Module 3: Phytophthora ramorum
Module 2: Disease Management
Welcome Module 1: Biology, symptoms, and Diagnosis Conclusion Take the Exam Further Information

What is Phytophthora?

Life Cycle

Disease Cycle

Seasonal Activity

Species Profiles

Virtual Dying Plant

Practice Questions 

Glossary

 

Systematic Approach to Diagnosing Phytophthora Diseases

Leaf roll and wilt of Rhododendron occurs with low temperatues which could be mistaken for Phytophthora disease.
Leaf roll and wilt of Rhododendron occurs with low temperatues which could be mistaken for Phytophthora disease.

Photo by J. Green.

Wilting and shoot dieback are common symptoms with Phytophthora disease as shown by this Rhododendron infected with Phytophthora citricola.
Wilting and shoot dieback are common symptoms of Phytophthora disease as shown by this Rhododendron infected with Phytophthora citricola.

Photo by M. Putnam.

It is difficult to determine whether a plant is infected with Phytophthora. Other factors—such as other plant pathogens, or drought or sunburn—can cause plant damage that looks like Phytophthora symptoms.

It is impossible to tell which species of Phytophthora are present based solely on plant symptoms. A correct diagnosis can be made only with the help of professional plant pathologists and laboratories.

In the field or greenhouse, you can begin the process of diagnosis by:

  • Defining the problem in terms of symptoms and signs
  • Defining the cause of the problem in terms of living or nonliving factors

Symptoms are an indication of disease by reaction of the host, such as changes in the plant’s growth or appearance. For example, the symptom of leaf spots on Rhododendron can be caused by various fungi, sunburn, too much fertilizer—or by Phytophthora.

Signs are an indication of disease from direct observation of a pathogen, insect, or their parts. Signs can include fungal fruiting bodies, bacterial slime, or the presence of insects or mites. Remember that signs of a Phytophthora infection, such as spores, are not visible to the naked eye, but signs of other factors that damage plants often can be seen.

Define the Real Problem
Determine what the plant should look like at this time of year. For example, a typical Rhododendron generally has large, flat, leathery, dark green leaves that are held out from the stem to intercept sunlight.

Next, examine the entire plant and surrounding plants, and determine what parts of the plant are affected.

Describe what the plant actually looks like; use symptoms and signs in your description. For example, leaves may be rolled, drooping down, and appear “off color” or a faded green. Wilting such as this may be due to rotted roots, a girdled trunk, or lack of water. A good examination of the diseased plant will help focus your attention on the most important area(s). Also look for other indications of the cause(s) of the damage; for example, mechanical damage, chemical residues, and records of extreme weather.

Leaf spot symptoms cauzed by Phytophthora ramorum on Rhododendron.
Leaf spot symptoms caused by Phytophthora ramorum on Rhododendron.

Photo by N. Grunwald.
This Rhododendron leaf spot was cauzed by sunburn. While the damage intensified, it never progressed to any other part of the leaf, especially the mid-vein.
This Rhododendron leaf spot was caused by sunburn. While the damage intensified, it never spread to any other part of the leaf, especially the mid-vein.

Photo by R. Regan.
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