J. S. Boyce; Principles of Plant Pathology, Forest Science, Volume 4, Issue 4, 1 December , Pages , This book is both a forestry and a plant pathology, this reference deals with the study of the problems and damage to forests due to plant diseases, insects, fire. Principles of Forest Pathology. This book focuses on the practical aspects of forest diseases and on practical measures to minimize damage and loss. Forest Pathology is a reference book that deals with the study of the problems and damage to forests due to: plant diseases, insects, fire, weather, and animals.


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There are a large number of guiding principles in Plant Pathology, which are often difficult to formulate because biology has so few absolutes. Here are 10 principles, which may aid in understanding, diagnosing and controlling diseases of ornamental plants.

Principle I - Disease is a malfunctioning of a plant, which results from a continuous irritant by a pathogenic agent. This definition of plant disease includes organismal causal agents which attack plants such as fungi, bacteria, mycoplasma, viruses, nematodes and parasitic plants.

However, for ornamental plants abiotic diseases far outweigh the traditional causes of plant diseases. Principles of forest pathology ornamental plants the Plant Pathologist must first examine such abiotic diseases as: Diseases and the plant organs they effect. How the disease affects a plant and what organs it causes to malfunction can aid in the diagnoses and control of plant diseases Fig.

A common mistake made by homeowners is to assume that dying leaves are caused by leaf diseases. These two types of disease are often difficult to separate simply by viewing the gross symptoms of the plant.


The plant stem must be severed to look for telltale stains in the stem caused by wilt diseases and the roots must be examined for symptoms of root decay. Cankers are a fourth type of disease which are usually characterized by conspicuous lesions on principles of forest pathology stem or branches.

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This type of disease does not interfere with the water movement in a plant but with the movement of sugars and other substances in the phloem. With this rudimentary knowledge of the disease it is possible to narrow your diagnosis to the plant organs which are affected.

A search of these organs for further symptoms or signs of disease will usually result principles of forest pathology a proper disease diagnosis. However, isolation or identification of biotic pathogens is required for a final diagnosis.

Principles of forest pathology.

Principle II - Disease results from an interaction of the virulence of the pathogen, susceptibility of the host, and the conduciveness of the environment. The concept that plant disease is not caused by a single organism or disease agent is overlooked by most people.

Plant disease is instead caused by a combination of three factors: The disease triangle shows visually that a very weak and inefficient pathogen could cause substantial disease if the environment is conducive or if the host is extremely susceptible. Disease triangle Principle III - Conditions which favor plant growth and health commonly favor disease.

One of the most common misconceptions among horticulturists and the public is that healthy plants are somehow more resistant to plant disease and so abundant water and fertilizer are often treated as pesticides to insure plant health.

Principles of forest pathology this may be true for weak or opportunistic plant pathogens it is not true for most competent pathogens.

For example, high levels of nitrogen fertilizers, which result in rapid growth of plants, is often prescribed to improve plant health.

Principles of Forest Pathology | Forestry | Agriculture | Subjects | Wiley

Principles of forest pathology only does this commonly result in ammonia toxicity, but high levels of nitrogen are known to exacerbate disease by many pathogens such as Rhizoctonia, Pythium, Phytophthora, Fusarium, Armillaria, Sclerotium, Pseudomonas, Corynebacterium, powdery mildews, rusts, cyst nematodes and many others.

Principle IV - Overwatering and underwatering plants both exacerbate disease.

Perhaps the most common plant problems, for most home owners and for ornamental plants principles of forest pathology general, results from improper watering.

Plants damaged by growing in water saturated soil can often be diagnosed by the foul hydrogen cyanide odor of the soil.

Principles of Forest Pathology

Plants which are chronically drought stressed usually have leaves which have brown necrotic principles of forest pathology or edges. However, most people are not aware that overwatering or underwatering can predispose plants to disease.

Predisposition is the environmental modification of plant resistance making the plant more susceptible to disease. Exposing roots to saturated water conditions for as little as 18 hours damages the sensitive root membranes and results in leakage of nutrients from the root cells.

This is called root exudation and many pathogens are attracted to root exudates and will preferentially attack roots which are leaking exudates. Overwatered plants often become more susceptible to root rot fungi such as Phytophthora and Pythium.