Cucurbits, in particular squash, cucumbers, watermelons and pumpkins, are produced as high value crops in Alabama, Florida, and other parts of the southern U.S. To meet the growing market demand, cucurbit production has increased significantly in the South over the past eight years, and Florida leads the nation in squash production. However, outbreaks of viral diseases and insect vectors (aphids and whiteflies) have become chronic and persistent threats to the profitability of cucurbit production in the region.
Current management strategies involve weekly calendar-base insecticide sprays from crop establishment to final harvest. This practice is unsustainable because insecticides are not effective in reducing virus spread. Furthermore, continuous use of insecticides increases the potential for resistance and destroys pollinators and other beneficial organisms. Other available options including resistant cultivars are inadequate or not effective against multiple viruses. The goal of this project is to assist southern cucurbit growers in combating these key pest problems by developing and implementing effective, system-based Integrated Pest Management (IPM) tactics. The ultimate goal is to enhance economic viability of cucurbit production in the South.
The goal of this project is to assist southern cucurbit growers in combating these key pest problems by developing and implementing effective, system-based Integrated Pest Management (IPM) tactics, including:
- Protective barriers;
- Plant defense system stimulating soil microbes, habitat manipulation through biologically active ground cover plants, and biocontrol agents;
- On-farm integration and evaluation of economic feasibility of adoption of the tactics to help understand how individual tactics interact and complement each other in a complex agroecosystem;
- A comprehensive outreach plan, which will facilitate the implementation of the program through training, education and technology transfer.
Obj 1. Evaluate a protective barrier as a mechanical tool to manage insect vectors of cucurbit diseases
Field trials were conducted at a commercial organic vegetable farm in Tuscaloosa, AL during spring 2018. The following treatments were evaluated: 1) Season-long row cover with an introduction of bumblebee hives at the start of flowering; 2) Season-long row cover with the ends opened at the start of flowering to enable pollinator access, and the ends closed after 10 days; 3) Complete removal of row cover at the start of flowering; and 4) No row covers (control)
Treatment plots consisted of raised beds, each 3 ft wide by 30 ft long for a total of ~ 30 plants. The beds were covered with white plastic mulch and seedlings 3-4 weeks old were hand transplanted. Proteknet “Biothrips” Insect netting row cover (0.35mm × 0.35mm mesh; 89% light transmission; 62% porosity; Weight: 0.74 oz. per sq. yd.) (Johnny’s Selected Seeds, Winslow, ME) was installed immediately after the transplantation and completely sealed along all edges. Hoops made of PVC pipes were used to support the row cover and form a low tunnel system over the plants. Pollinator bumblebees purchased from a commercial insectary (Koppert Biological systems. Inc. Howell, MI. http://www.koppert.com/). In treatment 1, when the crop begins to flower, the bumblebees were released under the row cover which was then re-sealed until the end of the crop season. In treatments 2 and 3, the row cover was placed over the crop as described in treatment 1. At the beginning of flowering, both ends of the row cover were opened for 10 days in treatment 2 whereas they were removed completely in treatment 3. Standard production practices were followed to establish the crop. Treatments were replicated four times and arranged in a randomized complete block design (RCBD). Plots were evaluated weekly by sampling five randomly selected plants from each plot and the data were analyzed using ANOVA followed by Tukey-Kramer HSD test (P < 0.05).
Obj. 2. Evaluate environmentally sustainable tactics for managing aphids and whiteflies on cucurbits including plant defense-boosters, habitat manipulation through biologically active ground cover plants, and biocontrol agents
Zucchini squash ‘Cash Flow’ (Siegers Seed Co., LaBelle, FL) was used as a cash crop while African marigold (Tagetes erecta L., Asteraceae) ‘Crackerjack’ (Stokes Seeds, Buffalo, NY), cowpeas (Vigna unguiculata (L.) Walp., Fabaceae) ‘Mississippi Silver’ (Urban Farmer, Westfield, IN), and (Lobularia maritima (L.) Desv., Brassicaceae) ‘Tall White’ sweet alyssum (Urban Farmer, Westfield, IN) were evaluated as companion plants.
Plants were drip irrigated and fertigated weekly after germination. To synchronize the maturity periods of all plant species, companion plants were 3-4 weeks old before planting the squash.
The experimental design was a randomized complete block with four replications. Each plot was 6-m × 4.4-m separated by 7-m of bare soil (buffer zone) on all sides. Each plot comprised of three raised beds (1.06-m apart). Plants were sown in double rows of 22 plants per row, at 30-cm intervals, on 18-cm high and 91-cm wide beds covered with black plastic. Treatments were defined by the type of companion plant grown in the middle row. These include marigolds, cowpeas, marigolds+cowpeas, alyssum, marigolds+A. swirskii, and alyssum+A. swkirskii. The three controls include one with no companion plant and M-Pede (Gowan Company, Yuma, AZ) used in the squash plants for insect control; a second one with no companion plant and A. swirskii released in the squash; and the third one without any type of pest management in the squash.
Alate and apterous aphids were sampled weekly using the leaf-turn method which involved gently turning over three leaves per plant and counting the number of aphids observed (Nyoike and Liburd 2010). Alate aphids were monitored using tomato cages holding one clear pan trap (PackerWare®) per plot with each pan trap containing approximately 250 ml of 5% detergent solution (Colgate-Palmolive Co., New York, NY). The detergent solution was refilled weekly.
Adult whiteflies were monitored weekly using two yellow sticky traps (Great Lakes IPM, Vestaburg, MI) per plot. Traps were left in the field for 48 hours and the numbers of adult whiteflies were recorded. Immature whiteflies were monitored weekly by collecting leaves from randomly chosen squash. One 4-cm diam. leaf discs were taken from each plant and the number of immature whiteflies was recorded. Adult and immature thrips were counted together in the leaf samples and sticky traps.
For each plot, two leaves showing disease symptoms were assayed for four aphid-transmitted cucurbit viruses (PRSV-W, WMV, ZYMV, and CMV) and one recently reported whitefly-transmitted virus (CuLCrV) using DAS-ELISA or TAS-ELISA, and PCR (Nyoike et al. 2008). The whitefly-transmitted virus was assayed only during the fall when symptoms were observed.
Additionally, Squash Silverleaf (SSL) physiological disorder was monitored weekly by randomly selecting squash plants and scoring them with an arbitrary index from 0-5 where 0 = a healthy plant and 5 = all leaves were completely silvered (Yokomi et al. 1990).
Total marketable yield was estimated by harvesting and weighing all fruits per plot in the field until crop termination. Unmarketable overgrown squash and fruit showing injuries from pickleworms or viruses were weighed separately. Total marketable yield estimates were compared among treatments.
The height (soil to the highest flower) and width (two opposite leaves) of the squash was measured twice during the season.
Repeated measures analysis was performed for all response variables. The number of insect pests per plot were fitted using the PROC GLIMMIX procedure following a Poisson distribution with LAPLACE adjustment. Averaged SSL indexes, plant size, and squash yields per plot were compared among treatments using the PROC MIXED procedure. No transformation was used for these variables. Models considered the fixed effect factors of treatment, week, and their interaction, together with a random effect of the block.
Comparisons of means among treatments at each week were obtained by requesting LSMEANS from each procedure. All models were fitted using SAS 9.4 (SAS Institute, Cary, NC 2013).
Obj 1. Evaluate a protective barrier as a mechanical tool to manage insect vectors of cucurbit diseases
The data from the first year of field trial showed a significant reduction in insect densities and crop damage in all three row-cover treatments compared with the uncovered control. Complete crop damage was recorded in the uncovered control plot within 4 weeks after planting. Removal of row-cover to expose the crop to natural pollinators at flowering resulted in increased crop damage from squash bugs. However, no significant differences in total marketable yields were recorded among the all row-cover treatments. Although releasing bees under row-cover did not seem to improve pollination or increase yield, extending the time the plants were covered significantly reduced crop damage. Overcrowded space under the row-cover may have restricted pollinator movement in the closed tunnels, resulting in poor pollination. In 2019 field trial, we plan to double the tunnel size to create adequate space for pollinator movement and improve pollination as well as crop yield.
Early results of the study suggest that row-cover strategy can provide complete protection of squash against insect pests and diseases. The row-cover strategy could be a potential pest management tactics especially for organic or naturally grown vegetable farmers who don’t use synthetic pesticides.
Obj. 2. Evaluate environmentally sustainable tactics for managing aphids and whiteflies on cucurbits including plant defense-boosters, habitat manipulation through biologically active ground cover plants, and biocontrol agents.
Aphids were present in the squash in high numbers by the beginning of the sampling period (three WAP) suggesting that they colonized the plants at a very early developmental stage when plants had less than five true leaves and were less than 25-cm high.
Cowpeas were highly attractive to cowpea aphids. High numbers of winged and apterous aphids were recorded mostly during the spring and fungus proliferation occurred due to continuous secretion of sap on top of the leaves and plastic mulch. Mixing the cowpeas with marigolds continue to harbor large numbers of aphids. Cowpeas were attractive to an aphid species (A. craccivora) that is not commonly found in cucurbits and it had a spill-over effect on the squash planted near cowpeas alone or those mixed with marigolds.
Overall, low incidence of aphid-transmitted viruses was observed in the treatments, but most of the samples that tested positive for viruses were found in the treatments with high numbers of aphids due to the presence of cowpeas.
Whitefly populations colonized the squash at the early stages of the crop, just like aphids, yet, no clear establishment pattern was observed across treatments.
High numbers of whitefly immatures feeding on the squash worsened SSL disorder symptoms resulting in multiple plants showing extensive silvering between primary and secondary veins. Similarly, high CuLCrV incidence was observed across treatments in the fall.
Low numbers of adult whiteflies were recorded in treatments where no companion plants were used but A. swirskii was released. Immature whiteflies in the same treatment did not show low abundances compared with other treatments. The same reduction in whitefly immatures was observed in the fall when A. swirskii was released in the absence of companion plants. The same tendency was observed in the treatment with A. swirskii introduction and marigolds used as companion plants; however, no significant differences were identified among treatments.
The presence of the predatory mite had no apparent effect on the SSL ratings in both spring and fall, and only the squash treated with M-Pede showed significantly lower levels of SSL during the fall season. CuLCrV was detected in all treatments with no apparent differences.
There was no clear establishment pattern identified for thrips species during the experiments given that the number of thrips recorded in yellow sticky traps were not consistent with the numbers recorded in pan traps. Only the number of thrips present in the companion plants were consistent through the seasons. Marigolds were highly attractive to various thrips species and marigolds used alone or mixed with cowpeas consistently showed the highest numbers of thrips present in flowers across treatments. African marigolds seemed to play a role as a trap crop not only for aphids but also for thrips species.
Significantly higher populations of aphids were observed in the marigolds compared with treatments where alyssum was planted as a companion plant. Yet, the spilled-over effect observed between cowpeas and squash was not observed for marigolds and squash. This means that when high aphid numbers were present in the marigolds, the squash had a low or intermediate aphid infestation. Marigolds seemed to play a role as a trap crop for aphids.
Marigolds acted as a host for Orius spp. since adults and immature stages of the predators were found in their leaves and flowers. Orius spp. were drawn to the marigolds in search of shelter and food because thrips were constantly present in marigold flowers and may have served as the primary food source for this hemipteran Orius spp. was also able to use marigolds as host for reproduction and development.
The lack of significant differences in size and leaf numbers of the squash with companion plant vs control suggests that competition between squash and companion plants was minimized by planting the latter using the same approach as the crop plants i.e., on top of raised beds and not in the middle of the beds or occupying additional space between the squash. By introducing companion plants this way, competition for water, nutrients, and the light were reduced, and additional time and labor was not needed. Overall, the highest marketable yield was obtained from plants treated with M-Pede.
The Alabama Vegetable IPM project is under the Alabama Extension Commercial Horticulture Team program as a multistep educational campaign. For the current project the following steps to information dissemination, project marketing, and evaluation have been established:
Step 1. Technology awareness: This step targets sharing basic project objectives and issues faced by diversified farms in Alabama where cucurbits are commonly grown. These are mainly done through short IPM presentations and indoor workshops in spring and fall. These are also events where continuous needs assessments through written surveys and general group discussions are completed by growers that results in quick feedback about major production issues, nature of the audience, etc. We reached approximately 599 participants via 20+ events statewide and also regionally.
Step 2. Technology demonstration: We established one large cucurbit IPM demonstration plot in Clanton, AL. This site was used for data generation and training of Extension Agents and Extension Coordinators on the Alabama Extension Commercial Horticulture as a capacity-building measure. In future years, we plan to invite some key producers to see the demonstrations and begin the technology refinement process (next step). Demonstrations are great at significantly improving Extension Agent knowledge and awareness of key pest issues and IPM tactics. We also plan to use the research location as additional key producer and Extension personnel training. Data and photos from demonstration plots are utilized for blog articles, videos, and other communication projects (e.g., marketing materials like bookmarks) in the pipeline. The evaluation system at this level included pre/post-tests which indicates very high levels of learning among the Extension personnel and key producers.
Step 3. Technology refinement: This step occurs on the producer fields with key producers (evaluation in commercial settings). With the initial field training on overall pest identification in cucurbits and scouting tactics, basic evaluation surveys indicate over 90% use of information since the need for the information is high. During the future technology transfer and refinement process to be carried out shortly, we will use the pictures and videos of producers adopting IPM recommendations as a motivation for other producers to adopt improved crop production and pest management practices.
Step 4. Full technology adoption: This will be achieved at the full execution of the program with key and secondary adopters of IPM information. Steps 3 and 4 will be evaluated using a standard written evaluation form and a scannable Qualtrics survey that Extension Agents use to capture major impact cases where growers benefited from the information.
Educational & Outreach Activities
Participating Extension Specialists, Regional Extension Agents and County Coordinators organized several events to train cucurbit vegetable producers. These include workshops (6 events, n = 161), field days (2 events, n = 15), IPM presentations (15 multicounty events, n = 438), and IPM exhibits in multiple states (3 events, n = 200). Extension surveys in Alabama indicated that producers would lose over 50% of their vegetable crop yield if they did not use recommended IPM practices. About 88% of respondents were strongly satisfied with the IPM training.
Extension surveys in Alabama indicated that producers would lose over 50% of their vegetable crop yield if they did not use recommended IPM practices. About 88% of respondents were strongly satisfied with the IPM training.
The integration of nonchemical pest management tools such as physical barriers, biocontrol agent, and living ground cover in reducing viral disease pressure will contribute to sustainable cucurbit vegetable production in the South. Our efforts focus on management of viruses transmitted by aphids in a non-persistent manner, which are among the most difficult pathogens to control in several high-value vegetable crops. Therefore, if our approaches are successful, they can be translated to other crops.