California Pest Rating Proposal for
Phytophthora cactorum (Lebert & Cohn) J. Schröt. 1886
Pest Rating: B
PEST RATING PROFILE
None. The current risk and status of Phytophthora cactorum in California are reassessed and a permanent rating is proposed.
History & Status:
Background: Phytophthora cactorum is an oomycete pathogen that has a very wide host range and can cause a wide range of disease symptoms including, root rot, collar and crown rot, fruit rot, and stem canker, usually in conjunction with other Phytophthora spp. in its hosts. Phytophthora root and crown rot disease are among the most important soilborne diseases of stone fruits (Brown & Mircetich, 1995). It is widespread in temperate regions of all continents and occurs in soils of natural forests, agricultural fields and orchards. It can persist and spread in different environments and is capable of surviving in the soil as a saprophyte and by producing resting spores.
Phytophthora cactorum is widespread in California and has been found in several counties (see: “California Distribution”). In California, P. cactorum has been found in several hosts: apple, avocado, apricot, American plum, European plum, Japanese plum, Myrobalan plum, sour cherry, sweet cherry, sweet almond, Mabaleb cherry, cherry laurel, peach, nectarine, pear, Southern California walnut, Northern California walnut, English walnut, strawberry, oval kumquat, sweet orange, kiwifruit, peony, rose, rhodendron, tomato, garden rhubarb, lilac, lily, calla lily, Didier’s tulip, tulip, garden snapdragon, western vervain, virbinum, blue blossom ceanothus, million bells, safflower, wild oats, daphne, white fir, Pacific madrone, chamise, manzanita, wild oats, coyote brush, incense cedar, beefwood, deodar cedar, eucalyptus, California buckthorn/coffeeberry, buckthorn, California flannelbush, toyon, common hop, holly, spicebush, carob, savin juniper, juniper, English laurel, redbay, Frasier’s photinia, chokeberry, Ponderosa pine, sticky cinquefoil, Formosa fire thorn, fire thorn, California live oak, valley oak, oak, cork oak, southern live oak, Indian hawthorn, redwood, giant sequoia, yew, and sticky monkey flower (French, 1989, CDFA Pest Damage Records). The pathogen has also been recovered from various habitats including flowing water, stream and ditch banks, residential and public gardens, recreational areas, orchards, forests, and nurseries (Yakabe et al., 2009; CDFA Pest Damage Records).
Hosts: Phytophthora cactorum has a very wide host range of plants belonging to several families including, Aceraceae, Apocynaceae, Apiaceae, Araliaceae, Cactaceae, Cucurbitaceae, Cornaceae, Ebenaceae, Ericaceae, Fagaceae, Geraniaceae, Grossulariaceae, Hippocastanaceae, Juglandaceae, Lauraceae, Liliaceae, Oleaceae, Pinaceae, Proteaceae, Polygonaceae, Rutaceae, Rosaceae, Salicaceae, Solanaceae, Sterculiaceae, and Violaceae (CABI, 2017).
Farr and Rossman (2017) include 1332 records of hosts for Phytophthora cactorum and its synonyms. Hosts include: Abies alba (silver fir), A. amabilis (Pacific silver fir), A. balsamea (balsam fir), A. balsamea var. phanerolepsis, A. concolor (white fir), A. firma (momi fir), A. fraseri (Fraser fir), A. magnifica var. shastensis (Shasta red fir), A. procera (noble fir), Abies sp., Acacia sp. (wattles/acacias), Acer spp. (maples), Actinidia chinensis (kiwi), A. deliciosa (fuzzy kiwifruit), Adenostoma fasciculatum (chamise), Aesculus hippocastanum (horse chestnut), Aesculus sp. (buckeye and horse chestnuts), Agonis flexuosa (Jervis Bay Afterdark), Alnus glutinosa (common alder/black alder), A. incana (grey alder/speckled alder), A. oregana (Oregon alder), Amygdalus persica (peach), Ananas comosus (pineapple), Anemone coronaria (poppy anemone/Spanish marigold), Angelica sp. (angelica), Annona cherimola (cherimoya), Antirrhinum sp., A. majus (snapdragon), Aquilegia sp. (columbine), Aralia cordata (spikenard), A. elata (Japanese angelica-tree), Arbutus menziesii (Pacific madrone/madrone), Arctostaphylos spp. (manzanita), Aster spp. (asters), Aucuba japonica (spotted laurel/Japanese laurel), Avena fatua (common wild oat), Baccharis pilularis (coyote brush), Banksia spp. (banksia), Begonia sp. (begonia), Beta vulgaris var. crassa (beets), Betula lutea (yellow birch), B. pendula (silver birch), Betula sp. (birch), Boehmeria spp. (false nettles), Brassica oleracea var. bullata (Brussel sprouts), Brassica sp. (mustard), Brassolaeliocattleya sp. (orchid), Bryophyllum pinnatum (airplant), Buxus sp. (boxwood), Cactus sp., Calceolaria integrifolia (bush slipperwort), Calceolaria sp. (sweetshrub), Calibrachoa sp. (million bells), Callistephus chinensis (China aster), Calocedrus decurrens (California incense cedar), Calycanthus floridus (eastern sweetshrub), C. occidentalis (spicebush), Calytrix angulata (yellow starflower), Capsicum annuum (cayenne pepper), C. frutescens (chili pepper), Carica papaya (papaya), Carthamus tinctorius (safflower), Carya illinoinensis (pecan), Castanea sativa (sweet chestnut), Castanea sp., Casuarina sp. (beefwood), Catharanthus roseus (Madagascar periwinkle), Ceratonia siliqua (carob), Cereus spp., Cattleya sp. (cattleya orchid), Ceanothus thyrsiflorus (blue blossom ceanothus), Cedrus deodara (deodar cedar), Ceratonia siliqua (carob tree), Chamaecyparis spp. (false cypress), Chrysalidocarpus lutescens (areca palm/butterfly palm), Chrysanthemum spp., Citrullus lanatus (watermelon; syn. C. vulgaris), Citrus aurantium (bitter orange), C. grandis (pomelo; syn. C. maxima), C. limon (lemon), C. limonia (Mandarin lime), C. sinensis (sweet orange), Citrus sp., Clarkia spp., Cleome spp., Cocos nucifera (coconut), Cornus sp. (dogwood), C. sericea (western dogwood), Cucumis melo var. inodorus (Kolkhoznitsa melon), C. melo var. reticulatus (galia melon), C. sativus (cucumber), C. pepo (field pumpkin), Dahlia sp., Daphne cneorum (rose daphne/garden flower), D. mezereum (February daphne), D. odora (winter daphne), Daphne sp., Dendrobium sp. (dendrobium orchid), Dianthus caryophyllus (carnation), Daucus carota (carrot), Diospyros kaki (persimmon), Diplacus aurantiacus (syn. Mimulus aurantiacus, sticky monkeyflower), Eriobotrya japonica (loquat), Echinochloa crus–galli (barnyardgrass), E. eyriesii, Epidendrum spp. (Epidendrum orchids), Erica hyemalis (cape heath), Eucalyptus spp., Fagus sp. (beeches) F. sylvatica (common beech), Fragaria spp. (strawberry), F. ananassa (strawberry), F. chiloensis (Chilean strawberry), F. vesca (wild strawberry), Frangula californica (coffeeberry/California buckthorn), Fraxinus spp., (ash), Fremontia californica (California flannelbush; syn. Fremontodendron californicum (California flannelbush), Fremontia sp., F. mexicanum (Mexican flannelbush), Fortunella margarita (oval kumquat), Galeandra baueri (orchid), Gladiolus sp., Glycine max (soybean), Hesperocyparis macrocarpa (syn. Cupressus macrocarpa, Monterey cypress), Heteromeles arbutifolia (toyon), Hibiscus spp. (rosemallows), Humulus lupulus (common hop), Ilex sp. (holly), Juglans californica (California black walnut), J. hindsii (Northern California walnut/Hinds’ black walnut), J. nigra (black walnut), J. pyriformis, J. regia (English walnut), Juglans sp., Juniperus procera (African juniper), J. sabina (savin juniper), Juglans. sp., Kalanchoe spp., Lactuca sativa (lettuce), Laeliocattleya sp. (orchid), Lilium spp. (lily), Lycopersicon esculentus (tomato; syn. Solanum lycopersicum), Malus domestica (apple), Malus sp., M. sylvestris (European crab apple), Mespilus germanica (medlar), Panax quinquefolius (American ginseng), Pelargonium spp. (pelargonium), Paeonia lactiflora (Chinese peony/common garden peony), Paeonia spp. (peony), Panax spp. (ginseng), Persea americana (avocado), P. borbonia (redbay), Photinia spp. (photonia/chokeberry), Picea spp. (spruce), Pinus spp. (pine), Populus alba (silver-leaf poplar), Potentilla glandulosa (syn. Drymocallis glandulosa, sticky cinquefoil), Prunus armeniaca (apricot/American plum), P. avium (sweet cherry), P. cerasus (sour cherry), P. dulcis (almond; syn. P. amygdalus), P. ilicifolia (hollyleaf cherry/evergreen cherry), P. laurocerasus (cherry laurel/English laurel), P. mahaleb (mahaleb cherry), P. mume (Chinese plum/Japanese apricot), P. persica (peach), P. persica var. nucipersica (nectarine), P. salicina (Japanese plum), Prunus sp., Pyracantha coccinea (scarlet firethorn), , P. koidzumii (Formosa firethorn), Pyracantha sp. (fire thorn), Pyrus communis (European pear), Quercus agrifolia (California live oak/coast live oak), Q. falcata (southern red oak), Q. lobata (valley oak), Q. petraea (durmast oak), Q. robur, (English oak), Quercus sp., Q. suber (cork oak), Q. virginiana (live oak), Rhamnus (Frangula) californica (California coffeeberry), Rhaphiolepis indica (Indian hawthorn), Rheum rhaponticum (false rhubarb), Rheum hybridium (rhubarb), Rhododendron spp., (azalea), Ribes spp., (currants), R. lobbii (Lobbs gooseberry), R. uva-crispa (gooseberry), Rosa sp. (rose), Salix sp. ( willow), Sequoiadendron giganteum (giant sequoia), Solanum (nightshade), S. lycopersicum (tomato), Syringa vulgaris (lilac), Syringa sp., Taxus sp. (yew), Theobroma cacao (cocoa), Tulipa sp. (tulip), Tulipa gesneriana (Didier’s tulip), Viola sp. (violet), Vanda sp. (Vanda orchid), Verbena sp., V. lasiostachys (western vervain), Viburnum spp., Vicia faba (fava bean/broad bean), Vicia sp. (vetch), V. unguiculata, Vigna unguiculata (cowpea; syn. V. sinensis), V. cylindrica (catjang), V. sesquipedalis (yardlong bean), Vitis vinifera (grape ), Zea mays (corn), Zantedeschia sp. (calla lily) (CABI, 2017; Farr & Rossman, 2017; French, 1989; CDFA Pest Damage Records).
Symptoms: Phytophthora cactorum attacks a wide range of host plants causing varied symptoms, depending on the host. Symptoms include root rot, collar and crown rot, fruit rot, stem cankers, leaf blight, wilts and seedling blights. This pathogen can cause pre- and post-emergence damping-off disease in several plant species. It has been reported to reduce sprouting and kill seedlings of beech, and cause seedling blight in Pinus spp., Salix scoulerana, and Robinia spp. (CABI, 2017).
On apple, pear and other woody hosts, P. cactorum causes crown, collar and root rot. Crown rot affects rootstock tissue from the graft union down to the tips of the primary roots, whereas collar rot affects the scion above the graft union or slightly above the soil line. Root rot refers to symptoms that appear beyond the proximal junction of primary roots to crown tissue (Cox, 2014). Above ground symptoms are indicative of an impaired root system and include general stunting with reduced terminal growth and small, chlorotic leaves. Symptom expression depends on the amount of infected crown or root tissue and their rate of destruction. Young trees are usually killed by the pathogen since their root systems and crown regions are not as developed as those of mature trees. Generally, crown rots advance rapidly and trees fall and die soon after the first warm spring. Their leaves wilt, dry, and remain attached to the tree (Adaskaveg et al., 2009; Gubler & Teviotdale, 2009). Trees with root rot slowly decline and eventually die over several seasons. At early stages of tree decline, removing the bark reveals orange to reddish brown necrotic lesions in cambium tissue. A thin, dark delineated margin is evident at the junction of healthy tissue and the expanding lesion which, over time, turns dark brown as it gets colonized by secondary fungi and bacteria. Symptoms can extend through the root system resulting in a lack of fibrous and feeder roots. Crown lesions can extend to the primary roots and up to the graft union, while collar lesions can extend up to a meter up from the graft union. On dissection, collar infections may appear striped in the inner phloem tissue and, sometimes, result in weeping though cracked barked tissue (Cox, 2014). Phytophthora cactorum also causes fruit rot in apple and pear, producing pale olive and dark brown lesions in apple and pear respectively. Those lesions are diffusely marbled or uniformly colored with softly delineated margins (Covey et al., 2014).
In Rhododendrons affected by Phytophthora root rot, roots become necrotic and leaves turn chlorotic, wilt, roll downwards parallel to the midrib, and eventually turn brown. In contrast, leaves of infected azalea become chlorotic, and then necrotic, but seldom wilt. Necrotic leaves eventually drop to the ground (Hoitink et al., 2014).
Infected trees may develop cankers on the stem or near the soil line with discoloration of infected bark, sometimes extending into the internal tissues (CABI, 2017).
Phytophthora cactorum can cause crown rot and root rot of strawberries. Initial symptoms typically include plant stunting and small leaves. Later, infected plants may collapse rapidly or gradually. When cut open, brown discoloration of the crown vascular tissue or entire tissue is apparent. While other Phytophthora species may be involved, P. cactorum is the most common species on strawberry (Koike et al., 2008). Fruit is also infected by P. cactorum resulting in leather rot disease. On green fruit, symptoms appear as dark brown areas or green areas with brown margins. As the rot spreads, the entire fruit turns brown with a rough texture that appears leathery. Infected mature fruit may be slightly discolored or turn brown to dark purple. Internally, vascular tissue to each seed is darkened, and in later stages of decay mature fruit becomes leathery. Infected fruit have unpleasant odor and taste. Under moist conditions, white mycelial growth may be present on the surface of fruit. Green and mature fruit eventually become shriveled mummies (Ellis & Madden, 1998).
Disease development: P. cactorum can survive for several years, mainly as oospores (sexual spores) in soil and mummified fruit. The pathogen can also survive as chlamydospores (thick-walled asexual spores) (Erwin & Ribeiro, 1996) in orchard soil or mycelium in host tissue (Cox, 2014). Similar to other Phytophthora spp., P. cactorum lives as a saprophyte in litter and in soil containing dead organic material and is favored by moist and moderate climates. In spring, and in saturated soil, oospores germinate to produce sporangia. In free water, zoospores are produced within sporangia and liberated into water. While oospores and chlamydospores form the primary inoculum, sporangia are the principal source of secondary inoculum (CABI, 2017). Free water is required for infection, however, a high incidence of disease can occur with as little as 2 hours or less of wetness at 17-25°C. Optimum temperature for infection is 21°C. The most favorable temperatures for sporangia production are between 15 and 25°C, and optimally at 20°C. No sporangia are produced at 10 and 30°C (Ellis & Madden, 1998). Sporangia can germinate directly or indirectly by producing zoospores. Zoospores allow a population to increase rapidly and disperse widely in films of free water. Zoospores are expelled from sporangia under suitable temperature and moisture conditions and swim by means of their flagella towards their host in response to root exudates. Once a zoospore comes in contact with a root it germinates producing a germ tube which penetrates the root directly under waterlogged soil conditions. More mycelium develops and eventually, oospores (sexual spores) are produced and serve as resting structures that can survive for several years. (CABI, 2017).
Transmission: Like most Phytophthora species, P. cactorum is soil-borne and water-borne and may be spread to non-infected sites through infected plants, nursery and planting stock, and seedlings, soil, run-off and splash irrigation and rain water, and contaminated cultivation equipment, tools, and boots. Under high moisture and windy conditions, sporangia may be airborne and important in spread of diseases such as leather rot of strawberry. The pathogen is not seed-borne but can be spread by infected seedlings and through soil or plant debris containing oospores or chlamydospores contaminating seed samples (CABI, 2017). Furthermore, irrigation water from canals, rivers, and ponds can be contaminated with Phytophthora spp. (Brown & Mircetich, 1995).
Damage Potential: Specific crop losses caused by Phytophthora cactorum alone may be difficult to assess as more than one species of Phytophthora may cause diseases with symptoms similar to those caused by P. cactorum and may be present in infected hosts. Nevertheless, P. cactorum is a serious pathogen of a wide range of plant species. Infections of 88-97% apple and pear nursery stock material in commercial nurseries has been reported (Jeffers & Aldwinckle, 1988), and P. cactorum has been frequently detected in several ornamental nurseries within California (Yakabe et al., 2009). Therefore, nurseries may be at risk and need to be monitored for this pathogen to ensure production and planting of disease-free nursery stock.
California’s native vegetation is also at risk of root and crown rot caused by P. cactorum and other Phytophthora spp., many of which are endemic (limited) to California, while some are rare, endangered, or threatened plants, e.g., Ribes spp. (currant/gooseberry), Monterey cypress, and Arctostaphylos spp. (manzanita) (Calflora, 2017; CNPS, 2017). Introduction of Phytophthora species are a threat to plant health in Bay Area restoration sites, where nursery stock is planted for flood control or to mitigate environmental impacts. Detections on madrone, toyon, oaks, sticky monkeyflower, and manzanitas in native stands indicate that P. cactorum is capable of becoming established in a variety of native plant habitats under a range of soil and environmental conditions and can have negative impacts on native vegetation.
Worldwide Distribution: Asia: China, India, Indonesia, Iran, Israel, Japan, Korea DPR, Republic of Korea, Laos, Lebanon, Malaysia, Pakistan, Philippines, Taiwan, Turkey, Vietnam; Africa: Egypt, Kenya, Morocco, Senegal, South Africa, Tunisia, Zimbabwe; North America: Bermuda, Canada, Mexico, USA; South America: Argentina, Brazil, Chile, Colombia, Peru, Uruguay, Venezuela; Europe: Austria, Belgium, Bulgaria, Croatia, Czech Republic, (former) Czechoslovakia, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Lithuania, Netherlands, Norway, Poland, Romania, Russian Federation, Russia (European), Serbia, Slovenia, Spain, Sweden, Switzerland, United Kingdom; Oceania: Australia, New Zealand; Central America and Caribbean: Cuba, El Salvador, Trinidad and Tobago (CABI, 2017; EPPO, 2017).
In the USA, Phytophthora cactorum has been reported from California, Florida, Maine, Michigan, Minnesota, New York, North Carolina, Ohio, South Carolina, Tennessee, Virginia, Washington (CABI, 2017; EPPO, 2017).
Official Control: Presently, Phytophthora cactorum is the “Harmful Organism Lists” for Egypt, French Polynesia, Guatemala, India, Israel, Lebanon, and Nicaragua, while, Phytophthora spp. is on the “Harmful Organism Lists” for Canada, French Polynesia, Mexico, Namibia, Seychelles, South Africa, and the Bolivarian Republic of Venezuela (USDA PCIT, 2017).
California Distribution: Phytophthora cactorum is widely distributed within California. From 2001-July, 2017, the pathogen was detected in Alameda, Butte, Contra Costa, Imperial, Los Angeles, Marin, Merced, Monterey, Placer, Sacramento, San Diego, San Francisco, San Mateo, Santa Barbara, Santa Clara, Santa Cruz, Siskiyou, Solano, Sonoma, and Stanislaus Counties (CDFA Pest Damage Records).
California Interceptions: None reported.
The risk Phytophthora cactorum would pose to California is evaluated below.
Consequences of Introduction:
1) Climate/Host Interaction: Phytophthora cactorum has already established a large distribution under moist and cool to warm climates in California.
Evaluate if the pest would have suitable hosts and climate to establish in California.
– Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
– High (3) likely to establish a widespread distribution in California.
2) Known Pest Host Range: The pathogen has a very wide host range.
Evaluate the host range of the pest.
– Low (1) has a very limited host range.
– Medium (2) has a moderate host range.
– High (3) has a wide host range.
3) Pest Dispersal Potential: Phytophthora cactorum, like other Phytophthora, has high reproductive capability under moist conditions. It is dependent on moisture for spore dissemination and plant infection. It is soilborne and may be spread to non-infected sites through infected plants, nursery and planting stock, and seedlings, soil, run-off and splash irrigation and rain water, and contaminated cultivation equipment, tools, boots, rivers, canals, and ponds. Therefore, it is given a high rating in this category.
Evaluate the natural and artificial dispersal potential of the pest.
– Low (1) does not have high reproductive or dispersal potential.
– Medium (2) has either high reproductive or dispersal potential.
– High (3) has both high reproduction and dispersal potential.
4) Economic Impact: Damage caused by Phytophthora cactorum alone may be difficult to assess as more than one species of Phytophthora may be associated with root and crown rot of host tree. Nevertheless, cactorum is a serious pathogen affecting production of several economically important hosts including, apple, pear, stone fruits, strawberry, ornamentals, and California native plants. Nursery productions could be at risk. Controlling the disease would include soil water management and use of resistant varieties, thereby requiring changes in cultural practices and increase in crop production costs.
Evaluate the economic impact of the pest to California using the criteria below.
Economic Impact: A, B, D, G
A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.
Economic Impact Score: 3
– Low (1) causes 0 or 1 of these impacts.
– Medium (2) causes 2 of these impacts.
– High (3) causes 3 or more of these impacts.
5) Environmental Impact: In conjunction with other Phytophthora, P cactorum may be a contributor to root and crown disease of environmental plants. California’s native vegetation is at risk of root and crown rot damage caused by P. cactorum and other Phytophthora spp. Certain native plants are endemic (limited) to the State, while some are rare, endangered, or threatened. The pathogen is capable of becoming established in a variety of native plant habitats under a range of soil and environmental conditions and can have negative impacts on native vegetation. Its association alone and with other Phytophthora spp. in infected forest and native tree and shrub hosts could result in lowered biodiversity, disrupted natural communities, and critical habitats. Also, it may significantly impact ornamental plantings and home/urban gardening.
Evaluate the environmental impact of the pest on California using the criteria below.
Environmental Impact: A, B, C, E
A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B. The pest could directly affect threatened or endangered species.
C. The pest could impact threatened or endangered species by disrupting critical habitats.
D. The pest could trigger additional official or private treatment programs.
E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.
Environmental Impact Score: 3
– Low (1) causes none of the above to occur.
– Medium (2) causes one of the above to occur.
– High (3) causes two or more of the above to occur.
Consequences of Introduction to California for Phytophthora cactorum:
Add up the total score and include it here. 14
-Low = 5-8 points
-Medium = 9-12 points
–High = 13-15 points
6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.
-Not established (0) Pest never detected in California, or known only from incursions.
-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
–High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.
7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)
Final Score: Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 11
Conclusion and Rating Justification:
Based on the evidence provided above the proposed rating for Phytophthora cactorum is B.
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USDA PCIT. 2017. USDA Phytosanitary Certificate Issuance & Tracking System. Retrieved June 6, 2017. 11:48:29 am CDT. https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp.
Yakabe, L. E., C. L. Blomquist, S. L. Thomas, and J. D. MacDonald. 2009. Identification and frequency of Phytophthora species associated with foliar diseases in California ornamental nurseries. Plant Disease, 93: 883-890.
John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health[@]cdfa.ca.gov.
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Posted by ls