Field tests were conducted on processing tomato varieties to confirm dodder resistance previously observed in greenhouse evaluations.

Dodder resistant tomato varieties identified in the greenhouse and planted in the field trial included CXD 233, H1100, H9888, H9997, SVR 024 2 0664, SVR 024 2 0665 and SVR 024 2 0662. CXD 234 was also included, as the company had observed this variety to be dodder resistant, however, in our greenhouse tests, it was sensitive to dodder. H9492 was included as the standard and the grower's variety was H9553 (also a dodder resistant variety). The trial was planted on March 15, 2005. Dodder attached to every tomato variety, with the greatest number of dodder attachments on H9553 (growers variety), H1100, and CXD 233. The least number of dodder attachments occurred on H9492, H9997, and SVR 024 2 0664. Two weeks ahead of harvest, dodder cover was nearly 70 percent on CXD 233 and around 30 percent on the CXD 234 and H9997.

As observed in the greenhouse, dodder was able to attach to seedling of most tomato varieties, but dodder could not form successful attachments to H9492, CXD 233, H1100, H9888, SVR 024 2 0664, SVR 024 2 0665 and SVR 024 2 0662 and eventually the dodder died. H1100 had about 60 dodder attachments, but by harvest, dodder was not visible. Tomato yields averaged about 40 tons per acre, with dodder reducing yield about 10 tons per acre on the CXD 233 and H9997 plots, but only about a 4 tons per acre reduction in the CXD 234 plots.

In 2004 greenhouse tests, several tomato varieties not previously identified as dodder resistant were observed to have high levels of dodder resistance; these included CXD 234, CXD 233, H1100, H9888, H9997, and SVR 024 2 0664. In 2005, we planted these varieties and also SVR 024 2 0665, SVR 024 2 0662, and H9492 (standard) under field conditions, to confirm dodder resistance.

Planted march 15

The trial was planted on March 15 in a field north of Davis with a history of dodder infestation. Each variety was planted on a single bed, (9 beds total) and were 250 feet long. Tomatoes were seeded two lines per bed, on standard 60-inch beds. The grower seeded H9553 (also a resistant variety) in the remainder of the field. Tomato emergence was noted approximately 10 days later and initial dodder attachment was observed on April 4.

On April 11, 18, 25, May 10, 20, 31, June 10 and 29 tomatoes with attached dodder were marked and counted for the entire plot area (9 beds wide by 250 feet).

On May 10 and May 20 the number of dodder infestations that appeared to be increasing in size (growth greater than one foot from the initial point of attachment) was also counted. On June 29 and July 18 dodder cover was estimated for each variety. Tomatoes were hand harvested, graded into red, green and rotten fruit, and weighed to estimate yields on Aug. 4.

When dodder cover extended at least 10 feet of a tomato row, tomato yields were taken from these areas as well as from areas with no dodder present.

Dodder emerged and was able to attach to all the tomato varieties used in this study.

The number of dodder seedlings attaching to tomatoes increased over the first 3 or 4 weeks after crop emergence and then declined to varying degrees for all tomato varieties. With CXD 233, CXD 234, H1100, and H9997, the decline in the number of individual dodder infestations was the result of infestations growing together into large patches. In the other 5 varieties, the decline in dodder attachments was the result of dodder death after initial attachment. It is not clear if the dodder growing on H9553 would have declined or continued to grow, since tomatoes, with attached dodder were removed during hand weeding.

On May 10, prior to hand weeding, dodder cover on H9553 was increasing in size and were likely to survive. In previous field studies, H9553 has been resistant to dodder.

Coverage percent

The number of dodder infestations that were increasing in size was also counted and both CXD varieties, H1100, and H9553, had at least 10 dodder patches that were growing vigorously. By May 20, the number of these patches declined slightly, but this was primarily due to patches growing together to form one large patch for the dodder sensitive varieties and due to dodder decline in the tolerant varieties.

Dodder cover on CXD 233, CXD 234, and H9997 was 28 to 70 percent. Dodder cover on the remainder of the varieties was less than 5 percent for all except H9888 (6 percent) by July 18. H1100, which had a large number of attached dodder plants early in the season, had relatively low dodder cover by July, which indicates that this variety was not an ideal host for dodder growth, but dodder attachment was not inhibited. Dodder cover on the three “SVR” varieties and H9492, was near zero for both cover measurements, indicating good levels of dodder resistance.

Tomato yields varied by variety and also by dodder presence. CXD 234, SVR 024 2 0664, and H1100 all had weak stands at planting, due to large seeds not properly flowing through the planter. Thus, low yield on these plots was not the result of dodder, as much as a weak tomato stand. Dodder appeared to reduce yields 5 to over 10 T/A. Most of this yield loss is probably due to direct competition for resources. However, the dense dodder canopy may also hold in more moisture and possibly increase disease pressure, as we have often noted tomato canopy decline or death, when dodder is dense.

Based on previous greenhouse studies and this field study, it appears that SVR 024 2 0664, SVR 024 2 0665, SVR 024 2 0662, and H9888 are all resistant to dodder. H1100 may also be resistant, but the large number of infestations early in the season, particularly for a weak stand, still leaves some question on whether this variety is truly resistant. Although the two CXD varieties were observed to be resistant in company conducted field evaluations, they did not appear to be resistant in this trial. However, even the sensitive varieties in this study may be better than other varieties currently available, as dodder infestations only resulted in a 25 percent yield loss, whereas early work observed as much as 75 percent yield loss from dodder infestations.