Status: Probably native
Location: Throughout California
Hosts in California: Various 2- and 3-needle pines and some other conifers
Management: Management of Diplodia spp. on a landscape scale is usually impractical because of its ubiquity and its airborne nature. However, some success can be had in more controlled, intensively managed settings, such as seed orchards, nurseries, and plantations. The fungus often infects cones and produces fruiting bodies on them, so clearing away old, fallen cones is necessary for inoculum reduction; burning other forest residues might similarly help. Application of a contact fungicide such as Bordeaux mixture to developing shoots in late spring (April into May) can help prevent new shoot infections, although it will not necessarily prevent later-developing cones from becoming infected. Pruning obviously infected or dead branches and burning them can also reduce inoculum and sanitize affected stands. If Diplodia spp. are persistent in the local area because humidity is higher there than in the surrounding landscape, opening the stand through thinning or opening individual tree crowns by pruning may help to reduce humidity and thus the presence of the fungus.
Highlights: Diplodia pinea and Diplodia scrobiculata are problematic for pines throughout the world. They can also be occasional problems for other conifers when conditions are right, along with other Diplodia species such as Diplodia mutila, Diplodia seriata, and Diplodia corticola. On pines, Diplodia pinea causes serious branch and shoot blight and can kill susceptible or highly stressed trees. Diplodia spp. usually require wounds to enter pines, and once there they can wait latently until drought conditions stress trees sufficiently to encourage the fungus to become aggressively pathogenic. Diplodia pinea often infects numerous branches in the tree crown at once; it can be recognized by its habit of killing newly expanding shoots, leaving either dead, shorter-than-normal needles or dead, droopy ones at the very ends of the branches. Cut-open shoots may display pitch-soaked wood or tissues permeated with necrotic brown flecks. Diplodia scrobiculata is somewhat less aggressive and often attacks older branch wood, killing individual branches and rarely spreading through the crowns of mature trees. Both Diplodia species can readily kill seedlings and very small trees, causing black streaking throughout the sapwood and prolific fruiting on all killed tissues.
Until the early 2000s, Diplodia pinea and Diplodia scrobiculata were thought to be the same species with two very different morphological types. DNA analysis established that they were indeed separate species, and subsequent testing and observation have shown that for the most part they occupy different niches. Diplodia pinea is most prominent on ponderosa, gray, and other 3-needle pines in California, while Diplodia scrobiculata is a problem for most coastal pine species. Monterey pine provides an interesting case: while Diplodia pinea is becoming a growing problem for planted Monterey pine throughout many parts of the world, native, coastal populations in California are primarily parasitized by Diplodia scrobiculata. They are also present in other parts of the U.S. Diplodia scrobiculata is occasionally isolated from branch cankers on coast redwood, but pathogenicity tests have established that it is less aggressive toward redwood than many other members of the Botryosphaeriaceae family.
Ponderosa pine with large portion of the crown killed by Diplodia pinea
Ponderosa pine with top dieback and numerous shoot infections caused by Diplodia pinea
Ponderosa pine with large portion of the crown killed by Diplodia pinea
Bihon, W., Slippers, B., Burgess, T., Wingfield, M.J., and Wingfield, B.D. 2011. Diplodia scrobiculata found in the southern hemisphere. Forest Pathology 41(3), pp. 175-181.
Burgess, T.I., Gordon, T.R., Wingfield, M.J., and Wingfield, B.D. 2004. Geographic isolation of Diplodia scrobiculata and its association with native Pinus radiata. Mycological Research 108(12), pp. 1399-1406.
de Wet, J. Burgess, T., Slippers, B., Preisig, O., Wingfield, B.D., and Wingfield, M.J. 2003. Multiple gene genealogies and microsatellite markers reflect relationships between morphotypes of Sphaeropsis sapinea and distinguish and new species of Diplodia. Mycological Research 107(5), pp. 557-566.
Fabre, B., Piou, D., Desprez-Loustau, M.-L., and Marçais, B. 2011. Can the emergence of Diplodia shoot blight in France be explained by changes in pathogen pressure linked to climate change? Global Change Biology 17(10), pp. 3218-3227.
Peterson, G.W. 1981. Control of Diplodia and Dothistroma blights of pines in the urban environment. Journal of Arboriculture 7(1), pp. 1-5.
Stanosz, G.R., Smith, D.R., Guthmiller, M.A., and Stanosz, J.C. 1997. Persistence of Sphaeropsis sapinea on or in asymptomatic shoots of red and jack pines. Mycologia 89(4), pp. 525-530.
Stanosz, G.R., Blodgett, J.T., Smith, D.R., and Kruger, E.L. 2001. Water stress and Sphaeropsis sapinea as a latent pathogen of red pine seedlings. New Phytologist 149(3), pp. 531-538.
Dutch Elm Disease
Dutch Elm Disease is caused by an introduced non-native fungus, Ophiostoma ulmi (Buisman) Nannf. and Ophiostoma novo-ulmi Brasier. This vascular wilt disease which causes rapid death in native elm trees.
Location: California, US, Europe, New Zealand
Impact Significance: This introduced fungus has devastated native populations of elms that do not have resistance to the disease. Highly susceptible trees often die in a single year and spreads quickly in areas where monocultures existed for aesthetic reasons. This disease and others illustrates the value of plant diversity.
Hosts: Species of Elm (Ulmus spp.) – for example - American Elm (Ulmus Americana), red or slippery elm (U. rubra), rock elm (U. thomasii). Asiatic elms have higher levels of resistance to DED – Chinese elm (U. parvifolia), Japanese elm (U. davidiana var. japonica) and Siberian elm (U. pumila).
Dutch elm disease pathogens overwinter in the bark and outer wood of dying or recently dead elm trees and in logs as mycelia. These fungi are spread from these sites by their vectors – the elm bark beetle (Scolytus multistiatus) and the native elm bark beetle (Hylurgopinus rufipes). The female beetles borer into the bark of the dead and dying elm trees and logs to lay eggs. There the eggs hatch and larvae feed and pupate into adults. When they emerge, they are covered in thousands of sticky conidia of Ophiostoma ulmi or O. novo-ulmi. These newly emerged adults of S. multistriatus feed in the twig crotches of elm branches or the newly emerged H. rufipes tunnel into the bark of elm branches and trunks. As the beetle feeds, the fungal spores are deposited into the healthy, uninfected elm tree. The spores then germinate and grow into the xylem and produce millions of small conidia that spread through the xylem sap. These spores cause the formation of tyloses which block the water conducting vessels. This causes the characteristic symptoms of Dutch elm disease which is wilting leaves. The fungi also produce enzymes that degrade the cell walls and kill xylem cells which causes the brown discoloration just under the bark.
Elms can also be infected by Dutch elm disease pathogens by spores spreading from infected trees to healthy trees by root grafts. Root grafts form naturally between closely spaced elm trees. Large elm trees within 20 feet of each other have almost 100% chance of becoming infected through root grafts.
Seedlings and saplings escape and live long enough to reproduce, so the most susceptible elm species have not been threatened with extinction by Dutch elm disease. Though in the absence of effective disease management, Dutch elm disease increases in an affected population until that population is greatly reduced.
Cultural strategies – These strategies include avoidance of monocultures of elm trees, removal of all dying or recently dead branches, trees and cut wood. To be successful, inspection of all elm trees but be completed several times a year in the growing season and all wood must be burned, chipped or buried so it doesn’t provide a home for the beetle vectors.
Chemical strategies – many fungicides have been injected into infected trees or trees at risk of infection. These systemic chemicals are most effective if they are used to prevent new infections or to prevent the movement of the fungi into uninfested parts of the tree. These fungicides and treatments are expensive and none is completely effective for control of this pathogen.
Resistance breeding – several Asian elm species have moderate to high resistance and breeding programs in both Europe and the US have introduced resistance from these species into native elm species. Other programs have focused on identifying and cloning American elm specimens to have moderate resistance. These breeders strive to maintain the elegant vase shape of the American elm and as a result of decades of work, several hybrid and clonal elms are now available that have very good resistance to Dutch elm disease.
Dutch elm disease
Dutch elm disease
Fire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive disease of pome fruit trees and related plants.
Pathogen: Erwinia amylovora
Estimated economic impact of loss of antibiotics for fire blight control in organic orchards is $8-16 million per year.
Hosts: Fruit trees and related plants, including Pear (Pyrus species), quince (Cydonia), Apple, crabapple (Malus species), and firethorns (Pyracantha species).
Symptoms of fire blight can be observed on all above ground tissues including blossoms, fruits, shoots, branches and limbs, and in the rootstocks near the graft union on the lower truck. Blossom symptoms become water soaked and dull, greyish green in appearance and later shrivel and turn brown to black. During periods of high humidity, small droplets of bacterial ooze form on water-soaked and discolored tissues.
Shoot symptoms are similar to those in blossoms but develop faster. The tip of shoot may rapidly wilt to form a "Shepard's crook". Leaves on the diseased shoots often show blackening along the midrib and veins before becoming fully necrotic. When numerous diseased shoots are on a tree, it gives the tree a burnt appearance, hence the name. As the disease progresses up and down larger limbs and branches the bark will become darken and water soaked. In advanced stages, cracks will develop in the bark and the surface will be slightly sunken. Amber-colored bacterial ooze mixed with the plant sap may be present on the bark and infested wood under the bark will show streaked brown to black discolorations.
Primary inoculum are moved to blossoms from rain and or insects. Once on the blossoms, the population size in influenced by temperature and the number of blossoms infected. The bacteria then continuing infecting the plant shoots and larger branches where a small percentage of the pathogen can overwinter in annual cankers on diseased branches. Free bacterial cells are released onto the bark surface and insects are attracted to the ooze and disseminate the bacteria from cankers to flowers.
Management: Cultivar selection is the most effective control method for fire blight. Another essential control method is removal of expanding or overwintering cankers by pruning during the winter months. Sprays of antibiotics and copper compounds to blossoms to prevent infection is also effective.
Damage: The disease can destroy limbs and even entire shrubs or trees.
Management: Management of anthracnose can be very challenging due to the inoculum on infected foliage that may remain attached to the tree and those leaves that have fallen to the ground. Inoculum can also be present in infected twigs and can remain dormant until ideal conditions are present. Generally, in California, anthracnose is of minor importance but can cause substantial amounts of defoliation and dieback in years with frequent and abundant rainfall that continues late into the spring. Pruning and discarding dead stems and branches and thoroughly removing all fallen leaves in the autumn and spring can help reduce inoculum onsite, but these fungal spores can travel long distances so eradication of the fungus is unlikely in most situations. Maintaining tree vigor and health in important.
Location: California, worldwide
Impact Significance: Most often oak anthracnose causes only minor damage to landscape oaks, however, after long periods of wet weather early in the growing season, damage can be severe.
Hosts: Oaks (Quercus), Beech (Fagus), Chestnut (Castanea) and Linden (Tilia). In California hosts include: valley oak (Quercus lobate), Oregon white (Q. garryana), blue oak (Q. douglasii), California black (Q. kelloggii), coast live (Q. agrifolia), and interior live oak (Q. wislizeni).
Disease Cycle: Oak anthracnose causes dark pinpoint fruiting bodies and localized necrotic spots or lesions on young leaves and shoots. Some of these fungi may completely kill young leaves and twig under ideal conditions, though severity will vary with host, time of season and weather. Mature leaves are more resistant to the infection because they have developed their thick waxy cuticle so lesions on these leaves will be smaller in size. The dead leaves can remain attached to killed twigs and symptoms are usually scattered throughout the canopy of the tree.
The spores of anthracnose are spread by splashing and wind-blown rain. Spores are produced from fungal tissue that ruptured through the surface of the leaf, petiole and twigs. Twig infections can occur especially if wet conditions persist into mid-summer. Wet conditions are required for infection to occur. Immature host tissue is the most susceptible and prolonged periods of rain throughout the spring provide optimum conditions for disease development. These conditions are common in other areas where this pathogen is found and severe disease can often result, but in California, these conditions are uncommon and so is severe disease from anthracnose. Disease outbreaks when they do occur will subside by mid-summer when conditions become warmer and drier.
The fungus overwinters on fallen leaves and in lesions on infected twigs. When conditions are favorable for infection in the spring, temperatures from 15-20◦C, spores are produced in fallen leaves and infected twigs and initiate new infections on succulent tissues and the cycle repeats. Healthy trees that are defoliated early in the season usually have reserves to produce a second flush of foliage and long-term impacts are minimal, however, oaks weakened by other stresses can be adversely affected by anthracnose.
Location: California, worldwide
Impact Significance: Infection levels are normally not severe to significantly effect on plant health, except in small seedlings. The symptoms it causes are simply unsightly.
Hosts: Hardwood tree species and ornamentals, gymnosperms are not affected.
Disease Cycle: Powdery mildews obtain their nutrients from living cells and the intent is not to kill the host tissues they are using as a food source. The fungi overwinter as either a durable spore, cleistothecia, or in some cases as mycelium in infected buds. In the spring, ascospores are released from the cleistothecia and are blown or rain splashed onto the host leaves. The ascospores germinate and for a swollen structure, appressoria, that allows the fungi to penetrate the hosts epidermal cells. Haustoria, fungal structures, are formed within the living host cells to absorb nutrients and water that the fungus needs for growth and reproduction. Most of the fungal mycelium grows on the outer surface of infected leaves. Conidia are formed on this mycelium on the surface of the leaves and are then windblown to new young succulent shoots where they can initiate new infections. New infections are favored by warm days and cool nights and are inhibited by wet conditions. Depending on the species infections can occur in the late summer and fall or in the spring (B. trina).
Signs of powdery mildews on oak leaves will be dry, whitish fungal growth on the leaf surfaces. Young leaves are more infected than older leaves and high infections levels may cause symptoms of leaf distortion and discoloration. The symptoms can vary a bit depending on powdery mildew species. Some species will grow on the upper leaf surface where others will grow on the lower surface of the leaf. Overall the mycelium will be white and turn brownish with age. Cystotheca lanestris can cause systemic infections that cause witches brooms (abnormal clusters of shoots that are thickened, elongated and highly branched). Leaves on the witches’ brooms will be smaller and will die early. This broom will be white when the fungus produces spores an die or infected new shoots next season.
Powdery mildew will cause slow debilitation of the infected plant parts such as dwarfing, distortion, chlorosis, premature dying and browning of leaves, and subnormal growth rate.
Management: Infections of powdery mildew are not normally severe enough to significantly affect plant health though they might be unsightly.
Wayne, S., Lyon, H and Johnson, W. Diseases of Trees and Shrubs. Cornell University Press, 1987, 575 p.
Swiecki, Tedmund J. and Bernhardt, Elizabeth A. 2006. A field guide to insects and diseases of California oaks. Gen. Tech Rep. PSW-GTR-197. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture, 151 p.