Organic Late Blight Tomato Cultivar Evaluation

Tomatoes are a top crop for Pennsylvania vegetable farmers, where field-grown tomatoes are produced on 1,203 farms comprising 2,466 acres (Census of Agriculture, 2022). They are grown on more vegetable farms in Pennsylvania than any other vegetable. By acreage, tomato ranks fifth after sweet corn, potato, snap bean, and pumpkin. Using Census of Agriculture data (2022), the average tomato planting in Pennsylvania is about 2 acres, while the average farm size is 11.3 acres (Census of Agriculture, 2022). Tomatoes encompass a significant part, about 18% of acreage on average, of diversified vegetable farms. Additionally, we asked 304 growers at the 2011 Mid-AtlanticFruit and Vegetable Convention, "What specific crops should researchers focus on to maintain your operation's profitability and that of the Mid-Atlantic Vegetable and Fruit Industry?" Tomatoes came out as the top response of 18 crops listed.

When we asked 110 farmers at the 2019 Mid-Atlantic Fruit and Vegetable Convention to rate vegetable management activities by their importance, cultivar selection was rated third (behind irrigation management and crop rotation; Sánchez et al. 2021). All farmers benefit from using disease-resistant cultivars. However, their importance on organic farms is amplified because available fungicides are limited compared to other types of farms.

Late blight (caused by Phytophthora infestans) is a devastating tomato disease. Since the 2009 Late Blight pandemic in the Eastern U.S., this disease has been a yearly challenge. According to Fry et al. (2013), during the pandemic, “Many home gardeners and many organic producers lost most if not all of their tomato crop, and their experiences were reported in the mainstream press. Some CSAs (Community Supported Agriculture) could not provide tomatoes to their members.” Pennsylvania farmers and gardeners can select disease-resistant cultivars to avoid late blight. However, organic farmers have indicated that consumers do not always favor the appearance and flavor of late blight-resistant cultivars.

In 2024, we evaluated indeterminate and determinate late blight-resistant slicing tomato cultivars using organic practices to provide farmers with up-to-date information for successful late blight-resistant cultivar selection based on yield. We had also hoped to evaluate the cultivars based on appearance and taste; however, that part of the evaluation was unsuccessful (explained below).

Methods:

The study was conducted in central Pennsylvania at Pennsylvania State University's Russell E. Larson Agricultural Research Center in Pennsylvania Furnace in a field managed using synthetic fertilizers and pesticides before 2024.

The project consisted of two experiments, one evaluating seven indeterminate cultivars and the other six determinate cultivars (Table 1). For both experiments, four-week-old transplants were planted on June 5^th, 2024, into a plasticulture system consisting of raised beds and two lines of drip irrigation tape (T-Tape model 508-12-450, John Deere, Moline, IL) covered with embossed black plastic mulch (1 mil, Wrap Bros., Chicago, IL); one line was used for irrigation and the other for fertigation. Plots consisted of 6 plants planted with 1.5 ft in-row spacing and 10 ft between rows. Plants were provided with 1-1.5 acre-inches of water each week. Before pulling beds, organic granular fertilizer (Fertrell Blue N 5-1-1; Fertrell, Bainbridge, PA) was applied at a rate of 150 lb/acre N. Additionally, the planting was fertigated weekly with 2 lb/acre N of organic liquid fertilizer (Alaska Fish Emulsion 5-1-1; Home Depot, State College, PA). Diseases were managed with a copper-based fungicide (Kocide 3000-O; Certis Biologicals, Columbia, MD; 2 lb/acre), applied weekly, beginning September 3^rd. Weeds were suppressed by planting a cover crop of annual ryegrass (Johnnys Selected Seeds, Winslow, ME; 25 lb/acre) between planting rows and surrounding the field. Plants were trained vertically using a Florida Weave system with 1-inch square wooden stakes placed in the following pattern: plant, plant, stake, plant, plant, stake, etc. The first string was placed when plants were about 10 inches tall. Stings were placed twice more in the determinate experiment and thrice more in the indeterminate experiment.

Tomatoes were harvested weekly when they reached red maturity, starting on August 2^nd, 2024, and ending on October 3^rd, 2024. Immediately after harvesting, the tomatoes were categorized as marketable or unmarketable, counted, and weighed. Additionally, marketable tomatoes were sized as small (< 2 inches in diameter), medium (2 – 2.75 inches in diameter), large (2.75 – 3.5 inches in diameter), or extra-large (> 3.5 inches in diameter).

A randomized complete block design was used for both experiments, and each cultivar was replicated four times. Data was collected from 6 plants per cultivar per replication and analyzed using the mixed procedure with means separated using pdiff.

Sensory Evaluation:

We invited the Penn State Master Gardeners to evaluate the appearance and taste of the tomato cultivars during one of their September meetings. Unfortunately, interest in participating was low, and we could not conduct the sensory evaluation. We will attempt to hold a sensory evaluation on Pennsylvania State University's University Park campus if we receive funding to repeat this study.

Results:

We did not observe any incidence of Late Blight in either experiment.

Indeterminate Cultivar Evaluation:

'Mountain Rouge' was used as the standard.

'Strawberry Fields,' 'Big Beef,' 'Beef Maestro,' and 'Rose de Berne' produced more marketable tomatoes than 'Mountain Rouge' (Table 2). 'Damsel' and 'Abigail' did not have marketable yields that differed from 'Mountain Rouge.' The mean marketable yield by weight was higher for 'Strawberry Fields,' 'Big Beef,' and 'Beef Maestro' than for 'Mountain Rouge.' All other cultivars were not different from 'Mountain Rouge.'

The number of unmarketable tomatoes was higher from 'Damsel' and lower from 'Strawberry Fields' than 'Mountain Rouge.' All other cultivars were not different than 'Mountain Rouge.' Mean unmarketable yields by weight were lower from 'Strawberry Fields' than 'Mountain Rouge.' All other cultivars were not different than 'Mountain Rouge.'

Table 2. Marketable and unmarketable yield of seven indeterminate Late Blight-resistant tomato cultivars evaluated in an organic production system in Pennsylvania Furnace, Pennsylvania, at Pennsylvania State University's Russell E. Larson Agricultural Research Center in 2024.

*Values are the means of six plants per replication and four replications. 'Mountain Rouge' (bolded) is the standard to which all other cultivars were compared. Values followed by different letters or within a column are statistically different at P ≤ 0.05. Values in green text are significantly higher than 'Mountain Rouge those in red are significantly lower.A randomized complete block design was used for the experiment, with each cultivar replicated four times. Data were analyzed using the mixed procedure, and means were separated using pdiff. Please read this article for more information on interpreting tables like this: Science, Tomatoes, and How to Read a Table

Overall, not many tomatoes were categorized as extra-large (Table 3). None of the cultivars produced different extra-large yields by number than 'Mountain Rouge.' Mean extra-large fruit weight was higher from 'Abigail' than 'Mountain Rouge.' All other cultivars were not different than 'Mountain Rouge.'

Most tomatoes were categorized as large- and medium-sized. 'Strawberry Fields,' 'Beef Maestro,' and 'Big Beef' had more large tomatoes by number and weight than 'Mountain Rouge.' All other cultivars were not different than 'Mountain Rouge.'

The mean number of medium-sized tomatoes was higher from 'Strawberry Fields' and 'Rose de Berne' than 'Mountain Rouge'. All other cultivars were not different than 'Mountain Rouge.' Mean large-sized fruit by weight was highest from 'Strawberry Fields', 'Rose de Berne', and 'Big Beef' than 'Mountain Rouge'.

Few tomatoes were categorized as small. In fact, for many cultivars, the mean number and weight of small-sized tomatoes was zero. 'Strawberry Fields' produced a larger mean number of small-sized fruit than 'Mountain Rouge.' No other differences compared to 'Mountain Rouge' were observed in the small category.

Table 3. Marketable yield by size of seven indeterminate Late Blight-resistant tomato cultivars evaluated in an organic production system in Pennsylvania Furnace, Pennsylvania, at Pennsylvania State University's Russell E. Larson Agricultural Research Center in 2024.

*Values are the means of six plants per replication and four replications. 'Mountain Rouge' (bolded) is the standard to which all other cultivars were compared. Values followed by different letters or within a column are statistically different at P ≤ 0.05.Values in green text are significantly higher than 'Mountain Rouge' those in red are significantly lower.A randomized complete block design was used for the experiment, with each cultivar replicated four times. Data were analyzed using the mixed procedure, and means were separated using pdiff.

Determinate Cultivar Evaluation:

'Mountain Merit' was used as the standard.

'Defiant,' 'Stellar,' and 'Iron Lady' produced more marketable tomatoes than 'Mountain Merit' (Table 4). No other cultivars were different than 'Mountain Merit'. Mean marketable yield by weight was higher from 'Defiant' and 'Stellar' than 'Mountain Merit.' All other cultivars were not different than 'Mountain Merit.'

'Defiant' and 'Stellar' produced more unmarketable tomatoes than 'Mountain Merit.' All other cultivars were not different than 'Mountain Merit'. No differences between cultivars for mean unmarketable yield by weight were observed.

Table 4. Marketable and unmarketable yield of six determinate Late Blight-resistant tomato cultivars evaluated in an organic production system in Pennsylvania Furnace, Pennsylvania, at Pennsylvania State University's Russell E. Larson Agricultural Research Center in 2024.

*Values are the means of six plants per replication and four replications. 'Mountain Rouge' (bolded) is the standard to which all other cultivars were compared. Values followed by different letters or within a column are statistically different at P ≤ 0.05.Values in green text are significantly higher than 'Mountain Rouge' those in red are significantly lower.A randomized complete block design was used for the experiment, with each cultivar replicated four times. Data were analyzed using the mixed procedure, and means were separated using pdiff.

'Mountain Gem' produced more extra-large fruit by number and weight than 'Mountain Merit.' All other cultivars were not different than 'Mountain Merit' (Table 5).

No differences compared to 'Mountain Merit' were observed in the large category.

'Stellar' and 'Defiant' produced more medium fruit by number and weight than 'Mountain Merit.' All other cultivars were not different than 'Mountain Merit'.

No differences compared to 'Mountain Merit' were observed in the small category.

Table 5. Marketable yield by size of six determinate Late Blight-resistant tomato cultivars evaluated in an organic production system in Pennsylvania Furnace, Pennsylvania, at Pennsylvania State University's Russell E. Larson Agricultural Research Center in 2024.

*Values are means of six plants per replication and four replications. 'Mountain Rouge' (bolded) is the standard to which all other cultivars were compared; Values followed by different letters or within a column are statistically different at P ≤ 0.05.Values in green text are significantly higher than 'Mountain Rouge' those in red are significantly lower.A randomized complete block design was used for the experiment, with each cultivar replicated four times. Data were analyzed using the mixed procedure, and means were separated using pdiff.

Discussion:

To confirm our results are repeatable, we need to conduct the study again. The results and this discussion should be viewed as preliminary.

Indeterminate Evaluation:

We observed a large amount of unmarketable tomatoes with most cultivars. Specifically, six out of the seven indeterminate cultivars produced higher unmarketable yields by weight compared to marketable ones, while five out of the six determinate cultivars also yielded more unmarketable than marketable tomatoes by weight. Fruit with large blossom end scarring, blossom end rot, and tomato pox were culled. The greatest cause of culled fruit was concentric and radial cracking and rain check. We have observed an increase in water-related disorders, where a hot, dry start to the season led to issues such as blossom end rot, followed by early fall rain events that became more extreme and frequent, resulting in higher rates of concentric and radial cracking and rain check. For example, in June, the average temperature was 70.5°F, compared to the historical norm of 68.1°F, and rainfall was only 2.58 inches, significantly below the historical average of 4.09 inches. July also followed this pattern, with an average temperature of 74.7°F (compared to a normal 72.2°F) and total precipitation of 2.41 inches versus the typical 3.79 inches. This hotter, drier summer contributed to early-season disorders. In August, temperatures stabilized at the historical average of 70.5°F, but rainfall surged to 6.03 inches, well above the historical 4.16 inches, intensifying water-related disorders in the later part of the season.

'Abigail' tomatoes with blossom-end scarring (right), a contributor to the unmarketable fruit in this trial. Photo: Tom Butzler

The mean unmarketable yield per plant exceeded the mean marketable yield for 'Rose de Berne', 'Damsel', 'Mountain Rouge', and 'Abigail' by number and 'Damsel', 'Mountain Rouge', and 'Abigail' by weight. Seed source descriptions of 'Abigail', 'Rose de Berne', and 'Strawberry Fields' mentioned minimal cracking or resistance or tolerance to cracking. We observed the expression of this trait to be strongest for 'Strawberry Fields' in our evaluation. Indeed, 'Strawberry Fields' produced significantly less unmarketable fruit per plant than any other cultivar evaluated and a higher marketable yield per plant than 'Mountain Rouge.'

Fruit from 'Strawberry Fields' was red at maturity. For markets demanding pink fruit, 'Rose de Berne' produced more marketable tomatoes than 'Mountain Rouge.' By marketable weight per plant, all four pink-fruited cultivars ('Rose de Berne', 'Damsel', 'Mountain Rouge', and 'Abigail') were not different from each other. It should be noted that all four produced significantly lower marketable yield per plant by weight than the red-fruited cultivars.

'Strawberry Fields' produced a higher marketable yield (number per plant) and lower unmarketable yield (pounds per plant) compared to other indeterminate late blight-resistant tomato cultivars. Photo: Tom Butzler

The commonality between all the cultivars we evaluated was that they were slicing tomatoes and had resistance to Late Blight. They differed in size (medium and large), shape (standard and heirloom), and fruit color at maturity (red and pink).

Very few tomatoes were extra-large. We didn't observe any cultivar produce a different number of extra-large fruit per plant than 'Mountain Rouge'. 'Abigail' produced a higher weight of extra-large fruit per plant than 'Mountain Rouge'. 'Abigail' was described as producing the largest-sized (10-16 oz) fruit in this evaluation (Table 1). Fruit were an heirloom type. They were largely flattened in shape and tended tohave gnarly blossom ends. Many fruit were discarded because of the extent of the blossom end scars. 'Abigail' may be more suited for home gardens.

Most fruit fell in the large- and medium-sized categories. The red-fruited cultivars produced more large-sized fruit per plant by number and weight than 'Mountain Rouge', while the pink-fruited ones were not different. Any of the red-fruited cultivars may be good options for large-sized fruit. The pink-fruited cultivars produced a smaller amount of large-sized fruit per plant; however, they may be grown for markets demanding pink fruit. Using other visual and flavor characteristics may help determine which cultivars to grow for large-sized fruit. We noted, for example, that 'Beef Maestro' produced a blocky fruit compared to the other cultivars, and 'Strawberry Fields' produced very uniform fruit.

'Strawberry Fields' and 'Rose de Berne' produced more medium-sized fruit per plant by number and weight than 'Mountain Rouge'. These may be good options for red- and pink-fruited cultivars, respectively.

Very few fruit were categorized as small. Additionally, very few statistical differences were observed in this category. None of the cultivars we evaluated were described by seed companies as small-fruited. We did not observe enough small-sized fruit to be able to make a cultivar recommendation for this category.

Determinate Evaluation:

Less visual differences were present between the determinate cultivars than the indeterminate ones. All cultivars evaluated were red-fruited, medium-sized slicers.

Marketable yields were higher by number per plant from 'Defiant', 'Stellar', and 'Iron Lady', and by weight from 'Defiant' and 'Stellar' than 'Mountain Merit'. As observed in the indeterminate evaluation, excessive water-related issues (cracking and rain check) were the primary cause of culled fruit. Apart from 'Defiant', all cultivars produced more unmarketable than marketable fruit per plant by number, and apart from 'Stellar', all cultivars produced more unmarketable than marketable fruit per plant by weight. 'Defiant' and 'Stellar' produced more unmarkable fruit per plant than 'Mountain Merit'. However, no differences were observed by mean unmarketable weight per plant. Overall, 'Defiant' and 'Stellar' appear to be the best options based on marketable yield.

Notably, most 'Defiant' culls had yellow shoulder. While some other cultivars also exhibited this disorder, ‘Defiant’ seemed to be especially prone. Tomatoes have a high need for potassium during fruiting. 'Defiant' may have a higher need relative to other cultivars.

'Mountain Gem' produced more extra-large tomatoes per plant than 'Mountain Merit.' In fact, 'Mountain Gem' was the only cultivar that produced more than one extra-large tomato per plant. 'Mountain Gem' appears to be the best option among the cultivars evaluated for producing extra-large fruit, although the amount of fruit in this category was small.

'Mountain Gem' stands out for its ability to produce extra-large tomatoes, surpassing other cultivars in both number and weight in the determinate late blight-resistant tomato cultivar trial. Photo: Tom Butzler

All cultivars may be good options for large-sized fruit as none differed from 'Mountain Merit.'

Overall, the largest category of fruit was medium-sized. 'Stellar' and 'Defiant' produced more medium-sized fruit by number and weight than 'Mountain Merit' and may be good options. 'Stellar' is reportedly resistant to Early Blight (caused by Alternaria solani) and Septoria Leaf Spot (caused by Septoria lycopersici),two other commonly encountered diseases in tomato production.

Too few fruit were small to recommend which cultivar to grow in this category.

Acknowledgments:

This work was only possible because we belong to a strong community of people who helped us succeed. Chris Sánchez prepped the field for planting, helped set up and clean up after every harvest, planted and maintained the cover crop, and took care of all post-harvest field cleanup. Austin Kirk supported field operations and helped make sure that we were set up for success. Chris Igo, Harvey Gilbert, Jeanie Kim, Tim Damico, Dave Wolfgang, Ann Shallcross, Leigh Steingraber, Tara Mather, Michal Stump, Dan Kottloski, Karen Kottloski, and Darlene Bachmann planted the field, assisted with the weekly harvests, and made Thursday morning harvests enjoyable. We also wish to acknowledge all the farmers who grow the food instrumental to our survival; you do not get thanked enough. We are grateful to the scientists who came before us and laid the groundwork for research like ours. Lastly, we express our sincere appreciation for the remarkable tomato plant (Solanum lycopersicum), which has inspired some of the best-tasting dishes we've ever eaten. Thank you!