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

Kentucky vegetable growers, as well as gardeners, will be seeing early blight and Septoria leaf spot of tomato in their plantings at this time. 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. Both diseases are worsened by wet, humid field conditions. These diseases 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

Cause and Disease Development

Early blight is caused by the fungal pathogens Alternaria solani and A. tomatephila. These pathogens overwinter in plant debris from the previous season, or sometimes in association with solanaceous weeds. Spores are initially splashed onto the lowest leaves, where symptoms first become apparent, but frequent rain or overhead watering can spread disease further into the plant canopy. Leaves, stems, and fruit may become infected at any stage of the growing season, but disease is most common on aging plant tissue.  Early blight is favored by moderate temperatures, high humidity, and frequent rainfall. Once established, the disease can spread rapidly in dense plantings. Most potatoes are also very susceptible to early blight, and symptoms, preventative tactics, and management are similar to tomato.

Symptoms

Early blight appears as dark-brown lesions with concentric ring patterns on leaves or occasionally stems, sometimes with a chlorotic (yellow) halo around lesions (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 dieback). Affected fruit develop dark-brown lesions with concentric ring patterns, most commonly at the stem end (Figure 2).

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

Figure 2: Fruit affected by early blight develop dark-brown lesions with concentric rings. (Photo: University of Kentucky Vegetable IPM Scouting Guide Team, UK)

 Septoria Leaf Spot

Cause and Disease Development

Septoria leaf spot is caused by the fungal pathogen Septoria lycopersici. The pathogen overwinters in plant debris or in association with solanaceous weeds. Disease initiates on the lowest leaves, but frequent rain and overhead watering will spread the disease throughout the plant. Only leaves, stems, or petioles may become infected, which can result in significantly reduced plant vigor and overall blighting. Because Septoria leaf spot and early blight are benefited by similar environmental conditions, the two diseases often co-occur.

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

Symptoms

Small circular lesions with darkened borders and tan-brown centers are characteristic of this disease. Septoria leaf spot is typically first observed in the lower plant canopy, but eventually spreads to new, younger growth. The number of spots increases as disease severity increases. Blighting (rapid dieback) may occur in severe cases, which may kill plants while leaving a few unblemished fruit (commonly occurs in garden tomatoes). Septoria cannot infect tomato fruit. The fungus produces additional spores in pycnidia, which can be seen as small, black specks in the centers of older lesions.

Management for Early Blight and Septoria Leaf Spot

Cultural practices

  • Select varieties with resistance or tolerance to early blight and/or Septoria leaf spot. Cherry and grape tomatoes are usually less susceptible than slicer or Roma-style tomatoes.
  • Do not set transplants with visible leaf spots.
  • Remove plant debris or weeds from growing areas.
  • Remove and destroy heavily infected, lower leaves from indeterminate tomatoes (gardens only). Do not remove leaves above tomato fruit.
  • 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 weeks (field) or 3 weeks (greenhouse) of setting plants.
    • Greenhouses: Apply mancozeb and/or copper on a 10- to 14-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.
    • Field: Apply mancozeb or chlorothalonil on a 7- to 10-day schedule early in the season; incorporate copper for bacterial disease management. Use shorter 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 100 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-1)
  • Greenhouse Sanitation (PPFS-GH-4)

 

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

 

 

 

Posted in Vegetables