Phytophthora
Edited by Len Phillips
Phytophthora is a genus of plant-damaging Oomycetes (water molds), whose member species are capable of causing enormous economic losses on trees and crops worldwide, as well as environmental damage to natural ecosystems. Phytophthora is morphologically very similar to true fungi yet its evolutionary history is quite distinct.
Symptoms
The leaves of plants affected by Phytophthora rot appear drought stressed. Trees or plants often wilt and die rapidly with the first warm weather of the season. Leaves may turn dull green, yellow, or in some cases red or purplish. Often, only plants in the most poorly drained area of the field or garden are affected. Phytophthora infections typically kill young trees because their root systems and crown areas are small compared to those of mature trees.
Symptoms may develop first on one branch or stem then spread to the rest of a tree or plant. Trees may decline over a period of years before finally dying or they may be killed in a single season. Slow decline occurs when the roots are attacked. Rapid decline occurs when the crown or basal stem is attacked and girdled. The damage completely encircles the stem in a single season.
Symptoms on roots and crowns may vary somewhat depending on the species of Phytophthora involved, the plant being attacked, the resistance of the plant variety, the soil moisture, and the soil temperature. In general, trees affected by Phytophthora develop darkened areas in the bark around the crown and upper roots. Gum or dark sap may ooze from the margins of the diseased trunk area. If bark tissue is carefully cut away, reddish-brown streaks or zones can be seen in the inner bark and outer layer of wood. No mycelium is visible in between the bark and wood in trees affected by Phytophthora, distinguishing this disease from Armillaria root rot, which is a true fungus.
Biology
Phytophthora species are soil-inhabiting pathogens that are favored by wet conditions. Species of Phytophthora produce resting spores that survive for years in moist soil in the absence of a suitable host. However, if the soil is completely dried out, these spores are less likely to survive for more than a few months. When a host is nearby and water in soil pore space is present, resting spores germinate to produce motile spores that can directly penetrate roots, branches, or crowns as long as free water is present. Wounds are not required for infection. Resting spores, decaying host tissue in the soil, and active disease-infected dead, sunken lesions/cankers can all be sources for new infections. The pathogen can be spread in splashing rain or irrigation water, in surface irrigation and runoff water, and by movement of contaminated soil, equipment, or plant parts. Flooded and saturated soil favors the spread of Phytophthora to healthy plants.
Some Phytophthora species are favored by warm weather, others by cool weather. Root rot of avocado, citrus, and tomato are favored by warm conditions, developing most extensively in late spring and early summer. Decay of crown, trunk, and branches of other tree species are favored by cool, wet conditions. These decays develop most rapidly in late fall and early spring.
Species
Approximately 100 species have been identified; although an additional 100-500 undiscovered Phytophthora species are estimated to exist. Phytophthora spp. are mostly pathogens of dicotyledons and are relatively host-specific parasites. Some of the most well-known Phytophthora diseases are described below:
Several species of soil borne pathogens of Phytophthora cause crown and root rot diseases of herbaceous and woody plants. Almost all fruit and nut trees as well as most ornamental trees and shrubs can develop Phytophthora rot if soil around the base of the plant remains wet for prolonged periods, or when planted too deep. Tomatoes, peppers, eggplant, and other vegetable crops can also be affected by Phytophthora rot.
Research beginning in the 1990s has placed some of the responsibility for European forest die-back on the activity of imported Asian phytophthoras. In trees and shrubs, the pathogen kills plants by growing from the roots up through the root crown and into the lower trunk, where it kills the inner bark and causes a browning of the outer layer of sapwood. Losses to Phytophthora are minimized by providing good soil drainage and selecting the most tolerant rootstocks or varieties available. In general, Phytophthora requires warm, moist soils in order to cause damage.
Management
The most important factor in reducing the threat of Phytophthora rot is good water management. Avoid prolonged saturation of the soil or standing water around the base of trees or other susceptible plants. Irrigate only as much and as often as necessary. In an orchard, keep track of the soil moisture around each tree and water only when necessary. If irrigating trees with sprinklers, use low-angle sprinkler heads and splitters to avoid wetting the trunk and lower branches. If using a drip system, place the emitters at least a foot away from the trunk. Avoid planting susceptible species on poorly drained or shallow soils. Water stress and/or salinity make some plant species more susceptible to infection when wet subsequently by irrigation or rains.
Provide adequate drainage by breaking through soil compaction and hardpan. In poorly drained soils, or in an area where Phytophthora is present, consider planting trees and shrubs on mounds. The mounds should be 8 to 10 inches high (20-22 cm). Planting depth after settling should be no deeper than as received from the nursery, with the upper roots near the soil level and the graft union well above the soil line. Raised beds provide good drainage in vegetable garden situations also. Do not install irrigated turf around the base of trees, remove all weeds, and do not water the crown area directly. Group plants according to their irrigation needs. Separate those needing frequent, light irrigation, such as potatoes and strawberries, from those needing infrequent, deep irrigation, such as tomatoes and melons.
At the first signs of above ground symptoms, examine the tree at the soil line for crown rot. Carefully cut away bark that looks infected. If crown rot is present, trees can sometimes be saved by removing soil from the base of the tree down to the top of the main roots and allowing the crown tissue to dry out.
Sanitation
It may be possible to slow the spread of Phytophthora within an orchard by avoiding movement of infested soil, water, and plant parts from an area where Phytophthora rot has developed. Surface and subsurface drainage water and anything that can move moist soil can carry the pathogen to a new area, including boots, car tires, and tools. If the physical setting allows drainage water to flow from infested to uninfected areas within the garden during wet weather, consider putting in drains to channel the water away from healthy plants.
Selection of planting stock
Plant only certified nursery stock from a reputable source, and choose the most resistant rootstocks or varieties available for the area. Carefully select individual plants that are free of symptoms and/or that come from healthy plots of material.
Chemical control
The most effective way of preventing Phytophthora rot diseases is to provide good drainage and to practice good water management. Along with the appropriate cultural controls, a fungicide may be used on a number of ornamental plant species to help prevent Phytophthora infections. When applied as a foliar spray it is absorbed by foliage and moves into roots. However, do not rely on fungicide applications alone to control root and crown rot diseases.
Sources
Edited by Len Phillips
Phytophthora is a genus of plant-damaging Oomycetes (water molds), whose member species are capable of causing enormous economic losses on trees and crops worldwide, as well as environmental damage to natural ecosystems. Phytophthora is morphologically very similar to true fungi yet its evolutionary history is quite distinct.
Symptoms
The leaves of plants affected by Phytophthora rot appear drought stressed. Trees or plants often wilt and die rapidly with the first warm weather of the season. Leaves may turn dull green, yellow, or in some cases red or purplish. Often, only plants in the most poorly drained area of the field or garden are affected. Phytophthora infections typically kill young trees because their root systems and crown areas are small compared to those of mature trees.
Symptoms may develop first on one branch or stem then spread to the rest of a tree or plant. Trees may decline over a period of years before finally dying or they may be killed in a single season. Slow decline occurs when the roots are attacked. Rapid decline occurs when the crown or basal stem is attacked and girdled. The damage completely encircles the stem in a single season.
Symptoms on roots and crowns may vary somewhat depending on the species of Phytophthora involved, the plant being attacked, the resistance of the plant variety, the soil moisture, and the soil temperature. In general, trees affected by Phytophthora develop darkened areas in the bark around the crown and upper roots. Gum or dark sap may ooze from the margins of the diseased trunk area. If bark tissue is carefully cut away, reddish-brown streaks or zones can be seen in the inner bark and outer layer of wood. No mycelium is visible in between the bark and wood in trees affected by Phytophthora, distinguishing this disease from Armillaria root rot, which is a true fungus.
Biology
Phytophthora species are soil-inhabiting pathogens that are favored by wet conditions. Species of Phytophthora produce resting spores that survive for years in moist soil in the absence of a suitable host. However, if the soil is completely dried out, these spores are less likely to survive for more than a few months. When a host is nearby and water in soil pore space is present, resting spores germinate to produce motile spores that can directly penetrate roots, branches, or crowns as long as free water is present. Wounds are not required for infection. Resting spores, decaying host tissue in the soil, and active disease-infected dead, sunken lesions/cankers can all be sources for new infections. The pathogen can be spread in splashing rain or irrigation water, in surface irrigation and runoff water, and by movement of contaminated soil, equipment, or plant parts. Flooded and saturated soil favors the spread of Phytophthora to healthy plants.
Some Phytophthora species are favored by warm weather, others by cool weather. Root rot of avocado, citrus, and tomato are favored by warm conditions, developing most extensively in late spring and early summer. Decay of crown, trunk, and branches of other tree species are favored by cool, wet conditions. These decays develop most rapidly in late fall and early spring.
Species
Approximately 100 species have been identified; although an additional 100-500 undiscovered Phytophthora species are estimated to exist. Phytophthora spp. are mostly pathogens of dicotyledons and are relatively host-specific parasites. Some of the most well-known Phytophthora diseases are described below:
- Phytophthora alni – causes alder root rot,
- Phytophthora cactorum – causes rhododendron root rot affecting rhododendrons, azaleas and causes bleeding canker in hardwood trees,
- Phytophthora capsici – infects Cucurbitaceae fruits, such as cucumbers and squash,
- Phytophthora cinnamomi – causes cinnamon root rot affecting woody ornamentals including aucuba, arborvitae, azalea, Camellia japonica, Chamaecyparis, dogwood, deodar cedar, forsythia, Fraser fir, heather, hemlock, high-bush blueberries, Japanese holly, juniper, leucothoe, Pieris, Rhododendron, Taxus, white pine, American chestnut and Australian Jarrah,
- Phytophthora fragariae – causes red root rot affecting strawberries,
- Phytophthora infestans – was the infective agent of the potato blight that caused the Great Irish Famine (1845-1849), and still remains the most destructive pathogen of potato crops. The soya bean root and stem rot agent,
- Phytophthora kernoviae – pathogen of beech and rhododendron, also occurring on other trees and shrubs including oak, and holm oak. First seen in Cornwall, UK, in 2003.
- Phytophthora palmivora – causes fruit rot in coconuts and betel nuts,
- Phytophthora parasitica – attacks Boxwood in a manner similar to other Phytophthora.
- Phytophthora ramorum – infects over 60 plant genera and over 100 host species – causes Sudden Oak Death which has very different symptoms and management than the species discussed here,
- Phytophthora quercina – causes oak death,
- Phytophthora sojae – causes soybean root rot. Phytophthora sojae has also caused longstanding problems for the agricultural industry. In general, plant diseases caused by this genus are difficult to control chemically, and thus the growth of resistant cultivars is the main management strategy.
Several species of soil borne pathogens of Phytophthora cause crown and root rot diseases of herbaceous and woody plants. Almost all fruit and nut trees as well as most ornamental trees and shrubs can develop Phytophthora rot if soil around the base of the plant remains wet for prolonged periods, or when planted too deep. Tomatoes, peppers, eggplant, and other vegetable crops can also be affected by Phytophthora rot.
Research beginning in the 1990s has placed some of the responsibility for European forest die-back on the activity of imported Asian phytophthoras. In trees and shrubs, the pathogen kills plants by growing from the roots up through the root crown and into the lower trunk, where it kills the inner bark and causes a browning of the outer layer of sapwood. Losses to Phytophthora are minimized by providing good soil drainage and selecting the most tolerant rootstocks or varieties available. In general, Phytophthora requires warm, moist soils in order to cause damage.
Management
The most important factor in reducing the threat of Phytophthora rot is good water management. Avoid prolonged saturation of the soil or standing water around the base of trees or other susceptible plants. Irrigate only as much and as often as necessary. In an orchard, keep track of the soil moisture around each tree and water only when necessary. If irrigating trees with sprinklers, use low-angle sprinkler heads and splitters to avoid wetting the trunk and lower branches. If using a drip system, place the emitters at least a foot away from the trunk. Avoid planting susceptible species on poorly drained or shallow soils. Water stress and/or salinity make some plant species more susceptible to infection when wet subsequently by irrigation or rains.
Provide adequate drainage by breaking through soil compaction and hardpan. In poorly drained soils, or in an area where Phytophthora is present, consider planting trees and shrubs on mounds. The mounds should be 8 to 10 inches high (20-22 cm). Planting depth after settling should be no deeper than as received from the nursery, with the upper roots near the soil level and the graft union well above the soil line. Raised beds provide good drainage in vegetable garden situations also. Do not install irrigated turf around the base of trees, remove all weeds, and do not water the crown area directly. Group plants according to their irrigation needs. Separate those needing frequent, light irrigation, such as potatoes and strawberries, from those needing infrequent, deep irrigation, such as tomatoes and melons.
At the first signs of above ground symptoms, examine the tree at the soil line for crown rot. Carefully cut away bark that looks infected. If crown rot is present, trees can sometimes be saved by removing soil from the base of the tree down to the top of the main roots and allowing the crown tissue to dry out.
Sanitation
It may be possible to slow the spread of Phytophthora within an orchard by avoiding movement of infested soil, water, and plant parts from an area where Phytophthora rot has developed. Surface and subsurface drainage water and anything that can move moist soil can carry the pathogen to a new area, including boots, car tires, and tools. If the physical setting allows drainage water to flow from infested to uninfected areas within the garden during wet weather, consider putting in drains to channel the water away from healthy plants.
Selection of planting stock
Plant only certified nursery stock from a reputable source, and choose the most resistant rootstocks or varieties available for the area. Carefully select individual plants that are free of symptoms and/or that come from healthy plots of material.
Chemical control
The most effective way of preventing Phytophthora rot diseases is to provide good drainage and to practice good water management. Along with the appropriate cultural controls, a fungicide may be used on a number of ornamental plant species to help prevent Phytophthora infections. When applied as a foliar spray it is absorbed by foliage and moves into roots. However, do not rely on fungicide applications alone to control root and crown rot diseases.
Sources
- University of California, “Phytophthora Root and Crown Rot”, IPM Home, 2011
- “Phytophthora”, Wikipedia.com, 2011