Phytophthora species (Root and crown rots of woody plants)

Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Pythiaceae

Scientific Name

Phytophthora

Common Name

Phytophthora root and crown rots

Author: Molly Giesbrecht, Texas A&M, Garrett Ridge, North Carolina State University

Pathogen

Phytophthora (from Greek phytón and phthorá; “plant-destroyer”) is a genus in the Phylum Oomycota, a fungus-like lineage of microscopic eukaryotes in the Kingdom Chromista , sometimes called the Kingdom Stramenopila. Many species of Phytophthora are soil- and water-borne plant pathogens capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. Phytophthora species cause various diseases on a wide range of hosts, but discussion here will focus on root and crown rots of woody plants.

Members within the Oomycetes, such as Phytophthora, resemble fungi morphologically with the production of filamentous hyphae and both sexual and asexual spores. Oomycetes, however, have cell walls made of cellulose and β-glucans, are diploid in their vegetative state, and at some point in their life cycle, produce heterokont zoospores with two unequal flagella.

Species of Phytophthora undergo sexual reproduction by the fusion of an antheridium and oogonium, resulting in the formation of a thick-walled oospore (Figures 1-2). Phytophthora species can also form multiple asexual structures: chlamydospores, sporangia, and zoospores. Chlamydospores are thick-walled survival structures formed by the pathogen under environmental stress and allow species of Phytophthora to survive adverse conditions in soil or plant material, making them difficult to eradicate. Sporangia are propagules that can germinate directly (producing mycelium) or indirectly by producing numerous zoospores that are motile in water (Figure 3). Zoospores are kidney shaped and typically around 10 μm in diameter (Figure 4). Each zoospore has two flagella growing from the concave (ventral) side that are used for motility in water. Zoospores can swim for hours and eventually encyst after agitation, contact with hard surfaces, or loss of energy reserves. The encysted zoosore then germinates producing hyphae if it is in contact with a suitable substrate.

Renewed interest in the genus Phytophthora over the past decade has led to the near doubling in the number of described species in this genus. Currently, more than 100 species are recognized and additional new species continue to be discovered.

Hosts and Pathogenicity

Phytophthora species parasitize a wide range of agricultural, ornamental, and forest plants and are ubiquitous in soil and water throughout the world. The most notorious species of Phytophthora is P. infestans, the pathogen responsible for potato late blight and the resulting Irish potato famine in 1845; a tragedy of war-like proportions in which Ireland lost 25% of its 8 million inhabitants to starvation and emigration. While the host range of P. infestans is limited to solanaceous crops and many Phytophthora species are relatively host-specific, other species have wide host ranges. Phytophthora cinnamomi is reported to parasitize more than 900 species of woody and herbaceous plants and P. nicotianae is reported to attack more than 250 species. Many forest trees, ornamental trees, and woody landscape plants are capable of being infected by at least one, if not several, species of Phytophthora.

Some of the most notable Phytophthora species associated with root and crown rots on woody plants include:

• Phytophthora alni—causes root and collar rot in alders
• Phytophthora cactorum—causes rhododendron root rot affecting rhododendrons, azaleas and related species in the genus Rhododendron. It also causes bleeding canker in hardwood trees and affects economically-important crops such as apple, pear, and strawberry.

• Phytophthora cinnamomi—causes root rot affecting woody ornamentals including arborvitae, azalea, Chamaecyparis, dogwood, forsythia, Fraser fir, hemlock, Japanese holly, juniper, Pieris, Rhododendron, Taxus, white pine, American chestnut and Eucalyptus (jarrah).

• Phyophthora cambivora—pathogen of European chestnut trees, beech, and some ornamental shrubs

• Phyophthora citricola—causes cankers and crown rot of avocado and root rot of conifers.

• Phytophthora citrophthora—causes root rot of citrus, pistachio, and peach as well as some ornamental species.

• Phytophthora cryptogea—infects numerous ornamental species.

• Phytophthora drechsleri— infects numerous ornamental species.

• Phytophthora gonapodyides—pathogen of European forest trees

• Phytophthora kernoviae—pathogen of beech and rhododendron, also occurring on other trees and shrub species including oaks, and holm oak. Distribution is confined to the UK.

• Phytophthora megakarya—one of the species causing black pod of cacoa, and is probably responsible for the greatest cocoa crop loss in Africa.

• Phytophthora nicotianae—causes disease in tobacco, onions, cotton, some ornamental species, and a number of tropical fruit crops (e.g. coconut and pineapple).

• Phytophthora palmivora—causes fruit rot in coconuts and betel nuts and disease in many palm species, and root, stem, and fruit rot in papaya (Carica papaya).

• Phytophthora ramorum—infects over 60 plant genera and over 100 host species; causes sudden oak death in several tree species, and ramorum blight in many ornamental and shrub species.

Signs and Symptoms

Typically, the first symptoms observed on plants infected by Phytophthora root and crown rots are above ground, in the form of foliar chlorosis or necrosis or twig/branch dieback (Figures 5-8). Beneath the ground, the outer bark of the roots and crown can be scraped away to observe whether the inner bark or outer xylem is necrotic. The outer tissues of the root will easily slough off and there may be a noticable lack of secondary and tertiary roots. Root and crown tissues infected with Phytophthora will be necrotic and discolored gray, dark brown, or a distinct reddish brown, often delimited by a well-defined margin adjacent to a healthy tissue area (Figure 9). Pathogen structures are difficult to visualize without a microscope and therefore are not likely to be observed in the field.

Ecology

Phytophthora species are hemibiotrophic plant pathogens, having an initial biotrophic stage in which they feed on living host cells followed by a nectrotrophic phase when the pathogen kills the host cells to absorb nutrients from the dead tissue. This pathogen can persist for long periods in soil, mainly as oospores or chlamydospores, or as mycelium within infected plant tissue. When soil and plant tissue are wet, sporangia are produced, either as the result of germinating resting spores or as direct outgrowths of the active fungus within infected roots and crowns. These sporangia are filled with numerous zoospores, which are expelled into the soil during saturated conditions. Zoospores exhibit negative geotropism (movement toward the surface of the water) and are chemically attracted to root exudates. Inoculum is spread by rain splash and through runoff water.

Species of Phytophthora are either homothallic and are capable of sexual reproduction in single-isolate culture or heterothallic, requiring the presence of two isolates with opposite mating types (A1 and A2). Phytophthora species encompass a wide range of lifestyles, from strict soil inhabitants, to soil inhabitants that also infect above- or below-ground tissue, to those that have a strictly aerial existence. Those that cause root and crown rots are typically strict soil inhabitants. In woody plants, the pathogen usually persists in root tissues throughout the year. Disease spread is generally favored by mild temperatures (20-32°C) and wet conditions, but optimum conditions differ somewhat between species. Several cycles of asexual spore production can occur within a season, which can lead to the rapid spread of disease when environmental conditions are favorable.

Geographic Distribution

Some species of Phytophthora are global and ubiquitous whereas others are much more restricted in their distribution. The global movement and cultivation of plants and plant materials plays an important role in long distance dispersal, and has resulted in the discovery of several new Phytophthora species. This kind of spread is facilitated by the fact that, in many cases, the organisms are benign in their native environments, allowing their existence to go unrecognized until they are displaced to an environment in which they are not native or a non-native plant is grown in their native environment and they begin causing disease. This type of pathogen introduction has occurred time and time again. Examples involving species of Phytophthora include the first report of P. ramorum in the U.K. and the introduction of P. infestans to Europe.

Management

Cultural

Preventative management strategies are important in controlling Phytophthora diseases as there is often little that can be done after symptoms are observed. Scouting for symptoms below ground before above ground symptoms are evident can assist with earlier disease detection, in some cases leading to greater success when implementing subsequent management strategies. However, infected plants can also remain asymptomatic for weeks or months. Disease may remain difficult to control when a large portion of the root system is infected. Moisture levels can be regulated through alteration of watering practices, planting in raised beds, and choosing soil and a site which will permit adequate drainage. Planting disease-free plants, solarizing soil prior to planting, fertilizing properly, limiting soil movement, and ensuring proper planting depth, are other important cultural management strategies for Phytophthora root and crown diseases.

Host resistance to Phytophthora infection has been achieved at various levels in many ornamental landscape plant species through the use of breeding and genetic manipulation techniques and has been an important tool for managing these diseases. In particular, there are Phytophthora-resistant varieties of rhododendron, azalea, camellia, and others available.

Chemical

Several fungicides exist which can help to effectively control Phytophthora root and crown rot diseases; some of the most widely used and effective being azole, phenylamide, and phosphonate fungicides. Fungal resistance has developed in some of these fungicides after repeated treatments with the same product or products within the same chemical (FRAC) group, so it is always important to follow label instructions and rotate between chemical groups. Metham sodium is an effective fumigant, capable of dramatically reducing pathogen populations in the soil prior to planting. More general biocides such as sodium hypochlorite can be used to treat containers, surfaces, and equipment prior to planting and propagating to help prevent infection.

Diagnostic procedures

Diagnosis of Phytophthora root rot begins with observation of symptoms and environmental conditions which are conducive to disease in the field. Necrotic root and crown tissue can be brought back to the lab and diagnosed by various methods. Microscopic observation of tissue sections may reveal the presence of oospores and coenocytic hyphae (Figure 10). Diagnosis to the species level will require culturing of the organism or application of serological or molecular methods. Selective media, which inhibits the growth of bacteria and other fungi, in some cases including Pythium spp., is often utilized to assist with the isolation. Species of Phytophthora can be detected by plating diseased tissue onto a selective medium to allow hyphae to grow from the infected plant material when plates are stored in the dark at 20°C (Figure 11). The most commonly used selective media have corn meal or V8-juice bases and incorporate ingredients like pimaricin, ampicillin, rifampicin, and PCNB. Hymexazol may be added for better inhibition of Pythium species, but it also is restrictive to the growth of many species of Phytophthora.

Another method which can be useful in the isolation of Phytophthora, particularly when extensive decay and deterioration of tissues has occurred, is baiting of soil from diseased plants with pears and other fruits or various leaf tissues which are known to support the growth of a wide range of Phytophthora species. Baiting involves the use of plant material to recover Phytophthora spp. from soil or water. Zoospores are attracted to leaves and other common baits. While baiting is not quantitative it is a sensitive method of detection because it samples a large volume of soil or water. Rhododendron and camellia leaves or leaf discs commonly are used in this process. The bait tissue is floated on the surface of the water or water-soil suspension in the laboratory for several days. Tissue which develops lesions is subsequently plated onto Phytophthora-selective media for growth and species determination (Figure 12).

Identification of Phytophthora species has traditionally been accomplished using morphological and cultural characteristics. The features of the sporangium, such as shape and the type of a papilla, are important in identification. Sporangia may be classified as papillate, semi-papillate, or non-papillate (Figures 13-14). The caducous or deciduous nature of sporangia and subsequent length of the subtending pedicel are key features of some species of Phytophthora. The homothallic or heterothallic nature and the resultant production or lack of production of oospores is often an important identifying attribute. Oospore morphology can also be a telling feature for some species.

Many species of Phytophthora require flooding with soil extract or distilled water under light to induce sporangium formation. Culture plates can be flooded directly or agar plugs from the colony margin can be taken and flooded within a smaller (60 mm diameter) petri plate. If no sporangia form within 24 hours, a brief chilling period followed by the return to room temperature may induce formation.

ELISA assays are a rapid and sensitive method of detecting Phytophthora in symptomatic tissues but they are only able to detect to the genus level and they become more likely to fail to detect the pathogen the longer the plant tissues have been dead. ELISA based tests for the genus Phytophthora are available in microwell and solid state format from Agdia and Adgen. ELISA tests may cross-react with some Pythium species and cause false positive results.

Molecular techniques have become an effective means for identifying species of Phytophthora. These DNA‐based techniques separate and identify morphologically similar species of Phytophthora. The internal transcribed spacer regions (ITS) of rDNA and the mitochondrion-encoded cytochrome oxidase (cox) I and II genes can be amplified by PCR, purified and sequenced. The resulting sequence can then be compared and matched with nucleotide sequences of identified isolates deposited in online databases such as Phytophthora-id and Phytophthora-db.

Resources and References

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Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)