Vegetable Production Update- July 20th, 2018

— Written By

Vegetable Production Update- July 20th, 2018

From Dr. Amanda Strayer-Scherer (Postdoctoral Research Scholar, Plant Pathology, Tomatoes) and Inga Meadows (Extension Associate, Plant Pathology Specialist)

Background: The information presented in this vegetable production update contains a summary of the details discussed in the one-hour, biweekly vegetable production conference calls entitled Veggie Calls. These open forum sessions were developed to allow extension personnel (agents, area specialized agents [ASAs], and specialists) to discuss challenging cases, seek advice on recommendations, report emerging issues, and share training opportunities related to vegetable production. These calls are hosted by Inga Meadows, Dr. Amanda Strayer-Scherer, and/or Dr. Lina Quesada-Ocampo every two weeks throughout the vegetable production season.

Attendees (description) of the July 20th Veggie Call: Inga Meadows (Extension Associate, Vegetable and Herbaceous Ornamental Pathology) and Amanda Scherer (Postdoctoral Research Scholar, Plant Pathology).

Vegetable Production Updates:

  1. Bacterial spot on field tomatoes in Western North Carolina.
  2. Early blight on field tomatoes in Western North Carolina.
  3. Downy mildew on basil in sentinel plots at the Mountain Research Station in Haywood County. See the Pest News alert for more information.

Upcoming Events:

  1. Mountain Horticultural Crops Research and Extension Center 2018 Tomato Field Day August 16th (field tours, lunch, research initiatives, taste tests, and more!) (Mills River, NC).

Pest News and other Announcements:

  1. Inga Meadows (Extension Associate, Vegetable and Herbaceous Ornamental Pathology): Explained to the group that we plan to publish summaries of the Veggie Calls with important pest information in the NCSU Extension Pest News and will keep track of the names of the attendees to give them credit in the published summaries. These published summaries will be accessible to vegetable producers, ASAs, county extension agents, extension specialists, and researchers with an interest in North Carolina’s vegetable production industry.
  2. Amanda Scherer (Postdoctoral Research Scholar, Plant Pathology): Amanda is a new postdoctoral research scholar with Inga Meadows at the Mountain Research Station in Waynesville, NC. Part of her research is to look at the pesticide sensitivities of the bacterial spot and early blight of tomato pathogens.
    1. For bacterial spot, they are focusing on tomato transplant producers and are asking for help from extension agent in collecting greenhouse tomato samples with bacterial spot symptoms.
    2. For early blight, they are focusing efforts on collecting tomato samples with early blight symptoms from commercial fields in North Carolina.
    3. These samples will help provide us with valuable information on how to better advise growers in North Carolina to manage these two diseases. If you are interested or available to help them with this project, then they can send shipping labels and simple instructions for collecting samples. Please contact Amanda (alstraye@ncsu.edu) or Inga (inga_meadows@ncsu.edu) for more information.

Additional Information on the Vegetable Plant Pathogens Mentioned in this Veggie Call: 

Bacterial Spot is caused by four species of Xanthomonas (X. euvesicatoria, X. gardneri, X. perforans, and X. vesicatoria) and occurs worldwide wherever tomatoes are grown. In North Carolina, X. perforans is predominant species associated with bacterial spot on tomato. Due to diversity within the bacterial spot pathogens, the disease can occur at different temperatures and is a threat to tomato production worldwide. Disease development is favored by temperatures of 24 to 30 °C (75 to 86 °F) and high precipitation. Bacterial spot can cause leaf and fruit spots, which causes defoliation, sun-scalded fruit, and yield loss.

  • Symptoms: In general, spots are dark brown to black and circular on leaves and stems. However, leaf lesions are initially circular and water-soaked and young lesions may be surrounded by a faint yellow halo. Spots rarely develop to more than 3 mm in diameter. Lesions can coalesce causing a blighted appearance of leaves and a general yellowing may occur on leaves with multiple lesions. Fruit lesions begin as small, slightly raised blisters, which become dark brown, scab-like, and slightly raised lesions as thy increase in size.
  • Signs: On leaflets, bacterial spot can be easily confused with the early symptoms of bacterial speck, early blight, gray leaf spot, target spot, or Septoria leaf spot. When Xanthomonas is present, bacteria will ooze (also referred to as bacterial streaming) from infected tissue and can be observed under a light microscope. Bacterial streaming will not be observed in lesions caused by fungal pathogens.
  • Management: Management of the disease focuses on preventive control measures throughout the season. The most effective management strategy is the use of pathogen-free certified seed and disease-free transplants. Seeds may be treated with sodium hypochlorite, hydrochloric acid, or hot water to reduce the potential for seedling infection. In transplant production greenhouses, minimize overwatering and handling of seedlings. Trays, benches, tools, and greenhouse structures should be washed and sanitized between seedlings crops. Greenhouse tomatoes can be sprayed with bacteriophages, copper-based bactericides, or streptomycin. Streptomycin CANNOT be sprayed in field tomatoes. For field tomatoes, copper in combination with mancozeb and plant activators, such as acibenzolar-S-methyl, can be used.
  • Resources:
    1. Jones, J. B., Zitter, T. A., Momol, T. M., and Miller, S. A. (Eds.). 2014. Compendium of tomato diseases and pests(pp. 39-40). St. Paul, MN: APS press.
    2. Meadows, I., and Henson, M. 2017. Bacterial Spot of Pepper and Tomato. North Carolina State University. Plant Disease Fact Sheet. Retrieved May 16, 2018.

 Basil Downy Mildew is caused by the fungus-like, oomycete pathogen Peronospora belbahrii. The pathogen prefers cool temperatures (~60°F), high-humidity, and moisture (6-12 hours of moisture which usually occurs as morning dew, rain, or overhead irrigation). In North Carolina, the disease typically begins in June and will last throughout the growing season. Spores can be transported from state to state via air currents and the pathogen can also be seedborne.

  • Symptoms: Yellowing or browning of leaves will occur and can be confused with nutritional problems. However, the underside (“downy” side) of infected leaves will be covered with dark, brownish-black to black spores.
  • Signs: Typically, dark spores can be observed on the underside of infected leaves without a microscope or hand lens. Sporangiophore structures bearing sporangia can be observed using a dissecting microscope.
  • Management: The use of pathogen-free seed is recommended to prevent the introduction of the disease into an operation. Remove infected plants. Reduce moisture by avoiding overhead irrigation, water early in the morning to allow foliage to dry and increase air circulation between plants by using adequate spacing. Some varieties such as red basil, Thai basil, lemon basil, lime basil, and spice basil are less susceptible. For the latest fungicide recommendations for basil downy mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.
  • Resources:
    1. Quesada-Ocampo. 2015. Basil Downy Mildew. North Carolina State University. Plant Disease Fact Sheet. Retrieved July 26, 2018.

Early Blight of tomato is a damaging disease caused by Alternaria linariae (formerly known as A. solani). The disease occurs in humid climates and semiarid climates with frequent dews that provide enough moisture conducive for disease development. Currently, A. linariae host range is limited to tomato and infection can result in severe defoliation and reduced fruit quality, number, and size.

  • Symptoms: Early blight occurs on leaves, stems, and fruit. Small, brownish-black lesions first appear on older leaves and surrounding tissue may become yellow. Unlike bacterial spot, lesions enlarge rapidly (6 mm or larger) and dark-brown lesions may be encircled by concentric rings. As the number lesions increases, infected plants can become defoliated, exposing the fruit to sunscald. Stem lesions on seedlings are small, dark, and slightly sunken which enlarge to form circular or elongated lesions with concentric rings with light centers. Fruit lesions are large, dark brown to black with concentric rings and may cover the entire fruit.
  • Signs: Mycelia are septate and branched and become dark with age. Conidia (asexual spores) are beaked, muriform (multicelled), dark, 12-20 x 83-117 µm in size, and can be borne singly or chains of two.
  • Management:  Although there are no early blight resistant varieties, it is recommended to use varieties that are more tolerant to early blight and maintain plant vigor with adequate fertilization to reduce susceptibility. Remove volunteer weeds that cans serve as a source of inoculum. For the latest fungicide recommendations for early blight, please consult the 2018 Southeastern US Vegetable Crop Handbook.
  • Resources:
    1. Jones, J. B., Zitter, T. A., Momol, T. M., and Miller, S. A. (Eds.). 2014. Compendium of tomato diseases and pests(pp. 39-40). St. Paul, MN: APS press.
    2. Meadows, I. 2015. Early Blight of Tomato. North Carolina State University. Plant Disease Fact Sheet. Retrieved May 16, 2018.