NC STATE UNIVERSITY
College of Agriculture and Life Sciences
Department of Plant Pathology
 

PP728 Pathogen Profile

Phytophthora cactorum

Pathogen Profile created by Cary Rivard

Requirement of PP 728, Soilborne Plant Pathogens

Department of Plant Pathology, North Carolina State University

Introduction

Phytophthora cactorum was first identified on cacti in 1870 by Lebert and Cohn (5). This oomycete is capable of infecting an extremely large number of hosts (2, 5), and is problematic in low-lying or wet field conditions. It can limit production for many economically-important crops such as apple, pear, rhododendron, azalea, and strawberry (1, 5, 6). P. cactorum can cause root, collar, and crown rots, as well as foliar and fruit infections. This report will include information related to the soilborne diseases that it causes.

Host Range and Distribution

P. cactorum can infect more than 200 species in 160 genera including strawberry and a number of woody ornamentals and fruit crops.  This pathogen occurs worldwide, but is most common in temperate regions (2, 5, 9).

Isolation

Phytophthora spp. can be baited from the soil or from water.  Many baiting techniques are not species selective, however, and microscopic methods may be needed to identify species.  P. cactorum can be baited using apple fruits, seedlings or sunflower seedlings (3).  P. cactorum can also be isolated from the root or stem tissue of infected plants and cultured on selective media (3). Several selective media exist for Phytophthora.  All of these utilize pimaricin, which inhibits most nonpythiacious fungi.  A variety of macronutrient sources can be used including corn meal agar, sucrose, or V8© juice (3).

Characteristics and Identification

On potato dextrose agar media, P. cactorum forms a white, loosely matted colony (4).  Sporangia are distinctively papillate, and are usually borne terminally.  Sporangial shapes vary widely, and can range from broadly ellipsoidal, obpyriform, or ovoid to spherical (4, 5).  A distinctive characteristic of this pathogen compared to other Phytophthora in Waterhouse group I is caducous sporangia with pedicels less than 40 mm (5).  Sporangia are often clustered, and sporangiophores are normally simple.  Each sporangium may contain more than 50 zoospores (4).  Chlamydospores can develop in culture on V8 juice-carbonate broth when incubated for 20 days at 4° C, but may not be produced at other temperatures or on other media.  Generally, chlamydospores are terminal, but may occasionally be intercalary.  P. cactorum is homothallic, and oospores can be found as within the tissue of certain host crops and in culture.  All antheridia are paragynous and are generally spherical or club-shaped.  Oogonia are usually hyaline and smooth-walled, and oospores are plerotic (4).

Oospore in tissue

Disease Cycle and Epidemiology

The specific epidemiology of this pathogen may vary with the crop that it infects.  The most important propagule for this pathogen is zoospores, which originate from hyphae or germinating oospores and sporangia (6).  In many cases, this pathogen may enter a field through infected transplants (5, 6, 8).  Infection by P. cactorum usually occurs during warm periods with prolonged wetness.  Motile zoospores are released from sporangia during saturated soil conditions and enter through wounds (8).  Once the zoospore reaches a host, it infects and developing hyphae of the fungus colonize the host.  Development of symptoms may vary depending on host species, but are usually the result of conducive environmental conditions.

Oospores of P. Cactorum

Diseases and Symptoms

Although P. cactorum can parasitize a wide range of plant species, pathogenicity may vary widely across hosts.  This pathogen can cause a number of symptoms and diseases depending on the host.  Symptoms can include: root and collar rots, fruit rots, cankers, leaf blights, wilts, and seedling blights.  Three of the most economically-important pathosystems are discussed below.

P. cactorum attacks the roots and trunk of apple and pear trees.  Although it is rarely a problem in pear, Phytophthora crown, collar, and root rot is a major problem in apple orchards (6).  Crown rot occurs when the infection is below the soil line, and typically affects the rootstock.  Collar rot occurs above the soil line on the lower trunk of the scion.  In both cases foliar symptoms are indicative of root or vascular dysfunction.  Affected trees exhibit poor terminal growth and become stunted (6).  Upon removal of the periderm, the inner phloem tissue is typically necrotic and orange- to red-brown, and may be dark brown during latter stages of infection.  Trees infected with either of these diseases usually decline over several seasons and eventually die (6).

P. cactorum causes crown and root rot in strawberry, and can also cause leather rot of the fruit as well.  Infection of the crown by P. cactorum can cause rot and dysfunction of the vascular system (7). Symptoms typically develop during early-mid summer.  Young leaves turn bluish and will often wilt suddenly.  Wilting will quickly spread throughout the plant, leading to plant death. The crown may break at the upper end, and necrosis occurs throughout.  Brown discoloration of the crown is diagnostic for this disease, and in most cases, symptoms appear first at the upper part and spread downward.(8) 

In Rhododendron, P. cactorum and other species of Phytophthora can cause root rot or shoot dieback.  Root rot can cause the foliage of infected cutting to develop a dull green cast, and wilting may occur.  Symptomatic cuttings typically have reddish brown roots as well (2).  Necrosis occurs in the roots, and leaves become chlorotic.  Leaves may roll downward parallel to the midrib, and will eventually die (1).  The roots of older plants may regenerate, but as the pathogen advances into the stem, browning of the cambium occurs, and the plant eventually dies (1).

Management

In all examples of the diseases caused by P. cactorum, an integrated approach is most effective to reduce damages caused by this pathogen (1, 6, 8).  Prevention and sanitation are extremely important as this pathogen is most-often introduced through infected propagative material (transplants, plugs, cuttings, etc.).  Cultural control may be beneficial as adequate soil drainage and low soil pH may help to reduce disease.  Furthermore, soil moisture extremes can be especially conducive to disease development (1).  Resistant cultivars may be utilized in hosts where it is available.  Fosetyl Al, metalaxyl, and etridiazol are recommended for chemical control (1,5).  Mefenoxam has also been recommended for strawberry, but resistance can be a problem if not implemented into an IPM program (7).  Biological control with either Enterobacter aerogenes or Trichoderma has been successful as well (5).

Management of P. cactorum in strawberry production

Additional Links:

References:

  1. Benson, D.M., H.A.J.Hoitink, and A.F. Schmitthenner.  1986.  Phytophthora Root Rot.  Compendium of Rhododendron and Azalea Diseases.  Eds. D.L. Couyier and M.K. Roane.  APS Press.  St. Paul, MN, USA.
  2. Benson, D.M. and R.K. Jones.  2001.  Diseases of Woody Ornamentals and Trees in nurseries.  APS Press.  St. Paul, MN, USA.
  3. Dhingra, O.D. and J.B. Sinclair.  1995.  Plant Pathology Methods.  2nd Edition.  Lewis Publishers.  Boca Raton. 
  4. Ellis, D.B.  1998.  Leather Rot.  Compendium of Strawberry Diseases.  2nd Edition.  APS Press.  St. Paul, MN, USA.
  5. Erwin, D.C. and O. K. Ribeiro.  1996.  Phytophthora: Diseases Worldwide.  APS Press.  St. Paul, MN, USA. 
  6. Fujita, D.B.  1990.  Crown, collar, and root Rot.  Compendium of apple and pear diseases.  Eds. A.L. Jones and H.S. Aldwinkle.  APS Press.  St. Paul, MN, USA.
  7. Louws, F.J.  2004.  Phytophthora crown rot: widespread in 2004.  Southern Region Small Fruit Consortium: Pest Alert. http://www.smallfruits.org/SRSFC_News/Phytophthora0404.pdf
  8. Seemuller, D.B.  1998.  Crown Rot.  Compendium of Strawberry Diseases.  2nd Edition.  Eds. J.L. Maas.  APS Press.  St. Paul, MN, USA
  9. Sinclair W.A. and H.H. Lyon.  2005.  Diseases of Trees and Shrubs.  2nd Edition.  Cornell University Press.  Ithaca, NY, USA.
All information in this site included for educational purposes. May, 2007
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