Vegetable Diseases to Scout for: Early Blight & Septoria Leaf Spot

Kentucky vegetable growers should be on the lookout for early blight and Septoria leaf spot of tomato. As the most common diseases of tomato in Kentucky, homeowners or growers not on a preventative spray program always have at least some Septoria leaf spot and early blight pressure. Both diseases commonly occur in fields under wet, humid conditions. They may also occur in greenhouses or high tunnels, particularly near side walls or when plants are grown in natural soil without plastic mulch. Prevention, early identification, and management will help reduce plant losses.

Here are the symptoms to look for, preventative tactics, and brief suggestions on how to treat crops once disease is confirmed.

Early Blight

Early blight is caused by the fungal pathogens Alternaria solani and A. tomatephila. These pathogens overwinter in plant debris from the previous season. Spores are initially splashed onto the lowest leaves, where symptoms are first apparent, but frequent rain or overhead watering can spread spores throughout the plant. Leaves, stems, and fruit may become infected at any stage during the growing season, which can result in fruit or plant loss.  Early blight is favored by moderate temperatures, high humidity, and frequent rainfall. Once established the disease can spread rapidly in dense plantings. Potatoes are also susceptible to early blight, and symptoms, preventative tactics, and management are similar to tomato.

Symptoms

Dark-brown lesions with a concentric ring pattern develop on leaves or stems, sometimes with a chlorotic (yellow) halo around the lesion (Figure 1). Older leaves are usually affected first, with the disease spreading up the plant to affect newer growth. Lesions enlarge and may coalesce to result in blight (rapid death). Affected fruit develop dark-brown lesions with a concentric ring pattern, typically at the attachment end (Figure 2).

Figure 1: Early blight results in the development of dark-brown lesions with concentric rings or bulls-eye patterns. (Photo: Kim Leonberger, UK)

Figure 1: Early blight results in the development of dark-brown lesions with concentric rings or bulls-eye patterns. (Photo: Kim Leonberger, UK)

Figure 2: Fruit affected by early blight develop dark-brown lesions with concentric rings (Photo: Yuan-Min Shen, Taichung District Agricultural Research and Extension Station, Bugwood.org)

Figure 2: Fruit affected by early blight develop dark-brown lesions with concentric rings (Photo: Yuan-Min Shen, Taichung District Agricultural Research and Extension Station, Bugwood.org)

Septoria Leaf Spot

Septoria leaf spot is caused by the fungal pathogen Septoria lycopersici. The pathogen overwinters in plant debris from the previous season. Spores are initially splashed onto the lowest leaves, but frequent rain and overhead watering will spread the disease throughout the plant.

Figure 3: Tomato plants infected with Septoria leaf spot develop circular lesions with darkened borders and tan-brown centers on stems, petioles, and leaves. (Photo: Kenny Seebold, UK)

Figure 3: Tomato plants infected with Septoria leaf spot develop circular lesions with darkened borders and tan-brown centers on stems, petioles, and leaves. (Photo: Kenny Seebold, UK)

Only leaves, stems, or petioles may become infected, which can result in significantly reduced plant vigor. Septoria leaf spot is favored by moderate temperatures, high humidity, and rainfall. Once established the disease can spread rapidly in dense plantings of tomatoes.

Symptoms

Small circular lesions with darkened borders and tan-brown centers are characteristic of this disease (Figure 3). The number of spots increases as disease severity increases. Blighting (rapid death) may occur in severe cases, which may kill plants while leaving a few unblemished fruit. The fungus produces additional spores in pycnidia, which are small, black specks seen in the centers of older lesions.

Early Blight & Septoria Leaf Spot Management

Cultural Practices

  • Select varieties with resistance or tolerance to early blight and/or Septoria leaf spot
  • Do not set transplants with visible leaf spots
  • Remove plant debris or weeds from the growing area
  • Remove and destroy heavily infected leaves from indeterminate tomatoes
  • Use drip irrigation (instead of overhead watering) to reduce leaf wetness
  • Improve greenhouse ventilation to reduce humidity
  • Use recommended plant spacing to facilitate air movement and leaf drying
  • Practice crop rotation

Chemical Approaches

Start plants on a preventative fungicide program within 2 (field) or 3 (greenhouse) weeks of setting plants.

  • Greenhouses: Apply mancozeb and/or copper on a 7 to 10 day schedule early in the season. If moderate disease pressure continues as harvest approaches, replace mancozeb with a systemic fungicide; otherwise continue with copper every 7 to 10 days. For specific systemic fungicide options with shorter preharvest intervals, see ID-36, page 18.
  • Fields: Apply mancozeb or chlorothalonil on a 7 to 10 schedule early in the season; incorporate copper for bacterial disease management. Shorten spray intervals under rainy conditions. Use a systemic fungicide tank-mixed with a protectant (mancozeb, chlorothalonil) at the third spray, and alternate between protectants and systemic fungicides as the season continues. A sample fungicide program for tomatoes may be found on page 97 in ID-36.

As always, all label recommendations must be followed when applying fungicides to crops. Pay particularly close attention to pre-harvest intervals.

Resources

  • IPM Scouting Guide for Common Pests of Solanaceous Crops in Kentucky (ID-172)
  • Home Vegetable Gardening (ID-128)
  • Vegetable Production Guide for Commercial Growers (ID-36)
  • Managing Greenhouse & High Tunnel Environments to Reduce Plant Diseases (PPFS-GH-01)
  • Greenhouse Sanitation (PPFS-GH-04)

 

By: Kim Leonberger, Extension Associate and Emily Pfeufer, Extension Plant Pathologist

 

Posted in Vegetables