In south Florida over the last few years several samples were diagnosed positive for a new disease called lethal viral necrosis (associated with sugarcane mosaic virus, SCMV) on Saint Augustinegrass, cultivar ‘Floratam’. The following are the most frequently asked questions regarding this disease:
Which is the most affected plant?
Saint Augustinegrass ‘Floratam’ cultivar is the principal host of the disease. More than 90% of the residential lawns in south Floridaare planted with this cultivar! Saint Augustinegrass ‘Floratam’ was released in 1973 by the University of Florida and Texas A&M University as an improved cultivar resistant to another virus St. Augustine Decline Virus and to chinch bugs. All other cultivars of Saint Augustinegrass besides Floratam show only mosaic symptoms but typically do not die when infected with lethal viral necrosis.
Why is lethal viral necrosis a problem?
For the majority of lawns lethal viral necrosis usually kills ‘Floratam’ in three or less years.
What are the symptoms?
Early symptoms include a mosaic pattern on the leaf that becomes necrotic (turns brown and dies prematurely) over time. The symptoms present blotchy and streaky patterns of yellow and green color. Turfgrass tends to have broken yellow streaks running between the veins on an otherwise green blade. Cultivars other than Floratam such as Palmetto and Bitterblue will only show the yellow streaky patterns, but do not die from the virus.
Early symptoms (H. Mayer)
Leaf blade with dieback due to the virus (H. Mayer)
Saint Augustinegrass with the virus. H. Mayer
When does the grass show the symptoms?
Floratam St. Augustinegrass is a tropical grass and grows much more vigorously during the warmer and wetter months. Also, when the temperature drops to approximately 65 F the grass will start to show more severe symptoms including plant death. The symptoms and dieback typically start in the fall/winter and continue through the spring.
Can grass that is not showing symptoms be a source of the virus?
Yes, if the grass is a known host of the virus. Lawns may not be showing obvious symptoms, but may contain the virus. Symptoms may be especially difficult to see during the warmer and wetter months.
Are other lawn grasses susceptible to lethal viral necrosis?
Yes, but it does not kill them. Grasses that are known hosts of lethal viral necrosis include other St. Augustinegrass cultivars like ‘Palmetto’and ‘Bitterblue’, and also, bermudagrass, paspalum, bahiagrass, and ornamental fountain grass (Pennisetum spp.). Zoysiagrass is not a host. Other monocots including crabgrass, sorghum, corn and sugarcane can also be hosts to the virus..
Bitter Blue is resistent to the virus. B. Schall
How is the disease spread?
The virus is spread in the moist plant sap from infected grasses. Exposed plant sap occurs mostly when lawns are freshly cut. Lawn mowers, trimmers, equipment wheels, and other similar equipment pick it up at that time. Once the sap and clippings dry out, they no longer transmit the virus to new grass. The virus does not survive for long outside plant tissue. Mowing when lawns are wet can extend the viability of the virus on equipment because it keeps the plant sap hydrated longer. It is also believed that aphids and plant hoppers could spread the virus.
Can wheels from vehicles or lawn equipment spread the disease?
Yes, if wet plant sap from affected freshly cut lawns is carried to unaffected, but susceptible lawns.
Infected ‘Floratam’ field. Visible dead patches along the wheels of the mowers.
Does the virus survive in soil?
No, once the virus is out of the plant tissue, and the sap dries, the virus is destroyed.
Do clippings need to be removed from freshly mowed lawns?
No, clippings dry out very quickly and then are not a source of the virus when dry. ly, Decaying clippings recycle some of the nutrients back into the lawn.
Are there any chemicals, fungicides or pesticides that can be applied to lawns to cure the lethal viral necrosis?
No, unfortunately there is no cure for the virus.
So, if no chemical control is an option how do we manage the disease?
Grass clippings and plant sap should be blown off mowing equipment on affected sites. Equipment should then be sprayed until wet with recommended sanitizers, and allowed to dry to destroy any virus that may remain on the equipment. A good management technique for commercial lawn maintenance companies is to mow lethal viral necrosis affected lawns as the last lawns of the day, and then sanitize. In theory, newly planted lethal viral necrosis infected sod could be a source, but currently none of the sod farms tested so far by the University of Florida Extension Plant Pathology Lab has been positive for the virus.
What are the current recommended mower and trimmer sanitizing materials?
In areas where the disease is known to occur, potentially contaminated equipment parts should be cleaned, dried and sprayed down using the following:
- 1 part PineSol type disinfectant with 3 parts water
- Potassium peroxymonosulfate & Sodium Chloride (Virkon S) mixed at a 2 percent solution·
- Quaternary ammonia products ·
- Physan 20 ·
- Lysol ·
- 1 part household bleach with 9 parts water. Caution: bleach rusts steel.
Can disinfectant materials be applied to lethal viral necrosis affected lawns for control?
No, the materials are surface disinfectants, and would not destroy the virus inside the living plant tissue. In addition, they are not legally labeled for disease management on lawns, and many are toxic to lawn grasses.
Are there other management options?
Replace dying Floratam St. Augustinegrass with ‘Bitterblue’ or ‘Palmetto’. Both are currently shown to be resistant to the virus. ‘Palmetto’ is somewhat finer textured than ‘Floratam’, and both will require slightly different management, especially regarding fungal problems. Lawn areas can be completely resodded with recommended cultivars, or they can be “plugged” with smaller pieces into existing affected ‘Floratam’ lawns. Plugging allows the resistant cultivars to fill in as the ‘Floratam’ declines and dies. Neither ‘Bitter blue nor Palmetto can be planted from seed. Do not replant ‘Floratam’ on the same site. ‘Bitterblue’ is an older variety that has been used since the 1930s.
Overseeding the affected ‘Floratam’ with a cool season grass like ryegrass can be a temporary aesthetic measure for the winter snowbird season.