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Septoria Leaf Spot

Are your tomato leaves drying up prematurely?

Many tomato growers across West Virginia may have noticed a disease which initially exhibits as numerous tiny spots on lower leaves that enlarge over time turning the leaf yellow and eventually killing the entire leaf. According to WVU Plant Pathology Extension Specialist Mahfuz Rahman, this is due to a common tomato disease called Septoria leaf spot (caused by  Septoria lycopersicae).

Frequent rain, high humidity, and dew deposits on tomato leaves lead to rapid disease development that starts on the lower leaves and moves upward. Septoria leaf spots appear as numerous brown spots (approximately 1/16 to 1/8 inch in diameter) on the leaves with gray or tan centers and dark brown margins. As the spots mature, dark brown pimple-like structures called pycnidia (fruiting bodies of the fungus) appear inside the spots. These pycnidia can easily be seen with a hand lens. Septoria leaf spots do not affect stems or fruits, although loss of leaves due to severe disease may expose fruits to sunscald.

Septoria leaf spot may be confused with early blight, which is caused by  Alternaria solani. Early blight is characterized by a few (5 to 10) brown circular spots up to half an inch diameter with concentric rings or ridges that form a target-like pattern surrounded by a yellow halo. As the disease progresses, stem and fruit also become infected forming dark, sunken spots. Dark, sunken cankers with concentric rings may also appear at or above the soil line on stems.

How to manage Septoria leaf spot

  1. Remove lower infected leaves from the garden and bury or burn them immediately.
  2. Any effort to keep the leaves dry should help prevent the spread of the disease (e.g. plastic canopy for small areas).
  3. Organic growers may use copper hydroxide (Kocide 101) or Serenade weekly.
  4. Conventional growers may use chlorothalonil (Daconil 2787), azoxystrobin (Quadris, Amistar, etc.), or mancozeb (Penncozeb, Dithane, Manzate, etc.).

Late blight of tomato and/or potato

Late blight, caused by the fungal-like organism  Phytophthora infestans, has been detected in several locations throughout the state. Once the disease is in the field or garden, management options should include Ridomil Gold MZ alternated with Catamaran fungicide if the organism is sensitive to Ridomil Gold. WVU Plant Diagnostic clinic can help with proper disease identification and sensitivity testing of the organism to Ridomil Gold.