By: Bonnie L. Grant, Certified Urban Agriculturist
Mediterranean plants like rosemary impart herbal elegance tothe landscape and aromatic flavor to cuisine. Rosemaryis a relatively stoic plant with few pest or disease issues but occasionallythey do have some problems. Sick rosemary plants need an accurate diagnosticprior to treatment for adequate control. Learn about the most common rosemarydiseases and how you can combat any problems.
Rosemary disease control is almost unnecessary since theyare naturally resistant to almost all common plant plagues. However, fungaldiseases of rosemary do occur as well as a couple of bacterial infections. Thebest defense is good cultural care and proper siting.
Questions regarding whether your rosemary is sick or not canbe answered by first giving a thorough inspection of the plant. If plant stems,leaves or tissues are discolored, it could be from the feeding activities of certainpests. Check carefully for tiny invaders.
If you see no insects, a closer look is required to decidewhich common rosemary diseases might be infecting the plant. To prevent disease,make sure your plants have plenty of circulation and are planted in a well-drainingarea. If overly wet soil occurs frequently, consider moving the plants tocontainers or raised beds.
The most common fungal diseases are rootrot and powderymildew. The latter occurs in warm, wet periods and is characterizedby a dusting of whitish, fine spores on all parts of the plant. It is mostprevalent when the plant is in semi-shade and temperatures are 60 to 80 degreesFahrenheit (16-27 C.). An organic fungicide spray or a DIY mixture of bakingsoda and water can help combat the fungus.
Root rot will almost always kill the plant. The rosemarywill become limp and terminal leaves and stems die off. This is because theroots are no longer able to uptake and move nutrients and water to the plant.Dig up the plant and prune out any infected roots and dust with fungicidepowder. If the entire root system is black and mushy, discard the plant.
Bacterial diseases are less common but may arise infavorable conditions and in contaminatedsoils.
Blight infections are both fungal and bacterial, and resultin patchy leaf growth and yellowish spots. High humidity, too little sun andlack of circulation are promoting factors. Prune to increase circulation andensure the plant is in a sunny location.
Leaf spot is another disease that may stem from fungal orbacterial pathogens. Brownish black spots appear and the stems will wilt. Avoidwatering plants overhead.
In most cases, rosemary disease control is a simple matterof correctly siting the plant, good care and common sense. These are hardyperennials and rarely have any issues.
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Rosemary (Rosmarinus officinalis) is one of the most popular culinary herbs in Europe and the United States and is often used as an ornamental plant. The upright varieties can become large shrubs, and the prostrate varieties are used as ground cover or in rock gardens. This fragrant plant with small blue flowers attracts bees, butterflies and hummingbirds to the garden. It is hardy in U.S. Department of Agriculture plant hardiness zones 7a through 10b.
How To Use This Problems Section
The chart is organized to give you a quick and dirty summary of the possible symptoms that you may encounter. Those problem causes for which we have full files will be linked to those files. Those causes with no link will have a paragraph below the chart helping you deal with that particular problem.
|Leaves Yellow or Brown, Drop Off||Cultural Problems|
|Leaves Stippled Yellow||Spider Mites|
|Plants Weakened, Foliage Yellowed||Whiteflies|
|Growth Stunted, Leaves Damaged||Mealybugs|
|Older Leaves Rot||Botrytis Blight|
Rosemary roots in soil that is constantly moist or soggy, will suffocate, causing its foliage to turn brown and die. Check the soil in the container if it is too wet, stop watering immediately. Allow the rootball to dry out and trim off the dead stems. During the winter, when the plant is not growing vigorously anyway, avoid watering on overcast days. A well-draining potting medium such as a cactus mix is good for this situation.
Leaves Stippled Yellow by Spider Mites
Spider mites are tiny spider-like pests about the size of a grain of black pepper. They may be red, black, brown, or yellowish-white. Mites feed by sucking plant juices, removing chlorophyll and injecting toxins which cause small white dots on the leaves, discoloring and distorting them. Foliage of mite infested plants becomes stippled, yellow, and dry, and sometimes fine webbing is visible.
Spray plants with a forceful spray of water every other day for 3 days, to knock the mites from the leaves. If they persist, spray them with a natural insecticide such as an insecticidal soap or neem product according to instructions on its label.
For more information see the file on Controlling Mites
Plants Weakened, Foliage Yellowed by Whiteflies
Adult whiteflies are tiny, mothlike insects about the size of a pinhead. They sometimes cluster on rosemary foliage in large numbers, sucking plant juices from leaf undersides. Shake an infested plant and they fly off like dandruff.
A minor infestation of these insects will not harm a mature, otherwise healthy plant. But young plants weaken, and their leaves turn yellow and die. To make things worse, honeydew given off by whiteflies may encourage mold growth as well. Control whiteflies by spraying them with a natural insecticide such as an insecticidal soap or neem product as directed on its label.
For more information see the file on Controlling White Flies
Foliage Damaged by Scale
Scale insects are related to mealybugs, aphids, and whiteflies. They appear as little bumps or scabs. These are really waxy outer shells, which protect scale insects while they suck sap from plant tissues. Various types of scale insects range in size from a small letter “o” to a capital letter “O”. They weaken plants and make them more susceptible to drought, cold stress and attacks by other insects or diseases.
Caught early a scale infestation can be scraped off with your fingernail or with a cotton swab dipped in rubbing alcohol. Sometimes it is simpler to clip off scale-infested plant tips and discard them in the trash. If done early in the season, this will not harm the plant and may, in fact, encourage it to branch and become bushier. If plants are severely infested, spray them with light horticultural oil which coats and suffocates the crawling insects as well as their eggs.
For more information see the file on Controlling Scale
Growth Stunted and Leaves Damaged by Mealybugs
Mealybugs are flat, wingless insects covered with a white, waxy powder. They collect in white, cottony masses on stems, branches and leaves. As they suck sap from their leaves and stems, plants lose their vigor. They grow poorly and may die if the infestation is severe. Ants are attracted to mealybugs because they excrete a sweet honeydew as they feed. The honeydew also encourages mold growth to which rosemary is prone.
Wash infested stems under the faucet and rub the bugs off or spray them with a natural insecticide such as an insecticidal soap or a pyrethrum based insecticide product as directed on the label. Mealybugs are also vulnerable to natural enemies such as lacewings and ladybugs, which eat the pests, and to tiny parasitic wasps, which lay their eggs inside them.
For more information see Controlling Mealy Bugs
Older Leaves Rotted by Botrytis Blight
Botrytis blight fungal infection rots the older leaves in the center of a rosemary plant. It flourishes in high relative humidity, poor air circulation, and cloudy weather conditions. Watch for yellowish brown, irregular leaf spots or water-soaked spots on plant stems. During humid weather, the fungus produces a fuzzy brown to gray growth on the surfaces of injured plant parts or dead plant material. When disturbed, the fungus emits a cloud of spores.
Remove and destroy infected plants and debris to control spread of the disease. Prune overgrown plants, and increase spacing between container plants to improve air circulation. Avoid mulching the plant beds with organic materials, because these harbor fungal spores. Instead, use crushed stone, pea gravel, or rocks, which dry out quickly and reflect warm sun onto the plants and reduce humidity still further.
For more information see the file on Controlling Fungal Disease
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