (Sivanto), biorational insecticides including Chenopodium extract (Requiem), Crop oil, and Chromobacterium subtsugae strain PRAA4-1 (Grandevo), and row cover (Remay) were evaluated. All insecticide treatments were evaluated using randomized complete design with four replications for each treatment. All insecticides were applied as foliar sprays using a tractor mounted sprayer at the rate of 50 gallons per acre using fan tip nozzles and guard to reduce drift. Row cover was used as a treatment for the first four weeks after planting or at first female flowering whichever is earlier. No insecticide sprays were undertaken for the row cover treatment until the covers were removed. Upon removal, weekly sprays with a conventional insecticide were undertaken.

Whitefly adults and immatures counts, virus incidence, virus severity, and yields were assessed as described in trial a.

Research results and discussion:

Results Objective 1.

Objective 1. Pre-planting integrated management

Mean adult whitefly and immature counts were several-fold greater in non-protected seedling trays than in seedling trays protected by insect-proof netting material.  In fact, eggs and immatures were reduced ten-fold lower in protected trays than in non-protected trays.  Virus incidence was also reduced at least 4-fold in protected trays than in non-protected trays. Treatment effects were realized more under the protected setting than when no protective nets were used.  No differences in virus incidence were observed between Verimark, Requiem, and Actigard treatments.  However, the virus incidence overall in 2018 was very low.

Results Objective 2.

a. Screening for tolerance to whiteflies and/or viruses in squash cultivars.

The cultivar evaluation trials in both locations in Quincy and Tifton, in Florida and Georgia, respectively were decimated by hurricane Michael, and data were compromised.  But, preliminary research indicated that out of the 20 cultivars evaluated, there was no evidence of resistance against CuLCrV in both yellow and zucchini squash cultivars.  Overall, the zucchini cultivars fared better than yellow squash cultivars.  The green zucchini squash cultivars though infected with the virus were more tolerant to the virus than yellow squash cultivars. 

2. Reflective mulch and companion crop and effects on whiteflies and viruses

Reflective mulch effectively suppressed whitefly adults and immatures over different sampling periods in fall 2018 than live (buckwheat) or standard white plastic mulch.  This translated to reduced virus severity, whitefly-induced silvering, and enhanced plant vigor with reflective mulch.  On several instances, the live mulch was comparable to reflective metallic mulch, but not always.  Also, at harvest, reflective mulch plots had more fancy and medium squash fruits when compared with either standard white or live mulch.  The fruit cull percentage was also lower in reflective mulch when compared with standard and live mulch. 

c. Insecticides, biorationals, and row cover: effects on whiteflies and virus incidence.

Row cover stood apart from all other treatments. Row cover usage significantly reduced the whitefly adults and immatures when compared with most insecticide treatments except for Sivanto and on occasions Verimark.  Even though these insecticides effectively suppressed whitefly, they were not as effective as row cover in suppressing virus incidence or reducing its severity.  Subsequently, yields of fancy fruits were greater when row cover was used than all other insecticides were used.  Incidentally, row cover usage resulted in the lowest cull percentage than all other treatments. In many instances, non-treated check was not worse than insecticide treatments. 

Numerous tables are included in the PPT file in reference to the results provided.

Discussion

Results reiterate that it could be beneficial to integrate pre- and post-planting management options to achieve better whitefly and virus suppression and boost yields.

There are several management tactics identified at each level such as reflective mulch, row cover, and increased virus tolerance in some instances.  Though effective, none of these options still has the potential to serve as stand-alone tactic.  Therefore, integration is critical.  After evaluating these tactics individually for another year, we will attempt at integrating the best tactics into a sustainable and effective management package.

 

SARE-report-results-

Participation Summary

Education

Educational approach:

At this stage involving a graduate student is providing opportunities for the student to learn more about sustainable management of a complex problem that involves whiteflies and viruses transmitted by them in various modes.  Also, involving undergraduate students in the process, increases their knowledge in agriculture and the relevance of holistic and sustainable pest management, and provides a good foundation for career choices.  

Educational & Outreach Activities

1 Curricula, factsheets or educational tools
1 Webinars / talks / presentations
1 Workshop field days
7 County extension meetings

Participation Summary

100 Farmers
10 Ag professionals participated
Education/outreach description:

Regional Presentation

A research and education talk was presented at the Southeast Regional Fruit and Vegetable Growers Conference in Savannah in January 2019.  More than 100 participants attended the talk.  This conference is the biggest fruit and vegetable production in the Southeastern United States. Around 3000 participants regularly attend this conference.

State County Extension meetings

Numerous County extension meetings (at least 10 in GA and a few in FL) were held between May 2018 to March 2019, highlighting the importance of whiteflies and viruses transmitted by them. The meetings were predominantly attended by growers and facilitated by County Extension Agents. 

Local or County Educational Programs:

Date

Type of Training and Location (County)

Attendance

03/19/2019

Management of tomato and cucurbit diseases in North Georgia (Rabun County)

22

03/18/2019

Management of vegetable diseases in North Georgia (Towns County)

35

     
     

02/14/2019

Cucurbit diseases and their management options (Turner County)

28

     

02/11/2019

Management of watermelon diseases (Wheeler County)

20

01/25/2019

Management options in vegetable crops (Colquitt County)

82

01/17/2019

Disease update on diverse vegetable crops (Echols County)

35

01/15/2019

Watermelon disease management in Tift County (Tift County)

98

   

Website, vegetable blog & Applications

Alerts on virus incidences and pertaining details were regularly conveyed through the blog https://site.caes.uga.edu/vegpath/.  Details are also being included in the mobile phone application available through both Android and Mac platforms (UGA vegetable APP Veg doctor).

Bulletin

A more generic bulletin “Whitefly-Transmitted PLANT VIRUSES in South Georgia” on whiteflies and whitefly-transmitted viruses (UGA Cooperative Extension Bulletin 1507) was prepared and will be regularly updated. 

Dutta, B., Myers, B., Coolong, T. W., Srinivasan, T., and Sparks, A. 2018 Whitefly-Transmitted Plant Viruses in Southern Georgia. University of Georgia Cooperative Extension Bulletin B1507. 

Learning Outcomes

4 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key changes:
  • Adopting metallic reflective mulch instead of plastic mulch

Project Outcomes

4 Farmers changed or adopted a practice
1 Grant received that built upon this project
1 New working collaboration
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.