For more information, contact Dr. Lindsey Thiessen (lthiess@clemson.edu)

Introduction

A novel pathogen, Neopestalotiopsis rosae (Neo), was introduced into South Carolina strawberry production in 2021; which has since caused significant losses each year to farmers across the state and the US. The disease was likely introduced through infected transplant material. Fungicides are unfortunately limited in their effectiveness for controlling disease in high pressure environments (e.g., high humidity, extended periods of rain). Identifying varieties with reduced susceptibility to disease is critical for developing a successful integrated pathogen management strategy for the region.

Disease Symptoms and Signs

Foliar Symptoms

Leaf spots may be the first symptom of infections in strawberry, appearing first as small brown necrotic lesions that expand to triangular necrotic areas that extend the end of leaflets. When conditions are conducive, small black dots (fruiting bodies) appear in the lesions. The fruiting bodies that are in the center of leaf lesions produce spores that spread to new areas of the plant or nearby plants, causing new lesions. Leaf spots can mimic several other pathogens, including Gnomonia leaf spot and Phomopsis leaf spot.

Crown/Root Symptoms

Crown and root infections cause similar symptoms to the aboveground portions of the plants. Plants may be stunted or wilted, and the leaves may show nutrient-deficiency symptoms. Infections ultimately result in the death of the plant, as it can no longer move water and nutrients into the leaves and fruit. Wilting symptoms may be most severe when plants are producing fruit.

Fruit Symptoms

Fruit lesions are often sunken and light-tan to brown. Soon after infection, the fruit lesions may appear similar to other fruit rots, but small black spots (fruiting bodies) form in the lesions.

2025-2026 Cultivar Trial

To identify cultivars that have greater tolerance to N. rosae infections, a field trial with mirrored horticulture evaluation plots were planted at the Musser Fruit Research Center. Cultivars were selected based on local interest and potential tolerance reported from other production region evaluations (Table 1). Disease trial plots and horticulture trial plots were planted October 15, 2025 into fumigated plasticulture and maintained separately throughout the growing season. Plots were treated with regional horticulture and integrated pest management practice recommendations; however, disease trial plots were not treated with fungicides. Plants were harvested twice per week between April 3 and May 22, 2026. Plants were inoculated using a water suspension containing N. rosae spores from locally collected, aggressive isolates (10,000 spores/ml) on November 8, December 19, January 15, February 19, and May 22, 2026. ‘Albion’ was used at the standard control cultivar for all analyses due to its susceptibility to a broad range of pathogens.

Table 1. Cultivars evaluated for their response to inoculations with N. rosae at the Musser Fruit Research Center between October 15, 2025 and May 22, 2026.

Cultivar	Photoperiod	Breeding Program
Surfline	Short Day	UC Davis
Monarch	Short Day	UC Davis
Albion	Day Neutral	UC Davis
Keystone	Day Neutral	UC Davis
Encore	Short Day	University of Florida
Ember	Short Day	University of Florida
Medallion	Short Day	University of Florida
Brilliance	Short Day	University of Florida
Ruby June	Short Day	Lassen Canyon
Galletta	Short Day	NC State University
Warrior	Short Day	UC Davis
GF Belle/X08	Short Day	Good Farms Genetics
SB_14_028-025	Short Day	Strawberry Sciences
Chandler	Short Day	UC Davis
Camarosa	Short Day	UC Davis
Fronteras	Short Day	UC Davis

First Year Trial Outcomes

Weather Influences. The 2025-2026 growing season was one of the driest on record for South Carolina. At the research farm, precipitation totaled 23.55 inches over 70 days for the duration of the study. In contrast, the 2024-2025 growing season had a total of 30.5 inches for the same time period. We also had several winter weather events that maintained row covers over plants in the field. One cultivar was significantly impacted by the winter weather and covering (Medallion), and was dropped from evaluations as there were too few plants to compare to others tested. Combining relatively low humidity and limited leaf wetness, opportunities for our inoculations to result in disease were limited.

Yield Impacts. Interestingly, there was limited impacts to yield, likely due to low disease pressure (Figure 3). The highest yielding variety in the plot was Monarch, and the lowest was GF Belle (X08). In general, high marketable yields were collected across the growing season.

Disease Evaluations. Disease incidence was low, and no plants showed symptoms of crown rot due to Neopestalotiopsis rosae infections. Low levels of disease were observed throughout the trial; however, infections were restricted to the lower canopy. After the final inoculation, final foliar disease incidence resulted in 100% incidence in most cultivar plots (each plant had symptomatic leaf lesions), but severity (not shown) was low so data. Keystone, Warrior, and Surfline plots all had significantly reduced foliar disease incidence compared to Albion (Figure 4).

Interestingly, fruit infections were variable across plots, potentially due to the limited disease pressure (Figure 5). Foliar disease susceptibility differences did not result in similar fruit infection patterns. Warrior was the only cultivar with significantly reduce fruit infections compared to Albion in addition to low foliar symptoms. Generally, fruit infections were low (less than 10%) across all cultivars and plots, which is likely the result of low disease pressure.

First Year Trial Conclusions

While there’s some general trends that align with other regional evaluations, the low pressure observed during the growing season has limited the confidence in responses observed from the different cultivars tested. Because the disease pressure was low, these varieties are being challenged in a greenhouse environment to get stronger support for levels of susceptibility. This year of testing has put further evidence that disease-free plants result in far better outcomes, particularly when weather conditions do not favor disease spread.