Organic Late Blight Determinate Tomato Cultivar Evaluation

By Elsa Sánchez, Ph.D. and Tom Butzler

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'.