Phytophthora cinnamomi

Pathogen profile created by Courtney Reuter
as a requirement for PP 728 – Soilborne plant pathogens

Introduction:
Phytophthora cinnamomi is an oomycete that occurs world-wide. The fungus-like organism causes severe root rot and dieback on Fraser firs, shortleaf and loblolly pines, azaleas, camellia, boxwood, and many other trees and woody ornamentals (5). The disease impacts a range of economic groups including nursery crops and avocado production, managed forests, and Christmas tree farms.

Host Range and Distribution:
Phytophthora cinnamomi is distributed world-wide and causes disease on hundreds of hosts, including azalea, rhododendron, camellia, boxwood, eucalyptus, avocado, pine, juniper, hemlock, spruce, fir, cedar, and cypress.

Isolation:
The fungus can be isolated from the soil and plant material either by plating on a selective medium or baiting. Direct soil plating involves suspending 1 g of infested soil in approximately 25 ml of distilled water. The soil to water ratio may be adjusted depending on the pathogen population levels. The soil-water suspension is transferred onto the surface of PARPH-V8 selective agar plates at a rate of approximately 5 ml of suspension per plate, then allowed to incubate for three days. PARPH-V8 contains 20 g agar, 200 ml filtered V8 broth, 800 ml deionized water, 50 g hymexazol, 5 mg pimiricin, 10 mg rifampicin, 250 mg ampicillin, and 125 mg a.i. pentachloronitrobenzene. After a three day incubation period at room temperature, the residue on the plates is washed off under running tap water with a spatula or finger pads so that the plate can be examined for colonies (4).

Alternative to direct soil plating, various methods have been developed to bait Phytophthora species from the soil, including the use of susceptible plants, leaf pieces, apples, pears, lupine radicals, and pine needles. Phytophthora cinnamomi is most reliably detected from fresh soil samples using camellia leaf pieces as bait. A subsample of the fresh soil is flooded in a deep Petri dish and flooded with 50 ml of distilled water. Five surface disinfested camellia leaf disks are floated in the plate and incubated in the dark for 72 hours. The baits are then removed, blotted on a paper towel, and plated on PARPH-V8 (5).

Identification:
Sporangia (Fig. 1) are ovoid, obpyriform with an apical thickening, tapered or rounded at the base, and terminally borne. Sporangia, which release motile zoospores (Fig. 2), are not readily produced in axenic culture. Chlamydospores (Fig. 3) are produced abundantly axenically and from infected tissue. They are borne from hyphae, and globose with thinner walls. Sizes range from 31 to 50 µm in diameter and are either terminal to intercalary in the mycelium. The fungus is heterothallic, requiring compatible types (A1 and A2) to sporulate sexually. Antheridia are amphigynous, averaging 19x17 µm. Oogonia are round with a tapered base, smooth, hyaline to yellow, with size ranging from 21 to 58 µm. Oospores are hyaline to yellow, and plerotic. Sizes range from 19 to 54 µm (4).

Disease Cycle and Epidemiology:
Chlamydospores, the survival spores of P. cinnamomi, may persist up to six years, and occur in infected roots, crown, and infested soil (2, 4). Host amino acids and root diffusates trigger chlamydospore germination; germination occurs optimally at a soil water matric potential between -5 and -15 mbars (1,6). P. cinnamomi grows and infects optimally at higher temperatures through the months of June and August, but is able to infect at temperatures as low as 15°C (2). Under optimal water and temperature conditions, chlamydospores germinate to produce sporangia, which release infectious zoospores (2). High water tables and excess irrigation provide suitable conditions for increased zoospore inoculum levels and subsequent root infections. Thus, excessive soil water increases the incidence and severity of disease (4). Zoospores are most readily released in soil water matric potentials higher than -5 mbar, or free-standing water. Hence, disease is not as common in sandy well-drained soils (2). Once a host is infected, the water flow through the xylem is reduced via wilt-inducing toxins such as ß-glucans and ß-glucan hydrolases. Unlike healthy plants, those infected with P. cinnamomi do not recover from the stresses of low soil moisture. Excessive use of nitrogen-based fertilizers further increases susceptibility to disease due to the increased uptake of water from the soil matrix (4).

Dispersal occurs via multiple avenues: ground water, streams, and irrigation, as well as infested potting soil, splash from pot-to-pot, infested pot bases on polyethylene, and diseased nursery stock.

Symptoms:
Root infected rhododendrons and azaleas and tree saplings develop above ground leaf chlorosis, necrosis, wilt, leaf curl, and death. Stem necrosis may not occur for many weeks after the development of wilting symptoms. Below-ground symptoms are most severe in poorly drained soils and include necrosis of young feeder roots and the lower vascular tissues around the crown and just below the soil line. Cankers may become visible at the base of 1-2 year old plants. The roots of older plants may recover from disease and may not develop a canker of the base of the stem. Older plants may remain symptomless, or display only mild dieback despite severe root rot (2).

Management:
Control is difficult because of the pathogen’s wide host range and ability to survive in symptomless or tolerant plants. Symptomless plants are a major source of spread to previously clean areas, which is a major problem for field nurseries (3,7). Preventative measures through sanitation are critical. The best field management practice is to prevent the introduction of the organism into the field through the use of clean seed and clean stock as well as utilizing well-drained sandy soils with a low pH (4). Both sporangium and zoospore production are inhibited at a pH of 3.3. However, at a higher pH of 4.0, sporangium production is still inhibited while zoospore production is not. (2). In container-grown plants, a low pH is not feasible because plant growth is limited. However, for cutting propagation, maintaining mist irrigation between 3.5 and 4.0 will control sporangium formation and thus prevent zoospore formation and disease (2). Control in container-grown plants can be achieved by using sterile potting soil, chemicals (Fosetyl Al, metalaxyl, or etridiazol), clean stock, and coarse, sloping gravel beds on which to place pots. Disease severity can be reduced in planted nurseries by planting in raised beds. Raised beds prevent free water from contacting the plant roots and promoting rapid drainage (4). Pre-planting fumigation may be effective, but it does not reach chlamydospores that may be present in the deeper soils (2).

1. Cook, R. J. and R. I. Papendick. 1972. Influence of water potential of soils and plants
on root disease. Annual Review of Phytopathology 10: 349-374.

2. Coyier, D. L. and M. K. Roane. 1986. Compendium of rhododendron and azalea
diseases. APS Press.

3. Diagnostic protocols for regulated pests – Phytophthora cinnamomi. 2004. EPPO Bulletin
34 (2): 201-207.

4. Erwin, D. C. and O. K. Ribiero. 1996. Phytophthora diseases world-wide. American
Phytopathological Society. St Paul. 562p.

5. Ferguson, A. J. and S. N. Jeffers. 1999. Detecting multiple species of Phytophthora in
container mixes from ornamental crop nurseries. Plant Disease 83(12): 1129-1136.

6. Reynolds, K. M., D. M. Benson, and R. I. Bruck. 1985. Epidemiology of Phytophthora
root rot of fraser fir: rhizosphere width and inoculum efficiency. Phytopathology 75: 1010-1014.

7. University of Illinois Integrated Pest Management Reports on Diseases. 1988. Phytophthora
root rot and dieback of rhododendrons and azaleas in the midwest. RPD No. 664.

NCSU Plant Pathology

Phytophthora cinnamomi Rands

	The culture morphology is characterized by a coralloid-type mycelium with prolific hyphal swellings, swollen vesicles, and terminal or lateral clustered protuberances. This organism is readily identified by its colony morphology (4).
Fig. 1. P. cinnamomi culture Dr. L. F. Grand


Wilting and mortality of Fraser fir seedlings, APSnet.org	Vascular discoloration caused by P. cinnamomi, Dr. L. F. Grand