Final report for FW16-035
TomatoCulture Year Two Annual Report - 2017
Summary and Grant Recap
This report comprises our second-year analysis of curly top virus (“curly top”) and its impact on plant outcomes among TomatoCulture heirloom tomatoes for the 2017 season. The farm is in Albuquerque’s North Rio Grande Valley. Curly top is transmitted by the beet leafhopper, the only known vector for this disease. We planted 37 different varieties of tomatoes (one hybrid, the rest heirlooms) into one of three zones, each divided into three different fields with different growing conditions: Either shaded with 30% shade cloth, in open sun, or interplanted with sunflowers for a dappled shade effect (Figure 1). We analyzed rates of curly top infection among the plants by field and condition, testing the hypothesis that shaded plants would have a decreased incidence of curly top due to the beet leafhoppers’ preference for feeding in bright sunlight. A subset of this hypothesis was to test if interplanting with sunflowers provides the same or better results than shade cloth in terms of deterring beet leafhopper feeding and reducing curly top disease. We also analyzed whether the shaded plants would be healthier overall and produce more fruit.
Figure 1 – 2017 three zones with three growing conditions in each.
Note – Black squares represent the locations where plant death occurred from curly top virus.
Our objectives did not change from year one:
- Determine if light shading deters the beet leafhopper from feeding on tomato plants and spreading the BCTV infection. Working with the Bernalillo County Extension service, TomatoCulture will monitor plant infection and mortality rates for each of the test plots and periodically send plant tissue samples to the plant diagnostics lab through NMSU to confirm BCTV infection.
- Compare shade cloth with inter-planted sunflowers to see which shading system works best in terms of effectiveness and cost. If we can confirm protection from shading, it will be valuable to understand if inter-planting tall sunflowers (assumed to be more cost effective) provides the same, less or greater benefit as fabricated shade cloth.
- Measure and compare plant yields to determine if there is a positive ROI for the increased labor and material cost associated with each shading system. Monitoring and comparing the fruit yields across the three different test plots will provide valuable information about the feasibility of an environmentally sound approach to a pest that can be very destructive to high-value cash crops.
- Determine if shading to control BCTV has other impacts, good or bad, on overall plant health and tomato yield. Some believe that shading sun-loving tomato plants slows plant growth, delays flowering and reduces yield. This experiment will allow us an easy way to test a secondary hypothesis: that shading in NM, which is at higher elevations and has high solar radiation, actually leads to less stressed plants, greater fruit set, and lower disease pressure overall.
As reported after year one, the 2016 farming season served up several confounding challenges that undermined the quality of our data and analysis, preventing us from truly testing our hypotheses. As a result, we set out in 2017 to correct the execution problems from the prior year to get a better data sample and draw more accurate conclusions. Here is a summary of the changes we made in year two:
- Changed mulching material from black to white plastic. The switch to white plastic mulch had the greatest positive impact on our success in 2017 because it eliminated heat build-up under our row cover fabric. The early protection under the row covers allowed for:
- Earlier planting. We planted the first week of May and did not have to replant any tomatoes.
- Good transplant establishment and strong growth during the first six weeks.
- Greatly extended time during which the young plants stayed under the row covers, protected from beet leafhopper feeding.
- Direct seeded sunflowers prior to transplanting tomatoes. Switching to directly seeding sunflowers (as opposed to transplants) earlier led to their much faster establishment and better shade protection at the appropriate time.
- Deployed a cover cropping strategy to eliminate fertility problems. We planted our 2017 tomatoes into a half acre plot that had been heavily cover cropped the prior year with a mix of clovers, hairy vetch, and oats. We also further amended the soil over the winter with trace minerals and extra N, P and K. Incidentally, the added nitrogen was overkill and not needed due to the large nitrogen contribution from the hairy vetch. (Tomatoes that follow hairy vetch do very well and could be the topic of another grant).
- Minimized fungal root rot diseases with Actinovate soil drench at time of planting. The product contains the active ingredient Streptomyces lydicus, a soluble powder biological fungicide for the control of root rot and damping-off fungi. In 2017, we saw no evidence of verticillium wilt and minimal problems with rhizoctonia, which had killed a significant number of our 2016 plants and made curly top identification more difficult.
- Finally in 2017, we planted a sample of each variety of tomato in all three sections to so that we could test each one according to the growing condition and eliminate any varietal effects on outcomes.
Figure 2 - Summary of 2017 results.
The following points summarize the outcomes of year two by zone and condition (Figure 2):
- The highest overall loss from curly top was the sunflower section in Zone 3 at 8%. It’s notable that almost all the curly top losses in this section were on the eastern perimeter of the field. In this zone, the full sun section had a 1% loss and the shade section had a 2% loss.
- In Zone 2, the shade cloth section had the lowest loss from curly top at 0%. The other two sections had the same loss rates at 2%. It might be worth repeating this study with samples in the interior of the field, taking out the edge effect, where the leafhoppers appear to stick to the edges of the field to do their feeding.
- Zone 1 was a tie, with all three sections having the same number of curly top losses.
- Across all three zones, the sunflower sections fared the worst in terms of overall plant health and losses. The sunflowers do not provide the same shade protection as shade cloth.
- All three shade cloth sections had the highest yields per plant as well as the highest overall yields.
2017 Key Observations
Beet Leafhoppers Could Care Less About Shade?
The data from our year-two curly top analysis testing our hypothesis that shading helps deter beet leafhoppers from feeding on tomato plants is, as in year one, inconclusive.
- Of the sections shaded with 30% shade cloth, only Zone 2 had the lowest losses from curly top – in fact this section did not lose a single plant from the disease (0%). The full sun and sunflower sections in this zone both lost two plants each to curly top (2%).
- In Zone 1, all three sections had the same number of curly top losses (3 each, or 3%). One observation stands out, however: 7 out of 9 curly top deaths across the three sections were along the western and southern edges of the field. All three curly top deaths in the shade cloth and sunflower sections occurred on the perimeter. Only the full sun section had two curly top deaths in the middle of the section (more on location of infection below).
- In Zone 3, the sunflower section had 6 deaths from curly top, all in the southeastern corner along the eastern edge of the field. The other two sections each had one loss from curly top.
Protect Your Plants!
We recorded 21 plant deaths from curly top virus in 2017 compared with 185 in 2016 (3% compared with 14%, respectively). The loss from the virus was so low in 2017 that it is almost statistically insignificant. We believe that another more important factor emerged as the primary disease prevention strategy resulting in this huge improvement: Covering plants with row cover and keeping them covered beyond the point of highest pressure from beet leafhopper activity. For our growing region, that time is around the third week of June. In 2017, we were able to keep our plants covered until late June compared with the third week of May in 2016. The conclusion here is that our success in avoiding curly top infection in 2017 was mostly due to the plants being covered for a much longer period.
Tomatoes Love a Little Shade
Although our research does not support shading as a curly top prevention strategy, we did note two significant benefits from shading tomato plants with 30% shade cloth:
- The shade cloth sections in all three zones had the lowest overall plant losses and;
- These sections had the highest yields per plant by significant margins.
These results are consistent with what we observed in 2016, as indicated by this excerpt from our 2016 Annual Report:
“Interestingly, per plant yields were highest among our shaded sections in Zones 2 and 3. The shaded section in Zone 1 had the second highest yield per plant. These results suggest that over the entirety of the season, the shading supports healthier plants overall. However, the one overriding factor could be soil fertility.”
We corrected for the uneven fertility in 2017 and ended up with all three zones showing significantly higher yields per plant under 30% shade.
These results confirm one of our secondary hypotheses that light shading helps improve overall tomato plant vigor, health and yields. In theory, plants under shade cloth have less heat stress, slower evapotranspiration, and cooler ambient temperatures during fruit set. They are more effective at fighting off disease and set more fruit during the hottest conditions.
Location, Location, Location
Another interesting observation from 2017 is worth noting. When we mapped curly top losses on the field grid (see Figure 1 above) we noted that most of the curly top losses (75%) occurred along the perimeter of the field. The southwest and southeast corners of the field were particularly vulnerable to beet leafhopper feeding and virus transmission, regardless of shading. The southern, eastern and western edges of the field are all exposed to other open fields, whereas the northern edge, with the least infection from curly top, butts up against a dirt road. The location of curly top infection indicates that beet leafhoppers may move in from adjacent open areas where host weeds exist and do not move much beyond the perimeter of the field. This observation warrants further study of prevention strategies.
Tomatoes Do Not Love Sunflowers
Two other diseases emerged in 2017 that impacted our results. Lab tests confirmed the presence of both Tobacco Mosaic Virus (TMV) and Early Blight (Alternaria Solani). The latter proved particularly devastating, especially in the sunflower sections. We achieved strong stands of sunflowers in all three zones because of successful direct seeding (and thereby achieved the desired shading effect). Unfortunately, the data indicates that the sunflower plants not only did not deter beet leafhoppers, but acted as disease vectors, spreading early blight (and other foliar diseases) to the tomato plants. In all three zones, the sunflower sections had the highest losses from foliar diseases, by significant margins. These sections also had the lowest yields.
After correcting execution errors that set us back last year, our second year provided a much better data set to evaluate our hypotheses and we were able to draw several conclusions from our research.
- It does not appear that shading tomato plants acts as a deterrent to beet leafhopper feeding, or if it does, the impact is minimal. What’s more important is keeping tomato plants covered with light row cover beyond the period of highest pressure from beet leafhopper activity. The use of white plastic mulch helps keep the plants cool under the row covers and allows them to thrive well into the summer. Our plants were healthy and pressing against the row covers when we finally uncovered them around June 20th.
- We will not interplant sunflowers with our tomatoes again because not only do they not provide the desired shade protection, the sunflowers brought severe fungal foliar diseases to the tomato plants in those rows. In our experience, interplanting with sunflowers had a strong negative impact on overall tomato health and productivity. Furthermore, the presence of sunflowers in those rows interfered with mowing, feeding, trellising, harvesting and clean-up, reducing productivity. We conclude that interplanting tomatoes with sunflowers does not provide the same or better shade protection than shade cloth.
- Light shading from 30% shade cloth resulted in the healthiest plants and the highest yields. Further study is needed to determine if the cost/benefit tradeoff is worth the extra effort and expense to cover the plants with shade cloth. Our year-two evidence further supports the year-one conclusion that shading with shade cloth provides tomato plants with protection from New Mexico’s harsh sun and dry heat, resulting in healthier overall plants and higher yields.
- The beet leafhoppers appeared to feed around the perimeter of the field and therefore the majority of our 2017 curly top infection was in the rows along the field borders. Repeating the shade study in the middle of the field and keeping a buffer around the edges may provide more conclusive results in terms of 30% shade cloth protecting against beet leafhopper feeding.
Education and Outreach
Lectures/Workshops: Bernalillo County Master Gardener keynote address, 2016 and 2017; Landscape for Life Backyard Farmer Series lecturer in 2017 and 2018; Albuquerque Tomato Fiesta lecturer, 2016 and 2017; Bernalillo County Master Gardener course lecturer, Tomatoes 101; Albuquerque Lavender Festival lecturer, 2017; Juan Tabo library lecturer on tomatoes
Publications: TomatoCulture Blog - 2 articles; author/contributor to "Down to Earth" gardening book
Public Outreach: Albuquerque Downtown Growers' Market booth; TomatoCulture Farm Tours for local farmers (Sol Harvest, A to Z farms) and backyard farmers, NMSU researchers (Drs. Stephanie Walker, Ashley Bennett, Israel Calsoyas), USDA Ag Agent Kevin Branum, USDA NRCS Rep Clint Chisler.