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

Attendees (description) of the June 8 Veggie Call: Nettie Baugher (Area Agent, Chowan and Perquismans Counties), Inga Meadows (Extension Associate, Vegetable and Herbaceous Ornamental Pathology), and Amanda Scherer (Postdoctoral Research Scholar, Plant Pathology).

Vegetable Production Updates:

Overall, there has been a lot of rain over the last couple of weeks, which has increased disease incidence.

1. Growers in northeastern North Carolina had around 6 inches of rain in the last couple of weeks. Producers with sandier soils seemed to fair better in terms of disease problems in crops.
2. A producer in northeastern North Carolina had reported several cucurbit diseases including Phythium root rot and powdery mildew on squash, and leaf spot and gummy stem blight in cantaloupe.
3. Although not a vegetable production issue, the heavy rains in northeastern North Carolina also shortened the harvest window for strawberry producers.
4. Cucurbit Downy Mildew has been reported in Duplin, Sampson, and Wilson County. For updates on the 2018 epidemic history of cucurbit downy mildew, please visit the Cucurbit Downy Mildew site for more information.
5. Due to the heavy rains, foliar infections of Phytophthora nicotianae has been reported on potatoes in multiple locations in Eastern North Carolina. P. nicotianae is normally a soil born pathogen and can cause tuber rot. However, in eastern North Carolina, severe foliar infections (foliar blight) have been observed and symptoms can look similar to late blight (P. infestans). Wet weather and high temperatures favor disease development. For more information please visit the Extension Plant Pathology portal.
6. In western North Carolina, Phytophthora nicotianae and P. capsici have been reported on tomatoes. Lesions look very similar to late blight, but P. nicotianae and P. capsici move only locally (within a field) and is not easily spread by wind and air currents like P. infestans. Producers are advised to put late blight products on (they are effective for all Phytophthora) at least until the weather turns warmer and drier.
7. Tomato spotted wilt virus (TSWV) has been reported on tomatoes in two counties in western North Carolina.
8. Symptoms of sun scald were reported on the foliage of greenhouse tomatoes in western North Carolina. When exposed to too much sun, tomato leaves will develop white or light tan, thin, crispy areas that are often irregular in shape. In the last couple weeks, there has been a lot of rain and not much sun. These symptoms were likely due to the plants being exposed to the sun after the long periods of cloudy and overcast weather due to the rain.

Upcoming Events:

1. Mountain Research Station Field Day on Thursday July 19, 2018 at 2 p.m. (research updates, trade show, entertainment, and more!) (Waynesville, NC).
2. Mountain Horticultural Crops Research and Extension Center 2018 Tomato Field Day August 16 (field tours, lunch, research initiatives, taste tests, and more!) (Mills River, NC).

Pest News and other Announcements:

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.

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.

• 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.
• For early blight, they are focusing efforts on collecting tomato samples with early blight symptoms from commercial fields in North Carolina.
• 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 Vegetable Production Update: 

Cucurbit Downy Mildew is caused by the fungus-like oomycete pathogen Pseudoperonospora cubensis. It infects forty species in twenty genera within the Cucurbitaceae family including cucumber, watermelon, melon, cantaloupe, squash, and pumpkin. High-humidity and moisture, and cool temperatures (~60 ℉) favor disease development. In North Carolina, the disease typically begins in June and lasts throughout the growing season.

• Symptoms: Leaf lesions appear as angular, yellow to brown spots on the upper side of the leaf. Leaf lesions are restricted by leaf veins. Under favorable disease conditions, the undersides of leaves may be covered in a mass of dark colored spores which is also called “downy growth”. Downy mildew can be confused with other foliar diseases such as Alternaria leaf blight, angular leaf spot, anthracnose, powdery mildew, and Phytophthora leaf blight.
• Signs: Under favorable disease conditions, the undersides of leaves may be covered in a mass of dark-colored spores which is also called “downy growth”. However, the color of the masses can range from colorless to gray-brown to deep purple depending on the density and age of the sporangia. Sporulation can be seen in the field with a 20x hand lens. They are most noticeable in early in the morning when dew is present or immediately following rainfall. Under a compound microscope, P. cubensis forms large (20-40 x 14-25 mm in diameter), lemon-shaped sporangia with a conspicuous papilla.
• Management: Plant early in the season to escape high disease pressure. Do not allow water to remain on leaves for long periods of time as this can favor disease development. Scout plants often and remove infected plants. If possible, plant tolerant varieties and protect the crop with fungicides. For the latest fungicide recommendations for cucurbit downy mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.

Resources:

1. Quesada-Ocampo, L. 2013. Cucurbit Downy Mildew. North Carolina State University. Plant Disease Fact Sheet. Retrieved June 14, 2018.
2. Colucci, S.J. and G.J. Holmes. 2010. Downy Mildew of Cucurbits. The Plant Health Instructor. Retrieved June 14, 2018.

Gummy stem blight (also known as black rot when affecting fruits) is caused by the fungal pathogen Didymella bryoniae (anamorph = Phoma cucurbitacearum). This pathogen infects all cucurbit crops such as cucumber, watermelon, melon, cantaloupe, squash, and pumpkin. High-humidity and moisture, and warm temperatures (61 to 75 ℉) favor disease development. In North Carolina, the disease can occur in seedlings and greenhouse transplants and throughout the growing season. However, the disease is most damaging in late June and July.

• Symptoms: The pathogen causes oval-shaped cankers on the stem and vine, which typically begin at the nodes. The cankers produce a characteristic grown, gummy exudate. Leaf lesions are light brown to nearly black, circular spots that start at the leaf margins and can expand to cover the entire leaf. Infected fruits display small water-soaked spots become large brown spots as they age. These spots can also exude the brown gummy secretion as found in stems.
• Signs: On dead tissue, more advanced lesions contain small black fruiting bodies called pycnidia, which can be seen with a 10x hand lens. Stem lesions often cause gummy, reddish-brown or black beads to exude.
• Management: Use pathogen-free seeds or seedlings and consider seed treatments. Seedlings should be inspected regularly for symptoms and infected plants should be removed. The pathogen can survive in soil and on un-decomposed plant material for 2 to 4 years. It is recommended to rotate all cucurbits out of infected areas for at least 2 years and remove all volunteers of related species. To promote the decay of plant debris, Deep plow infected areas post-harvest. For the latest fungicide recommendations for cucurbit downy mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.

Resources:

1. Quesada-Ocampo, L. 2013. Gummy Stem Blight of Cucurbits. North Carolina State University. Plant Disease Fact Sheet. Retrieved June 14, 2018.
2. Zitter, T. A. 1992. Gummy Stem Blight. Cornell University, Cooperative Extension Fact Sheets. Retrieved June 14, 2018.

Pythium root rot and damping-off has been observed in watermelon, pumpkin, summer squash, and winter squash. Several species of Phythium are associated with the two diseases. However, P. aphanidermatum, P. ultimum, and P. irregulare are most commonly associated with the disease. P. ultimum is a low temperature pathogen, but P. aphanidermatum requires higher soil temperatures (89.6 to 98.6 ℉) and low relative humidity for pathogenicity.

• Symptoms: In seedlings, a water rot develops in the taproot and hypocotyl at or near the soil line and plants will frequently collapse. Mature plants will have root and crown rot. Feeder roots are the first to affected and destroyed. Soon after, on to several brown lesions (0.3 to 2.0 cm in length) will develop on the lateral roots. In severe infections, the root cortex will be completely decayed, but the stele (central part of the root) will remain intact. As severity of the disease increases, leaves may become chlorotic and necrosis will move gradually outward toward vine tips. Due to defoliation, fruit may be sunburned. Also, plant that appear “healthy” may wilt suddenly during the day and may recover slightly at night, but plants will die in 2 to 4 days.
• Signs: All these Pythium spp. produce oogonia with apleurotic (have space between oospore wall and oogonium) oospores. However, distinguishing between species can be difficult using oogonia and oospores alone. Thus, the use of molecular methods for identification is recommended.
• Management: The pathogen can be easily transferred between cropping locations and possible routes for inoculum entry should be minimized. Thus, avoid the movement of contaminated potting mixtures and soil, avoid walking from an infected location to a clean one, and sanitize any contaminated irrigation equipment before reuse. Root rot can be managed by planting on raised beds to allow for maximum water drainage. In the greenhouse, solarization has shown some effectiveness in reducing inoculum density in the soil. The use of grafted cucurbit scions on to resistant rootstocks have been shown as an effective disease management strategy against root rot and damping off.

Resources:

1. Keinath, A. P., Wintermantel, W. M., and Zitter, T. A. (Eds.). 2017. Compendium of Cucurbit Diseases and pests (pp. 48-50). St. Paul, MN: APS press.

Powdery Mildew is caused by the fungal pathogens Podosphaera xanthii and Erysiphe cichoracearum. Disease can occur on all cucurbit crops. However, due to the incorporation of resistance genes, most commercial cucumber and melon cultivars are less susceptible. High-humidity (50-90%), moderate temperatures (68 to 80 ℉), dense foliage, and low light conditions favor disease development. However, dry conditions favor colonization, sporulation, and dispersal of the pathogen.

• Signs and Symptoms: White, powdery fungal growth first appears on older or shaded leaves and on the underside of leaves. Yellow spots may form on upper leaf surfaces opposite of powdery mildew colonies. The white, powdery growth can also appear on stems and petioles. Infected leaves will senesce (deteriorate) early, and fruit yield and quality can be affected. Due to plant stress induced by powdery mildew infections, speckling of fruit can occur in some cucurbits.
• Management: Provide adequate spacing between plants and remove any debris, weeds, and volunteers that can harbor the pathogen. When possible, the use of resistant cultivars is recommended. Fungicides should be applied to protect the crop from infection. For the latest fungicide recommendations for cucurbit powdery mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.

Resources:

1. Quesada-Ocampo, L. 2015. Cucurbit Powdery Mildew. North Carolina State University. Plant Disease Fact Sheet. Retrieved June 14, 2018.
2. McGrath, M. T. 2017. Powdery Mildew of Cucurbits. Cornell University, Cooperative Extension Fact Sheets. Retrieved June 14, 2018.

Tomato Spotted Wilt Virus (TSWV) is a member of the genus Tospovirus and has one of the widest host ranges of any virus. It is estimated to infect 800 plant species in more than 80 plant families including tomato, pepper, potato, celery, lettuce, legumes, annual and perennial ornamentals, and several weed species. It is transmitted in a propagative manner by seven species of thrips such as western flower thrips (Frankliniella occidentalis) and tobacco thrips (F. fusca).

• Symptoms: Symptoms can vary among hosts. In most cases, young leaves will turn bronze and later develop numerous, small dark spots. Stunting is also a common symptom and is more severe in young plants. Half or the entire plant maybe stunted. Tomatoes that are infected early may not produce any fruit. On plants infected after fruit set, may produce fruit with chlorotic ringspots. On green fruit, fruit may have slightly raised areas with faint, concentric rings. On ripe fruit, these rings are more obvious and can become red and white or red and yellow.
• Signs: Virus particles are 80 to 110 nm in diameter and membrane bound.
• Management: Due to the wide host range of TSWV and its vectors, management of this disease can be difficult. Do not grow ornamental plants in the same greenhouses as vegetable transplants started from seed. An integrated pest management strategy using UV reflective mulches, a plant activator (acibenzolar-S-methyl), and insecticides can provide some control.

Resources:

1. Jones, J. B., Zitter, T. A., Momol, T. M., and Miller, S. A. (Eds.). 2014. Compendium of tomato diseases and pests (pp. 96-97). St. Paul, MN: APS press.
2. Sherwood, J.L., German, T.L., Moyer, J.W. and D.E. Ullman. 2009. Tomato spotted wilt. The Plant Health Instructor. APS Press. Retrieved May 30, 2018, from APS: Tomato spotted wilt virus.