The same procedure executed for the 2018 cultivar experiment was used to calculate and re-inoculate the pots. Field grafting experiment. Field experiments were performed in a commercially-owned field south of Bakersfield, Kern County, CA. The field has historically been under consistent tomato production and typically experiences southern blight. The soil was a sandy clay loam with a pH of 6.37 and 2.19% organic matter. In 2017, the preliminary field experiment evaluated two cultivars under two graft treatments that were mechanically transplanted on May 15, 2017. Plots were 165 m long with 30.4 cm plant spacing and were arranged in a randomized complete block with 7 replications. Plants were irrigated with a buried drip system at a depth of 26 cm. The field experiment in 2018 and 2019 evaluated the same treatment structure as the 2018 and 2019 greenhouse experiments. The field experiments consisted of treatments arranged in a randomized complete block design with 6 replications with plots that measured 34 m long in 2018 and 30.5 m long in 2019. In both the 2018 and 2019 field trials, the plants were mechanically transplanted at a spacing of 60.9 cm in single row beds. Transplants were established with a towed water tank or sprinklers , then irrigation was switched to drip. Although not located within an active production field, the experiments were maintained by the commercial grower using standard practices for processing tomato in the southern San Joaquin Valley. Data Collection. In both the cultivar and grafting greenhouse experiments, Southern blight severity was rated using the following 0 to 7 ordinal rating scale: 0 = no disease symptoms; 1 = chlorosis of the older leaflets; 2 = wilting of the older chlorotic leaflets; 3 = wilting of the older leaves with a wilted apex; 4 = necrotic older leaflets with a wilted apex, black plastic plant pots and apex leaflets showing chlorosis; 5 = all leaflets are dry; 6 = all leaflets are wilted and dry with a chlorotic stem; and 7 = a dead plant that is completely wilted and dry .
Data was collected weekly after southern blight symptoms began to develop for the greenhouse cultivar experiments in 2018 and 2019. For the grafting greenhouse experiments, disease severity was rated every 2 weeks after southern blight symptoms began to develop for the 2017 and weekly for the greenhouse graft study in 2018 and 2019. For the field trials of objective , in 2017 data was collected weekly beginning six weeks after planting. Strike counts, defined as plants observable as infected or not infected, were collected from four 15.2 m sections per plot in the 2017 field trial. In the 2018 and 2019 trials, data collection began two and five weeks after transplanting after transplanting, respectively, and approximately every one to two weeks thereafter. In these trials, the status of each plant was individually recorded on each rating date. Plants that were wilting, collapsed, and lime-colored were rated as exhibiting southern blight symptoms . Other diseases were also observed in these trials. Plants with crisp leaves that roll or curl upwards with or without appearing stunted were rated as symptomatic of curly top, and plants with crinkled leaves having interveinal yellowing and typically with stunted growth were rated as symptomatic of unknown virus. Plants completely brown and dry were rated as dead. When a dead plant was observed, it was marked with a flag to ensure it would be counted on subsequent rating dates. Yield data was collected from 165 m long plots from the 2017 field trial on September 18, 2017. Yield data was not collected in 2018 due to quick collapse of plants ending in poor fruit quality for harvest. Yield data was not collected in 2019. Data Analysis. For the 2018 and 2019 cultivar and grafting trials in the greenhouse, the influence of experimental factors on southern blight severity was analyzed with generalized linear mixed models with PROC GLIMMIX in SAS 9.4 using the multinomial distribution and the cumulative logit link function.
The cultivar trial was analyzed as a nested model, with inoculum as a main effect and inoculum × cultivar as an interaction effect, because only a small set of the cultivars were evaluated in noninoculated control plots. The grafting trial was analyzed as a factorial. For both trials, rating date was included separately as an additional main effect and not included as an interaction to reduce complexity of attempting to model ordinal data. Block was included as a random effect for both trials. When interactions were significant, the effect of cultivar within inoculum and the effect of graft within cultivar were examined with the slice statement for the cultivar and grafting trials, respectively. Levels of significant main effects or interactions were separated by obtaining odds ratios for all pairwise comparisons with the model statement. Due to the large number of treatments in the cultivar study, odds ratios were summarized with the lsmeans statement in PROC PLM with the Tukey-Kramer adjustment for multiple comparisons. In the cultivar trials, the effect of cultivar among inoculated plants was analyzed using a dataset with noninoculated cultivars removed because odds ratios cannot be determined for interaction terms. Initial analysis of the greenhouse grafting trials did not detect statistical evidence for an effect of inoculum despite a total lack of symptoms in control pots, therefore all non-inoculated pots were removed for analysis. In addition, initial analysis of the 2019 cultivar trial did not find statistical evidence for separation of cultivars despite clear variation in the raw data. Therefore, three cultivars which possessed all 0 ratings on all dates were excluded from analysis. For the 2018 and 2019 field trials, individual plant status data was first subjected to quality control. In some cases, the same plant was rated with more than one disease over the course of each trial. This was generally due to lack of clarity of the symptoms when they are first observed or a secondary disease affecting plants following the first.
Quality control consisted of assigned the true or primary pathogen retroactively to all symptomatic ratings. Then, for dead plants, the cause of death was determined from ratings on previous dates when the plant was symptomatic but alive. Following quality control, ratings were summarized at the plot level. The total number of plants in each plot with a given rating was determined, and southern blight incidence was determined by adding the number of plants exhibiting southern blight symptoms and the number of plants dead due to southern blight. The influence of cultivar, graft, rating date, and all interactions on southern blight incidence in the 2018 and 2019 field trials was analyzed with a generalized linear mixed model in PROC GLIMMIX with the binomial distribution and the logit link function. Block was included as a random effect. The effect of graft within significant cultivar × graft interactions was examined with the slice statement. Means of significant main or sliced effects were separated using the least significant difference test with Tukey-Kramer adjustment for multiple comparisons with the lsmeans statement. For the 2017 field trial, yield and strike count data were analyzed with PROC GLIMMIX procedure in SAS v9.4 using the log normal and binomial distributions, respectively. The 2017 greenhouse experiments were analyzed as relative treatment effects with repeated measures using the nparLD package v2.1 in R v3.3.2 . An analysis of variance -type statistic was used to determine the effect of treatment, black plastic planting pots and means will be separated using 95% confidence intervals calculated from the nparLD package.Cultivar greenhouse experiment. By the end of the experiments, some inoculated plants from almost all cultivars had died from southern blight, but many plants did not develop any symptoms . The raw data showed that cultivars differed primarily in the number that did not develop any symptoms and that most died after exhibiting disease symptoms. One cultivar in 2018 and three in 2019 did not develop any symptoms. No symptoms were observed in any non-inoculated plants. In both trial years, analysis of fixed effects showed that the interaction of cultivar and inoculum had a significant effect on disease severity, and slicing these interactions showed inoculum of 10 sclerotia per 100 cm3 of soil had an effect on disease severity . There were few differences among cultivars in the multiple comparison analyses in both years, and relative differences among cultivars varied between years . In 2018, HZ 4707 had the lowest risk of developing disease, but was not different from SUN 6366, HZ 1428, and N 6428. In 2019, risk of both HZ 4707 and HZ 1428 was relatively low but was similar to several commercial cultivars and Texas A&M breeding lines. In contrast, N 6428 had the highest risk in 2019 but was not significantly different from six other cultivars. Cultivar N 6416 had the highest risk in 2018 and relatively high risk in 2019, but was not different from 9 or 12 other cultivars, respectively. Although Maxifort and Multifort were included as positive controls in 2018, their risk of developing southern blight was similar to all but 1 and 4 of the remaining cultivars,respectively. Of the Texas A&M breeding lines that were not excluded from analysis in 2019, 5635M and 5913M exhibited the least risk, but were not significantly different from the two remaining breeding lines and 8 commercial cultivars.
Grafting greenhouse experiments. The preliminary 2017 study showed under moderate inoculum pressure, disease severity was significantly higher in non-grafted HZ 5608 compared to HZ 5608 grafted to Maxifort, but was similar for H 8504 grafted and non-grafted . Disease severity was low in both 2018 and 2019 experiments. The Type III analysis of fixed effects detected a significant effect of grafting on disease severity in 2018 and 2019 . However, odds ratio estimates and confidence intervals of the pairwise comparisons control-standard and standard-tall were not sensical , and the control-tall comparison suggested that control had significantly greater odds to develop disease in 2018 but significantly lower odds in 2019. HZ 5608 had numerically higher incidence of southern blight compared to HZ 8504 for both replicate trials in inoculated pots across all grafted treatments , however a statistical effect of cultivar was not detected in either experiment. Field grafting experiment. Disease incidence was significantly lower on four of five rating dates in grafted plots compared to non-grafted in 2017 . On the final rating date, southern blight incidence was 52% and 58% lower in grafted compared to non-grafted plots. A significant effect of grafting was observed on yield, in which yield was 30.0% higher in grafted plots compared to non-grafted . In 2018 and 2019, disease severity was moderate to high . The Type III analysis of fixed effects on the 2018 field trial showed a significant interaction of cultivar and grafting on disease incidence, whereas in 2019 only the main effect of grafting was significant . For both cultivars in 2018 and in 2019, disease incidence was significantly lower in grafted plots regardless of height when compared to the non-grafted control. Mean incidence in non-grafted plots was approximately 7.5 and 11.5 times higher in 2018 and 2019, respectively, when averaged over cultivar and height of the graft union. Additionally, for HZ 8504 in 2018, incidence in tall grafted plots was significantly lower than incidence in standard plots . This numeric trend was also observed for HZ 5608 in 2018 and in 2019, but the difference was not significant.This study presents options for the management of southern blight of processing tomato in California. We found that grafting to resistant rootstocks dramatically reduced southern blight in processing tomato. Our finding agrees with previous literature on the benefit of grafting for management of southern blight and other diseases. In addition, our results suggest that raising the height of the graft union may reduce southern blight incidence. Finally, we observed variation in susceptibility to southern blight among commercial cultivars currently planted in California.While our findings in processing tomato agree with previous research in fresh market tomato, the utility of grafting to processing tomato production may be lower due to the relative costs and returns between the two systems. Although we did not perform a comprehensive economic analysis of production using grafted transplants, the current cost of F1 hybrid seed and the grafting operation exceeds returns under reasonable price and yield scenarios.