Organic Late Blight Determinate Tomato Cultivar Evaluation

Methods

This study took place in central Pennsylvania at Penn State's Russell E. Larson Agricultural Research Center in Pennsylvania Furnace during the 2024–2025 growing seasons. In previous years, the experimental field had been managed with synthetic fertilizers and pesticides.

We evaluated six determinate tomato cultivars (see the table below). Four-week-old transplants were planted in early June into a plasticulture system with raised beds covered in embossed black plastic mulch. Each bed had two drip irrigation lines: one for irrigation and the other for fertigation. Each test plot had six plants spaced 1.5 ft apart within rows, with 10 ft between rows. Plants received 1–1.5 acre-inches of water weekly. Before bed formation, we applied an organic granular fertilizer (Fertrell Blue N 5-1-1) at a rate of 150 lb/acre N. During the season, we fertigated weekly with 2 lb/acre N using an organic liquid fertilizer (Alaska Fish Emulsion 5-1-1).

To manage Early Blight and Septoria Leaf Spot, we applied a copper-based fungicide (Kocide 3000) weekly beginning in early September. Brown Marmorated Stink Bug was managed with an insecticide (Entrust; applied only in 2025), which was also applied weekly starting in early September. Annual ryegrass was planted between rows for weed suppression, and plants were trained vertically using the Florida Weave system.

We harvested tomatoes weekly once they had turned fully red, starting in early August and continuing until early October. After each harvest, we sorted the fruit into marketable and unmarketable categories, counted, and weighed them.

Marketable tomatoes were then sized into four categories:

• Small: less than 2 inches across
• Medium: 2 to 2.75 inches
• Large: 2.75 to 3.5 inches
• Extra-large: more than 3.5 inches

Most unmarketable fruit had Radial or Concentric Cracks or Raincheck. A few had other issues, such as Blossom End Rot, Zippering, or Yellow Shoulder ('Defiant').

We used a randomized complete block design for the experiment, with each cultivar replicated four times. We collected data from six plants per cultivar per replication. Data were pooled and analyzed using GLIMMIX with means separated at P ≤ 0.05 using Tukey's Honestly Significant Difference test. When year * cultivar interactions were significant, data were analyzed separately by year using the mixed procedure, with means separated at P ≤ 0.05 using the pdiff function.

Key Findings

We didn't see Late Blight during the two years of testing.

We used 'Mountain Merit' as the standard for comparison.

The number of marketable fruit was consistently higher from 'Stellar' compared to 'Mountain Merit'. All other cultivars were not consistently different from 'Mountain Merit'.

Excessive water-related issues (Cracking and Raincheck) were the primary cause of culled fruit. 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' consistently produced more culls than marketable fruit by number. 'Iron Lady', 'Defiant', 'Galahad', and 'Stellar' consistently produced a higher number of unmarketable fruit than 'Mountain Merit', while 'Mountain Gem' produced a lower number. By weight, none of the cultivars consistently produced a different amount of unmarketable fruit than 'Mountain Rouge'.

The amount of extra-large fruit was very small. None of the cultivars evaluated consistently outperformed 'Mountain Merit' in terms of extra-large fruit by number or weight. Other cultivars should be used when extra-large fruit is desired.

All cultivars may be good options for large-sized fruit, as none differed from 'Mountain Merit' in both years of the trial.

Overall, the largest category of fruit was medium-sized. 'Stellar' and 'Defiant' consistently produced more medium-sized fruit, both in number and weight, than 'Mountain Merit'.

Very few fruit were categorized as small. No cultivar consistently outperformed 'Mountain Merit' in terms of the amount of small fruit produced by number and weight.

Bottom Line

For higher marketable yield, consider 'Stellar'.

For extra-large and small fruit, consider cultivars beyond the ones we evaluated.

Consider any cultivar we evaluated for large fruit.

Medium-sized fruit dominated across all cultivars, with 'Stellar' and 'Defiant' outperforming 'Mountain Merit'.

If you choose 'Defiant', monitor potassium levels closely to reduce Yellow Shoulder risk.

Acknowledgments

This work was only possible because we belong to a strong community of people who helped us succeed. Chris Sánchez prepared the field for planting, assisted with setting up and cleaning up after every harvest, planted and maintained the cover crop, and handled all post-harvest field cleanup. Austin Kirk supported field operations and ensured 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.

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