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Combat Common Diseases in Your Favorite
Plants By Tom Ewing
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Love Veronica (Hebe) varieties but hate the inevitable die-back? Want Pyrus or Pyracantha but don’t want the fire-blight that is sure to pop up? Tired of the gall and die-back that is prevalent in Oleander? These beautiful plants thrive in Southern California, but so can the diseases that plague them. Don’t stop employing the plants, just learn to combat their problems. Many of the most popular landscaping plants suffer from bacterial disease and fungus infections. The key to combating them is to understand why the diseases occur and how we may be responsible for providing them a place to thrive. While lack of water, too much water, poor soil, the wrong climate, incorrect lighting, and even air pollution may cause your plants to whither and die, often the culprit is bacteria or fungus. Bacteria Bacteria are classified into three groups according to their shape: spherical bacteria are cocci, spirally curved bacteria are spirilla, and rod shaped or oval bacteria are bacilli. Nearly all of the several hundred bacterial diseases of plants are caused by the bacillus group. Under certain conditions, bacteria reproduce themselves rapidly simply by dividing in two. This may happen every hour, so that in a single day one bacterium can produce as many as 17 million new organisms. Other bacteria produce spores within themselves. Spores are minute and can survive a long time, even in the absence of water or at temperatures which could kill other organisms. When conditions are right, the spores germinate and begin a new generation of bacterial infection. Bacterial diseases fall into three categories: 1) Wilting, due to invasion of the plant’s vascular system or water conducting system. 2) Nerotic blights, which cause rotting and leaf spots as in fire-blight. 3) Overgrowth, or hyperplasia, as in crown gall. Bacteria can survive for months in an inactive state in plant tissue, and can survive years in the soil. Fungi Examples of common plant fungi are Rhizopus, which causes mold on fruits and berries, and Pithium, which causes damping-off, and is often seen in bedding plants. Practically every species of ornamental plant is more or less subject to attack by parasitic fungi. Some may cause little damage, while others kill the plant outright. With this basic understanding, it is easy to see why certain plants that appear healthy in the nursery, begin to die once they leave those controlled conditions. Most often, bacteria and fungi are already present in the plant or its soils. The nursery has controlled disease by regular drenching with anti-bacterial or anti-fungal products, or culling of any evidence of disease. When the conditions change, the problems manifest themselves again. For instance, many common bacterial infections are perpetuated by overhead water spray. Fire-blight is one of these bacteria. Called Erwinia Amylovare, this bacteria can be a serious problem on varieties in the Rosaceae family (Pyracantha, Photinia, Pyrus, Hawthorns, etc.). Symptoms usually appear as a sudden wilt of blossoms and leaves, turning dark brown to black suddenly. This disease is systemic and is definitely spread by frequent overhead water spray. For the disease to occur, the temperature must be mild, and there must be continuous moisture. Landscapers and designers should be aware of this when designing irrigation systems and surrounding plant materials with compatible requirements. Varieties that are prone to disease should be planted away from conditions that activate or perpetuate the problem. Too often you see Pyracantha, Nerium ‘Petite,’ or Veronica planted within Aptenia, iceplant, or other groundcovers that are irrigated frequently with overhead sprays. These groundcovers require frequent irrigation to establish themselves. The irrigation usually occurs from overhead and causes bacteria to activate in the susceptible neighboring plants. Common bacterial diseases are caused by a trio of the organisms: Phoma Exigua, Pseudomonas Syringae, and Fusarium Oxysporum. Most recently, Californians have seen damage from Phoma Exigua on the popular Nerium ‘Petite.’ The first symptom is dying lateral shoots. The leaves turn yellowish brown, then completely brown. Close inspection may reveal that the dying shoots are rising from below a pruning cut of a dead stem. Common Oleanders around the west suffer from a gall caused by the bacteria Pseudomonas Syringae. Infected galls or tumors should be pruned out, sterilizing shears between cuts. Cleanliness is key to reducing the spread of disease. Fusarium Oxysporum is the bacteria responsible for the uneven die-out often seen in Veronica (Hebe) species. Foliage of affected plants initially appears to dry unevenly and then die out. This disease has been frustrating enough to cause many nurseries to discontinue new crops. Common fungi diseases include Phymatotrichopsis Root Rot, Phytophthora Disease, Pythium Disease, Rhizoctonia Disease, Verticillum and Fusarium Wilts, Powdery Mildew, Rust diseases, Cytospora Canker, and Sooty Canker. Combating the problem
If you are too late to save a plant by changing it’s environment, then help stop the spread of the disease to healthy plants by eliminating those infected. Keep your plants healthy by providing them the appropriate nutrients. Well-fed plants are less susceptible to disease. Also look for resistant plant varieties. Tom Ewing is a consultant with Miramar Wholesale Nurseries, Southern California’s leading supplier of landscape plant material and supplies. MWN grows a wide-variety of perennials, shrubs, and trees at each of their three locations in San Diego, Irvine/Lake Forest, and San Juan Capistrano, to serve commercial properties, institutions, property managers, and commercial landscapers. In an acquisition in early 1999, MWN became a member of the San Diego region of TruGreen-LandCare, a ServiceMaster company.
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