Downy Mildew

Figure 1. Downy mildew on St. Augustinegrass leaves. Linear white streaks develop parallel to leaf veins. Surface of the steaks is slightly raised.

Causal Agent: Sclerophthora macrospora

Host Grass: St. Augustinegrass and zoysiagrass

Cause and Symptoms: While downy mildew is a frequent issue in warm-season turfgrasses, the symptoms and effects can vary greatly between grass species, as is the case with St. Augustinegrass and zoysiagrass. Leaf blades of infected St. Augustinegrass are easily and uniquely distinguished by white or yellow-green linear streaks running parallel to the leaf veins (Figure 1). These symptoms are similar to St. Augustinegrass decline (Figure 2) and iron deficiency (Figure 3). Leaves turn yellow and, in severe cases, there may be some browning of leaf tips. Wet weather promotes pathogen growth and the development of the white downy mycelium on leaves more commonly associated with the disease.

Figure 2. St. Augustinegrass decline produces chlorotic mottles and streaks on leaf blades

While symptoms are apparent on leaves for St. Augustinegrass, infected zoysiagrass can be identified is a different way. An unusual number of tillers develop from a single crown giving the appearance of a “witch’s broom.” Grasses experiencing these symptoms can be easily pulled out and frequently have stunted growth and yellowed leaves. Although disfiguring, it does not kill turfgrass. These symptoms expressed in zoysiagrass are similar to yellow tuft disease, a related condition found in cool-season turfgrasses caused by the same pathogen (Figure 4).

Control and Management:  Key cultural practices that reduce the occurrence and severity of the disease include:

• Maintaining drainage and adequate sunlight
• Avoiding soil compaction, thatch accumulation, and short mowing heights
• Applying preventative fungicides
  

  Figure 3. Iron deficiency on St. Augustinegrass cause interveinal yellow streaks on leaves.

Downy mildew is usually more severe in moist and shaded areas, especially where there is poor drainage. Avoid plantings of turfgrass in areas with prolonged periods of standing water or shade. For already established turf, installing subsurface drainages in low-lying areas will help with reducing the chances of this disease developing. Yet sometimes even sites with good drainage will see symptoms of this disease if there is extensive rainfall for several days.

Maintaining good soil conditions helps promote healthy grasses without excessive growth. Topdressing and aerification are recommend for reducing soil compaction and thatch accumulation, which allows for greater air and water penetration. Keeping mowing heights above 1 inch is also recommended as shorter grasses are more likely to result in severe disease symptoms when infected.

Figure 4. Yellow tuft in cool-season turfgrass. Clusters of chlorotic tillers are developed from the same crown. Yellow tuft plants are easily detached from the turf.

Fungicide applications for downy mildew will be most effective when used preventatively before disease symptoms develop and where the disease is a recurring problem. Fungicides do not work well once the disease symptoms appear.

Content edited by:

Young-Ki Jo (ykjo@tamu.edu, Professor and Extension Specialist)

Raleigh Darnell and Paul Goetze (Extension Assistant)

Department of Plant Pathology & Microbiology, Texas A&M AgriLife Extension Service

September 31, 2021