Insects, disease can devastate popular citrus

Photo provided by BOB SOUVESTRE -- Early season rust mites cause superficial injury to the rinds of these satsuma fruits. Fruit quality is not affected; only appearance.
Photo provided by BOB SOUVESTRE -- Early season rust mites cause superficial injury to the rinds of these satsuma fruits. Fruit quality is not affected; only appearance.

BY BOB SOUVESTRE

Citrus has become the most popular fruit tree in home landscapes due to our long period of relatively mild winter temperatures, wonderfully scented flowers, reduced maintenance needs and dependable production of high quality fruit.

With the increase in citrus plantings, insect and disease pressure has also increased. Some are of little consequence while others can result in devastation.

Some commonly occurring problems many homeowners observe on their trees include whitefly, leaf miner and aphids on newly developing growth and a darkening of the fruit skin by mites. More serious problems include canker, scab and greening which could affect an individual’s tree, their neighbor’s trees and potentially the commercial Louisiana citrus industry.

The LSU AgCenter has an excellent online 16-page publication that will help with tree selection and care along with insect and disease identification images and controls. Visit lsuagcenter.com and search for publication No. 1234, Louisiana Home Citrus Production.

There is a new USDA campaign for Louisiana citrus (saveourcitrus.org/louisiana) designed to help homeowners and citrus growers learn the difference and help identify life-threatening diseases.

In regard to citrus, the Asian citrus psyllid is now in 10 parishes and citrus greening has been found in Orleans and Washington parishes. Orange scab is now in 14 parishes. Citrus canker was confirmed in New Orleans in June 2013. These are serious threats to trees and Louisiana’s citrus industry. Please grow healthy citrus and use this website to report signs of citrus disease.

Torpedograss

More and more homeowners are experiencing their worst nightmare — Torpedograss. It seems this summer more calls about this noxious, invasive weed are being fielded by garden center personnel and Extension professionals. This is one of the most difficult weeds to control when found growing in the lawn and landscape.

Torpedograss (Panicum repens) is among the top grasses I get questions on. Even if introduced into a small area, this weed can rapidly spread to become a major problem. The name comes from the hard, sharp point on the rhizome that runs horizontally underground, like a torpedo going through the water. The rhizomes can travel a foot or more deep and the hard points are able to punch through landscape fabric and weed barriers.

In most situations, managing torpedograss and other grassy weeds can only be accomplished with diligent, repeated, frequent efforts. That means monitoring the situation often and promptly taking action anytime torpedograss is seen growing in an area.

Where you can just apply a herbicide to the foliage of the grassy weed, apply glyphosate at the highest label rate regularly as needed (Roundup, Killzall, Grass and Weed Killer, and other brands). This can be used near and around desirable ornamentals as long as you don’t get it on their foliage. Protect nearby plants by shielding them or cover them with plastic bags. Glyphosate is likely the best herbicide to kill torpedograss and other grassy weeds, but be prepared to do follow-up treatments if new shoots appear.

Where you cannot just spray the foliage of the torpedograss, use a selective grass killer. These products can be sprayed on ornamental plants and torpedograss or other grassy weeds, and they just hurt or kill the grass, not the ornamentals.

The herbicide fluazifop (Ferti-lome Over the Top, Ortho Grass B Gon, Fusilade, Ornamec and other brands) has a bit better activity on perennial grasses like torpedograss than the herbicide sethoxydim (Vantage, Hi-Yield Grass Killer, Poast).

But both are useful. Make sure the ornamentals in the bed are listed on the label as tolerant. If they do not appear on the label, there is a chance they might be damaged.

Do this as needed, following label directions carefully. These herbicides will only suppress, not kill, torpedograss and will kill most other grassy weeds.

In centipedegrass lawns, you can use the herbicide sethoxydim (Vantage or Poast) to suppress torpedograss and kill grassy weeds. It does not hurt centipedegrass if applied as directed. Repeated applications, at least three, through the summer will keep torpedograss suppressed, but not eradicated. If you ever stop spraying, it will come back.

In bermudagrass and zoysia lawns, several applications of the herbicide quinclorac, such as Drive (this is a commercial product) or Image Crabgrass Killer (homeowner version), will do a good job of actually killing the torpedograss with multiple applications. And they do a very good job on other grassy weeds.

No herbicides can selectively control torpedograss, bermudagrass and most other grassy weed in St. Augustinegrass. You can kill patches of grassy weeds that grow in summer with glyphosate (keep this off the desirable grass as much as possible).

When the grassy weed is brown, remove it and patch the damage with a new piece of sod. Doing this repeatedly over the years can maintain a lawn that primarily contains the desirable grass.

You also can use the “nuclear option.” Centipedegrass lawns or St. Augustine lawns severely infested with torpedograss may need total renovation. This requires spraying the lawn area with a high concentration of glyphosate, with the goal of killing off everything and starting over with a new lawn. Sometimes it takes two applications to get torpedograss killed off.

Renovation is absolutely the last resort and definitely not the cheapest route to travel. But it may be the most effective way to manage severe torpedograss problems in a lawn. Managing tough grassy weeds in the landscape takes persistent, repeated effort over the long term. There are no quick fixes or one-time applications that will properly deal with these weeds.

Got a gardening question? Write to Bob Souvestre, horticulturist with the LSU AgCenter, at Burden Center, 4560 Essen Lane, Baton Rouge, LA 70809, or email to bsouvestre@agcenter.lsu.edu, or call Master Gardeners at (225) 763-3990.