What Is It? | Facts in Depth | For the Professional Diagnostician
Shortly after infection, symptoms begin as tiny water-soaked lesions or chlorotic spots on the leaves. These spots are circular to angular in shape and gray to light brown in color, appearing “peppery,” with chlorotic halos. These spots expand and coalesce into irregular shapes. They eventually turn light brown in color and are confined along the leaf veins. Severely infected leaves are dry and become brittle as the disease progresses.
Peppery leaf spot on collard greens (left) and mustard (center; right).
Bacterial streaming will be observed under a microscope when the margin of a lesion is placed on a slide with a few drops of water and covered with a cover slip.
Examples of bacterial streaming under a microscope.
Often Confused With
- Bacterial leaf spot caused by Xanthomonas campestris armoraciae – The symptoms caused by bacterial leaf spot closely resemble those caused by peppery leaf spot. Both diseases start as small necrotic spots; however, bacterial leaf spot has more pronounced yellow halos than peppery leaf spot and is favored by warmer temperatures.
- Black rot caused by Xanthomonas campestris pv. campestris – In the early stages of black rot infection on cauliflower, numerous small brown to black spots appear resembling peppery leaf spot infection. As the disease progresses, soft rot develops which is not characteristic of peppery leaf spot.
- Downy mildew caused by Peronospora parasitica – Angular chlorotic lesions of downy mildew on the upper surface of leaves of leafy green crucifers may appear similar to peppery spot. The two diseases are favored by the same environmental conditions (cool, rainy). However, the diseases can be differentiated in the field by the appearance of mycelial growth and sporangia on the underside of downy mildew lesions. The growth is best observed early in the morning when humidity is high. Sporangia and sporangiophores can be easily observed under a dissecting microscope, or on a tape or water mount using a compound microscope.
- Pseudomonas F Medium: Cream colored, fluorescent colonies can be observed 24 to 48 hours after streaking or dilution plating.