Alejandra Jimenez Madrid, Director of the Plant Molecular Diagnostic Lab, University of Georgia

Survey of Neopestalotiopsis Leaf Spot and Fruit Rot Disease in the Southeastern US

Strawberry production in the southeastern United States has faced significant challenges in recent years, with plant diseases emerging as a critical concern for both growers and nurseries. Among these, Neopestalotiopsis leaf spot and fruit rot, caused by the novel fungal pathogen Neopestalotiopsis sp., has become one of the most pressing issues. This disease has been responsible for severe outbreaks in the region. Although multiple Neopestalotiopsis genotypes have been identified as potential pathogens on strawberry plants, their symptoms can vary quite a bit, making accurate identification essential. As such, producers are strongly advised to send samples to diagnostic labs or specialists for confirmation to ensure proper management recommendations are implemented.

Historically, Neopestalotiopsis species were known to be weak pathogens on strawberry, often isolated from roots and crowns of poorly established plants. However, recent outbreaks in commercial fields across the Southeast have highlighted the aggressive nature of the pathogen, leading to significant yield losses. The “aggressive strain” of Neopestalotiopsis sp. has been particularly damaging, causing severe leaf and fruit symptoms and even plant death (Fig. 1). Florida was the first state to report significant yield losses attributed to this aggressive strain, with symptoms including leaf spot, fruit rot, and general plant decline. A detection protocol for this strain was developed by Kaur et al. (2023). This aggressive strain has since been observed in multiple states, including Alabama, Georgia, Louisiana, North Carolina, South Carolina, and Texas.  Indeed, the disease has now threatened strawberry production almost everywhere in the Southeast.

Investigating the Spread of Neopestalotiopsis sp. in the Southeast

To assess the distribution of this pathogen, a multi-state survey was conducted in 2024 with the collaboration of state Extension specialists from Arkansas, Georgia, Louisiana, Mississippi, and North Carolina. The survey, which was supported financially by the Southern Region Small Fruit Consortium (SRSFC), was conducted by the University of Georgia’s (UGA) Plant Molecular Diagnostic Lab in Tifton, GA. A total of 40 strawberry samples were collected from the participating states, AR (n=5), GA (n=21), LA (n=10), MS (n=1) and NC (n=3), and fungal isolates were recovered and tested for the presence of the aggressive strain of Neopestalotiopsis.

Figure 1. Symptoms and signs of Neopestalotiopsis on strawberry samples sent to the Plant Molecular Diagnostic Lab at UGA-Tifton Campus

Out of 123 fungal isolates (Fig. 2) recovered from the 40 samples, 48% tested positive for the aggressive strain. The proportion of plant samples from which the aggressive strain was isolated varied among states: AR (20%), GA (43%), LA (90%), MS (0%) and NC (100%).  Notably, the aggressive form of the pathogen was detected in samples from all states that sent samples except Mississippi, from which only one sample was tested. The detection of the aggressive strain across a wide geographic area suggests that the disease has become a serious threat to strawberry production throughout the Southeast. Most of the samples we received in the lab exhibited typical symptoms such as irregularly distributed, different-sized spots (dark brown with light brown centers) and dark brown V-shaped necrotic areas starting at the leaf edge. Red discoloration of leaves and necrosis in crowns and roots were also observed on severely infected samples. Disease incidence reported by the clients ranged from >1% to 40%. Signs of the pathogen, including mycelium and spores, were also commonly observed in the symptomatic plant tissues (Fig. 1).

Figure 2. A. Colony morphology of Neopestalotiopsis sp. on artificial media after 14 days of incubation at 25°C. B. Spore (conidia) morphology observed under the microscope (x40).

Associations of Nursery Sources with Disease

It is believed that the aggressive Neopestalotiopsis sp. initially arrived in some fields with infected plants. The survey also investigated the potential role of nurseries in the spread of Neopestalotiopsis. Samples that tested positive for the aggressive strain originated from at least eight different nurseries. In most cases, it is not possible to know if plants were infected in the nursery or in the field, but collecting information about plant sources provides information that may be useful to growers and researchers in the future.

For more information on the nursery sources and cultivars represented, please read the 2024 report on the Prevalence and characterization of Neopestalotiopsis spp. on strawberry cultivars in the southeastern United States (PDF).

Challenges in Diagnosis and the Need for Molecular Tools

One of the major challenges with Neopestalotiopsis species is the difficulty in distinguishing the aggressive strain from non-aggressive forms of the pathogen based solely on morphological characteristics. The spore types of the aggressive and non-aggressive strains are very similar, making it hard to differentiate them visually. As such, molecular detection assays are the most reliable method and the only means for accurate diagnosis, although they can be expensive. Financial support provided in 2024 by the SRSFC allowed for free testing, enabling us to assess the distribution of the aggressive strain across the Southeast.

However, new research has revealed the presence of a “second aggressive” strain in Florida and Georgia, which is not detected by the Kaur et al. (2023) protocol but may also be contributing to the recent outbreaks in these states. This second strain, identified through molecular testing by the Peres Lab at the University of Florida, has further complicated the diagnosis and management of Neopestalotiopsis disease. Current diagnostic tools are not able to differentiate between the two aggressive strains, posing a challenge for growers and researchers. Efforts are underway to develop a new molecular test capable of distinguishing both aggressive strains from non-aggressive ones, which will enhance the usefulness of diagnostic results and enable better-targeted management strategies.

Management Strategies and Future Outlook

Given the aggressive nature of the disease and its rapid spread, it is essential for growers to implement effective management strategies to mitigate the impact of Neopestalotiopsis. These strategies may include removing infected plants, good sanitation practices, applying appropriate fungicides, and closely monitoring nursery transplants for early symptoms and signs of infection. Additionally, growers should continue to collaborate with Extension specialists and diagnostic labs to stay informed about the latest developments in disease management and detection.

The results of this study have highlighted the importance of accurate, timely diagnosis in preventing further spread of the pathogen and minimizing production losses. Molecular detection tools are invaluable for identifying the presence of the aggressive strain and guiding management decisions. However, the emergence of a second aggressive strain emphasizes the need for ongoing research and the development of new diagnostic methods to keep pace with the evolving nature of the disease.

Ultimately, the successful management of Neopestalotiopsis in the Southeastern U.S. will require a coordinated effort among researchers, Extension specialists, diagnosticians, nurseries, and growers. By combining molecular diagnostics, effective disease management practices, and proactive surveillance, the strawberry industry can better combat this emerging threat.

Citation

Kaur, H., Gelain, J., Marin, M. V., Peres, N. A., and Schnabel, G. 2023. Development of a molecular tool for identification of a new Neopestalotiopsis sp. associated with disease outbreaks on strawberry. Plant Dis. 107:1544-1549. https://doi.org/10.1094/PDIS-09-22-2117-RE