Laurel Disease (4)
I have both Prunus Otto Luyken and Skip Laurel and they have some disease that is causing leaves and branches to wither and die or to cause "holes" to appear in the leaves. See pictures. (Note since I can only send one picture at a time I will repeat this request four separate times.) Is there a insecticide or/of fungicide, etc. that I can use to resolve the problems? Thank you.
Montgomery County Pennsylvania
The Otto Luyken (Prunus laurocerasus "Otto Luyken") is a dwarf English laurel cultivar that grows well in U.S. Department of Agriculture plant hardiness zones 6 through 8. This compact, broadleaf evergreen shrub only reaches about 3 feet tall with 6-foot spreads and is commonly used as a hedge, as you described in your situation.
Unfortunately, this particular species has been susceptible to multiple diseases and infestations which appear to be infecting your plantings. You appear to have a bacterial infestation and in addition suffer from a pest and/or fungal problem/s. Some of the suggested treatments are life cycle dependent and therefore might not be available until the next growing season.
Bacterial problem One of your pictures suggests that the laurels are affected by a disease caused by bacterial pathogens (Xanthomonas pruni). This disease typically starts in early summer with little, brown sores appearing on the undersides of leaves. The spots grow and develop red borders and yellow halos. The spots eventually fall out of the leaves, leaving behind gaping holes and tattered leaf tissue. Many infected leaves look as though someone peppered them with buckshot.
Because chemical control of shot hole disease is hard to achieve for home gardeners, help prevent this disease from occurring by planting the shrub in a well-draining location. Prune out and destroy any infected plant tissue as soon as you notice it. Rake up fallen plant litter to prevent pathogens from being attracted to the area near your shrub.
Fungal Problem This English laurel cultivar sometimes suffers from root rot and powdery mildew diseases. Root rot pathogens (Phytophthora spp.) enter the roots and disrupt the transfer of water and nutrients. Although the fungi thrive in overly wet soil conditions, affected plants look drought stressed with wilted, discolored, stunted and prematurely dropping foliage. Infected branches and twigs typically suffer dieback while cankers or stains often appear on infected trunks. Various fungus species trigger powdery mildew, a disease that causes a white, powdery growth to form on leaf surfaces. Affected foliage may also appear stunted and misshapen.
Help prevent root rot by planting your Otto Luyken in a site with good drainage. Avoid saturating the soil around the base of your shrub, and water only when the soil feels dry to the touch. Planting your shrub in a full-sun location helps control fungal diseases since most pathogens prefer shady conditions and cooler temperatures. Severe powdery mildew infections may require fungicidal treatment. You could spray your shrub with a 2 percent horticultural oil solution. Following the directions on the product's label, thoroughly spray the upper and lower sides of the laurel leaves for the best results.
Pest Infestation In addition, shot hole borers (Scolytus rugulosus) still occasionally overrun this shrub. This tiny, dark-colored beetle grows to about the size of a grain of rice and possesses strong jaws for chewing its way into the inner wood of branches and trunks. Adult females lay eggs just underneath the bark, and the larvae emerge to start mining little galleries. Signs of a shot hole borer infestation include little, round holes oozing gummy sap mixed with sawdust. Boring damage typically causes twig dieback.
Chemical control methods don't work once shot hole borers infest your shrub since the pests live in the protected area just below the bark. Preventative treatment is also difficult because the pesticide products available to home gardeners don't work effectively. Prune out and destroy all infested plant parts. If the borers attack the trunk, remove the entire shrub.
Or, your cherry laurels may be subject to feeding by the peach tree borer and/or a scale insect.
Peach tree borer - You will have to look around the base of the shrubs. Remove the mulch from around the base of the plant. Look for dieback and the presence of gummosis and/or frass at the root crown. (Cast pupal skins may be visible at exit holes.) Make sure your mulch is no thicker than two inches and keep away from the base of the trunk to prevent boring insects. Here is a link to help you identify the peach tree borer. http://plantdiagnostics.umd.edu/level3.cfm?causeID=166
White prunicola scale - All active stages of the above insect suck plant cell contents from twigs and branches. Heavy infestations usually begin on outer branches. Dieback usually begins on small branches and usually is preceded by leaf yellowing and premature leaf drop. At this point, prune off heavily infested branches showing dieback. Use a soft toothbrush or soft scrubbrush to remove scale covers now.
You can use a dormant oil spray to manage light infestations. Use a dormant oil when deciduous trees drop their foliage usually (Dec-March). Target the spray where the scale is located. Spray on a windless day when the temperature is expected to remain above 40 degrees for 24 hours. Follow all label directions. Check for crawlers in mid-May. You may need to use a hand lens. You can use a summer rate of horticultural oil in June, if need be. Use horticultural oil instead of neem oil.
If you are still in doubt as to your specific diagnosis please contact your local extension office in Montgomery County and share the pictures and any samples with the volunteers at the Master Gardener office at that location. Thank you,
Thank you for your reply. Do you know the commercial names of the 2 percent horticultural oil solution that you mentioned that I might be able to obtain from local garden stores? Also would Bonide "Fung-oinl" be of any use?
There are several brands of horticultural oil, for the most part all are refined petroleum products; although there are some refined vegetable oils being used. Sunspray and Volck Oils are probably the most common in the trade. Read the label closely as Summer Fine oils tend to cause the least amount of burn. There are dormant oils as well that can only be applied to dormant plants. Products containing neem oil may also control some of the scale insects. Good luck. I hope you are able to save the laurels.