Primary vs. Secondary Stemphylium Leaf Blight Target Spot Lesions

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Christy Hoepting, Extension Vegetable Specialist
  • Agriculture
  • Vegetables

Stemphylium leaf blight (SLB) is a foliar disease of onion that causes tan, black or purple-colored target spots and leaf dieback. When uncontrolled, it results in excessive leaf dieback which causes reduced bulb size/yield and bulb quality. SLB can behave as both a primary and a secondary pathogen. When it behaves a primary pathogen, it attacks healthy green tissue, causing it to die and otherwise “causing trouble” in the crop. When SLB is behaving as a secondary pathogen, it attacks necrotic leaf tissue that has already been killed by something else, such as post-emergent herbicide injury (especially leaf burn), bacterial disease, or the lesions of IYSV and downy mildew. Secondary SLB is contained on this necrotic tissue and causes “no further trouble” in the crop. When scouting onion fields and evaluating fungicide trials, we distinguish the primary and secondary forms of SLB. Of course, it is the primary form that is of most concern.

Primary Form of SLB

The primary form of SLB is characterized by:

  • Target spots of any color (tan, black or purple) on green leaf tissue.
    • Once the green tissue has yellowed or become necrotic, it is important to look at the location of the target spot along the leaf. When the target spot occurs several inches from the leaf tip and the other side of the leaf is still green, then you know that the target spot originally occurred/infected green leaf tissue and is primary.
  • Black sporulation on target spots that occur on necrotic or green leaf tissue.
  • Target spots with a “greasy” or water-soaked appearance that occur on necrotic, yellowing or green leaf tissue.
  • Purple-colored target spots, unless they clearly are on necrotic tissue that was killed by something else.
close-up of 3-inch section of green onion leaf with a 1.5 cm in size light purple spot in the middle of the leaf.
A close-up of a 4-inch section of onion leaf where three-quarters is necrotic along the length and the remainder is green. In the middle of the leaf on the necrotic side is 1-cm in size tan spot.
: A close-up of 4 leaves of an onion plant. The outer leaf and leaf tips are yellowing and becoming necrotic. There are 8 1-cm in size black spots along the outer leaf.
a close-up of 3-inch section of green onion leaf. In the center is a 1-inch water-soaked spot where the center is becoming necrotic.
Close-up of an outer necrotic leaf of an onion plant. In the middle of the leaf is a 1-cm in size brown spot that is surrounded by water-soaked tissue.

Secondary Form of SLB

The secondary form of SLB is characterized by:

  • Tan (and sometimes purple) target spots on necrotic tissue, where the leaf tissue and target spots appear “dried up". These target spots may have been primary at one time but are not anymore.
    • If there are black spores on such target spots, then SLB is primary.
  • Target spots that occur on tissue that is necrotic from other causes, such as from post-emergent herbicides, especially where the outer leaf was burned.
  • Other examples of causes of necrotic tissue include where leaf is bent or kinked, IYSV lesions, necrotic disease center caused by downy mildew and necrotic leaves caused by bacterial disease.
A close-up of 6-inch section of necrotic onion leaf that has 6 dried-up 1-cm in size tan in color spots.
close-up of three outer leaves of an onion plant that are necrotic because they were burned by a post-emergent herbicide. The necrotic leaves are dotted with 0.5-cm in size black spots.
Close-up of two 6-inch sections on onion leaves. The outer leaf has a 3-inch necrotic section to the leaf tip, which has two 1-cm in size tan spots that formed near the middle of the leaf.
A close-up of a 4-inch section of green onion leaf that is bent at a 90-degree angle. There is white necrotic tissue at the bend where there is a purple spot.
close-up of the top section of an onion plant where two middle-aged leaves are white and dying back. On the one leaf there is a 1-cm in size black spot.