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Making the world a greener place

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Diamond Scale

Posted on November 9, 2013 at 9:54 PM Comments comments (263)
Despite the name, diamond scale is not an insect pest. Instead the fungus Phaeochoropsis neowashingtoniae (previously called Sphaerodothis neowashingtoniae) causes this common foliar disease, which derives its name from its characteristic diamond-shaped fruiting bodies.
Diamond scale attacks primarily the California fan palm (Washingtonia filifera) in coastal regions and the intermediate and interior valleys of California subject to marine influence; it rarely occurs in arid regions such as the Central Valley or the deserts of Southern California. Diamond scale can occur on hybrids of the Mexican fan palm (Washingtonia robusta), and the incidence and severity usually are proportionate to the amount of California fan palm in the hybrid. Diamond scale has not been observed on pure Mexican fan palm or any other species in California.
Identification and Damage
Initial infection sites are dark, water-soaked spots the size of a pinprick that eventually turn black and grow to shiny, diamond-shaped fruiting bodies 3/25 to 3/10 inch long by 3/50 to 3/25 inch wide. The fruiting bodies occur on the upper and lower surfaces of leaf blades and petioles, the stalk connecting the leaf base to the blade.
Diamond scale causes leaves to yellow and brown, then die prematurely, resulting in a reduced crown of leaves and an unattractive landscape subject. Older or lower leaves typically become the most infected, because the longer the leaf remains exposed, the greater the number of infections. Because of their more vigorous growth rate, young palms tend to have less disease and a fuller crown of leaves than older, less vigorously growing plants.
Disease severity often is cyclical. The dry, warm seasons of summer and fall favor California fan palm rather than diamond scale, so palms tend to grow quickly, producing leaves faster than the pathogen can colonize them. In contrast, the moist, cool seasons of winter and spring favor the pathogen over the host, so palms tend to grow more slowly, and the disease advances higher into the crown, resulting in a less-than-full set of leaves.
Heavily infected leaves have a black, sooty dust that rubs off easily when you brush against or handle them during removal, making the plant a nuisance to work around. Although not particularly lethal by itself, diamond scale reduces vigor and stresses the palm, leaving it vulnerable to other diseases such as pink rot.

Sooty Mold

Posted on November 9, 2013 at 9:51 PM Comments comments (42)
Sooty mold is the common name applied to several species of fungi that grow on honeydew secretions on plant parts and other surfaces. The fungi’s dark, threadlike growth (mycelium) gives plants or other substrates the appearance of being covered with a layer of soot.
Sooty molds don’t infect plants but grow on surfaces where honeydew deposits accumulate. Honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect can’t completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as “honeydew.” Wherever honeydew lands—e.g., leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary—sooty molds can become established.
Although sooty molds don’t infect plants, they can indirectly damage the plant by coating the leaves to the point that it reduces or inhibits sunlight penetration. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age (senesce) and die, causing premature leaf drop.
Fruits or vegetables covered with sooty molds are edible. Simply remove the mold with a solution of mild soap and warm water.
Most plants will tolerate a small insect population and light amounts of sooty mold. When sooty molds are present on any surface in the landscape, it indicates there is, or has been, a sucking insect population present in the vicinity. Control of sooty molds begins with managing the insect creating the honeydew. For example, populations of aphids usually are highest on succulent, new growth. In some situations a strong stream of water can dislodge the insects. Also fertilize and water to keep plants healthy but not excessively vigorous.
Another important consideration can be ant management. Ants are attracted to and use honeydew as a source of food. Because of this, they will protect honeydew-producing insects from predators and parasites in order to harvest the honeydew. In many cases, predators and parasites are sufficiently abundant and quickly begin feeding on and reducing populations of scale insects, aphids, psyllids, whiteflies, or mealybugs once ants have been eliminated. If populations fail to decline, apply horticultural oils, neem oil, or insecticidal soap to suppress the problem insects. One or more applications might be needed. For detailed information on managing these pests see the appropriate Pest Notes listed in References.

Powdery Mildew

Posted on November 9, 2013 at 9:48 PM Comments comments (83)
Powdery mildew is a common disease on many types of plants and is prevalent under the diverse conditions found in many areas of California. Different powdery mildew fungi cause disease on different plants. These fungi tend to infect either plants in the same family or only one species of plant.
You can recognize this disease by the white, powdery mycelial and spore growth that forms on leaf surfaces and shoots and sometimes on flowers and fruits. Powdery mildews may infect new or old foliage. This disease can be serious on woody species such as rose, crape myrtle, and sycamore where it attacks new growth including buds, shoots, flowers, and leaves. New growth may be dwarfed, distorted, and covered with a white, powdery growth. Infected leaves generally die and drop from the plant earlier than healthy leaves.
Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, water on plant surfaces for extended periods inhibits germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60° to 80°F and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95°F may kill the fungus.

Fire Blight

Posted on November 9, 2013 at 9:45 PM Comments comments (86)
Fire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive disease of pome fruit trees and related plants. Pear (Pyrus species) and quince (Cydonia) are extremely susceptible. Apple, crabapple (Malus species), and firethorns (Pyracantha species) also are frequently damaged. Fire blight is less common on hawthorn (Crataegus species), Spiraea, Cotoneaster, toyon (Photinia species), juneberry or serviceberry (Amelanchier species), loquat (Eriobotria), mountain ash (Sorbus species), and other related plants. The disease can destroy limbs and even entire shrubs or trees.
In spring, branch and trunk canker symptoms can appear as soon as trees begin active growth. The first sign is a watery, light tan bacterial ooze that exudes from cankers (small to large areas of dead bark that the pathogen killed during previous seasons) on branches, twigs, or trunks. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. However, most cankers are small and inconspicuous; thus infections might not be noticed until later in spring when flowers, shoots, and/or young fruit shrivel and blacken. The amount of fruit loss depends upon the extent and severity of the disease.


Posted on November 9, 2013 at 9:38 PM Comments comments (54)
Anthracnose is a group of diseases found on many deciduous and evergreen trees and shrubs; some trees such as sycamore, ash, and evergreen elms can be noticeably blighted. Often called leaf, shoot, or twig blight, anthracnose results from infection by any of several different fungi, including Apiognomonia errabunda, A. veneta, Discula fraxinea, Glomerella sp., Gnomonia sp., and Stegophora ulmea, depending on the tree attacked. Infections on deciduous plants are more severe in areas where prolonged spring rains occur after new growth is produced. Anthracnose fungi need water to be disseminated and infect; they do not spread under dry conditions.
Anthracnose symptoms vary with the plant host, weather, and time of year infection occurs. The fungi affect developing shoots and expanding leaves. Small tan, brown, black, or tarlike spots appear on infected leaves of hosts such as elm or oak. Dead leaf areas may be more irregular on other hosts such as ash. Sycamore anthracnose lesions typically develop along the major leaf veins. If leaves are very young when infected, they may become curled and distorted with only a portion of each leaf dying.
Generally, mature leaves are resistant to infection, but when conditions are favorable, they may become spotted with lesions. Heavily infected leaves fall prematurely throughout the growing season, and sometimes trees are completely defoliated. Early leaf drop is usually followed by production of more leaves. Twigs and branches also may be attacked and killed, resulting in a tree with crooked branches.
On some trees, cankers (infected areas that may or may not be surrounded by callus tissue) are another symptom of anthracnose infection. Cankers develop on twigs, branches, and the trunk, occasionally resulting in girdling and dieback. If defoliation, branch dieback, or cankering does not occur every year, anthracnose will not seriously harm plants. In California, anthracnose rarely causes permanent damage to plants except for elm trees.