I have had the unique opportunity to lend my eyes and expertise on various landscapes throughout the coastal barrier islands of Glynn over the last few weeks. Basically, I was searching for common disease, insect and nutritional issues and the plants most susceptible to the problems. It didn’t take more than 10 landscapes to see a pattern and certainly not an unexpected one. Certain common landscape plants that have been used for years are proving more and more difficult to grow as disease and insect resistance to chemicals have increased. When I see the plants in a landscape today, I see plants in more distress than years past. Unfortunately, having designed landscapes for many years, I know how easy it is to get into a pattern of using the same plants over and over. Of course, it helps that these plants remain readily available in the nursery trade. However, what has set our home landscape apart is our willingness to search out and experiment with new plant material, the desire to use what is native and a keen focus on high disease and insect resistance in combination with low maintenance. I have always loved research and am willing and able to educate myself on the new material that is constantly becoming more available. My husband also keeps us in the know with subscriptions to landscape industry literature and maintaining strong relationships with Georgia wholesale growers who are always looking for someone to experiment and give feed back.
Commonly found landscape plants in many coastal landscapes include American Holly and cultivars, boxwood, cabbage palm, camellia, Canary Island Date palm, cleyera, gardenia, Indian Hawthorne, ligustrum, magnolia (Southern), oleander, podocarpus, sago palm, viburnum, and Yaupon Holly. I used them frequently when I did local landscape design here in the 90s. Here are the problems I am seeing daily in 2019. If a plant doesn’t look healthy and green, most likely something is wrong.
Several different types of scale insects attack palms, hollies, ligustrum, oleander, magnolias and camellias in the home landscape. Scales hatch from eggs. The newly hatched soft-bodied young are called crawlers. They will move about on the plant, eventually finding a spot where they will insert their mouthpart and begin to feed on the plant sap. Over time, their legs disappear and a soft or hard covering will develop. They will then remain in the same place for their entire life cycle. Once their outer covering is formed, scale insects are very difficult to kill. Scale insects are most commonly found on the underside of the leaf, along the stem, but can also be seen on the surface. If the scale is on the underside feeding, the top of the leaf surface will show small yellow spots. As the infestation progresses, leaves and fronds can become brown and desiccated. Vigilance, identification of the scale, treatment at the crawler stage, pruning and removal of infested fronds of palms, proper watering and drainage, sufficient nutrition, organic and systemic sprays all play a role in controlling these insects.
Sandy soils of the coast have a difficult time retaining nutrients. A high pH also plays a factor in immobilizing nutrients in the soil so that a plant’s roots cannot absorb them. A soil test and foliar analysis will steer you in the right direction if your plant is showing a yellowing of the leaves or fronds. Most commonly we see manganese deficiency on sago palms (sago is actually a cycad and not a palm). Yellow spots on the new upper leaves appear first. As the deficiency progresses, leaves become more yellow, then brown and fizzled looking. It left unchecked, a sago palm will die from manganese deficiency. Applications of manganese sulfate are the cure. Canary Island Date Palms are most prone to magnesium deficiency. A palm tree suffering from this deficiency typically has leaves that turn yellow or orange on the outer edge of the oldest leaves while the center of the frond remains green. To cure magnesium deficiency, follow a regular fertilization schedule using a palm special fertilizer that contains magnesium. Always read and follow the directions on the label. Gardenia leaves that are turning yellow especially between the veins (which remain green) are most likely suffering from iron deficiency. Fertilize with an iron chelate. However, over and under watering and too much sunlight affect gardenia leaves can also affect leaf color but generally cause an overall yellowing of the entire leaf. The best means of avoiding nutritional deficiencies in most plant species is to provide plants with a moist, well-drained organically rich soil with a pH between 6-7.
I have seen terrible defoliation with the increased use of Viburnum Awabuki in landscapes, triggered by cool moist weather, especially foggy nights and mornings. Downy mildew causes light green to reddish-born leaf blotches, yellowing and eventual defoliation. I have never seen this problem on Walter viburnum (V. obovatum, native), or Sandankwa viburnum (V. suspensum). Good cultural practices are important to prevent the disease from getting out of control-avoid overhead or nighttime watering and water contact on the leaf surface; don’t too much fertilizer; avoid overcrowding; pick up diseased leaves. Fungicidal applications may be necessary but even with repeat applications and alternative products may not stop the progression and damage.
This is a fungal disease that has become a very serious issue. It causes rapid defoliation, decline and death to American and English boxwoods. It is very difficult and costly to control. Symptoms are sudden leaf drop or black streaking on stems. The key is prevention. Wet conditions, humid and warm temperatures are ideal for fungal spores to form. These sticky spores are transferred from plant to plant via leaf litter, grass clippings, lawn or garden equipment, birds, animals, wind and rain. Scientists are busy working on developing means of control boxwood blight.
Anthracnose of Sago Palm
This disease initially causes a portion of the leaf margin to yellow (chlorotic) and become necrotic. Infected leaves develop tan to reddish brown lesions usually associated with the leaf veins. In severe cases the plant will drop its fronds. Anthracnose can be confused with other problems so a laboratory analysis may be useful in determining the presence of anthracnose. Implementation of good cultural practices such as removal of diseased fronds can help. Be sure to keep the ground surface under the sago clean and free of plant debris. Check with the county extension agent for treating your sago with a fungicide.
Leaf spot diseases
Entomosporium leaf spot on Indian Hawthorne is common here. The first symptoms are tiny, round red spots on upper and lower sides of young leaves. These will expand into large irregular blotches. Severe infestation results in leaf drop. Avoid watering systems that wet the leaves; use drip irrigation. Space plants far enough apart for good air movement. Remove all fallen diseased leaves. Use of fungicidal sprays as new leaves emerge in the spring can help slow the spread of this fungal disease. Cercospora leaf spot on oleander appears as yellow spots on the infected leaves. The same cultural methods recommended above will help control the spread of this fungal disease.
This list of plants and plant problems is no way complete. My objective is to alert you, the homeowner, to problems I am seeing frequently so that you can have your landscape crew treat them or seek treatment yourself. Native plants, ornamental grasses, little to no turf and use of plants less prone to the problems above is possible in controlling disease and insect problems in coastal landscape design.