What Is It? | Facts in Depth | For the Professional Diagnostician
Brassica Diseases | Peppery Spot
Peppery Spot
Identification
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 dry and become brittle as the disease progresses.
Peppery leaf spot on collard greens (left) and mustard (center; right).
Pathogen Biology
Peppery leaf spot is seed borne and overwinters in plant debris and seed. Infection can occur when bacteria are mechanically transmitted by wind, direct contact, or water splash. Disease development occurs when temperatures are cool; therefore, peppery leaf spot is likely to be a problem in the fall and early spring. Symptoms first appear as tiny water-soaked lesions. As the disease progresses, these small lesions coalesce and become brown in color with chlorotic halos. Once peppery leaf spot is introduced into a cropping system, it is very difficult to eradicate.
Favorable Environmental Conditions
Cool temperatures around 65°F to 75°F are favorable for disease development.
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.
Scouting Notes
Peppery leaf spot is most prevalent in the fall and early spring when temperatures are cool and favorable for disease development. An early indication peppery leaf spot infection is the development of small water-soaked lesions on the foliage.
Thresholds
There is currently no threshold information for peppery leaf spot of crucifers.
Management Notes
- Certified pathogen-tested seed – Peppery leaf spot spreads by seed so it is very important that all plantings begin with clean, certified pathogen-tested seeds in order to prevent introduction of the disease into a new field. If seeds have not been tested and certified, consider hot-water or other sanitizing seed treatments to eliminate this and other pathogens from the seeds.
- Sound cultural practices – Bacterial disease spread by wind and water splash so avoid the use of overhead irrigation. Increasing plant spacing will help with keeping foliage dry so that the bacteria are less likely to move to nearby healthy hosts. If peppery spot is present in a field, remove and destroy diseased plants and rotate with non-leafy cruciferous crops for 1 to 2 years.
- Bactericides and biopesticides – Copper bactericides, such as copper hydroxide, may reduce the spread and severity of the disease, but can cause crop phytotoxicity. Copper applications may be alternated with biopesticides such as, but not limited to, those containing strains of Bacillus