Fungi and Fungi-Like Organisms

The "fungi" are long recognized causes of plant diseases. An argument may be made that they collectively represent the most important class of pathogens based on total numbers. There are estimates of 10,000 fungi cause plant diseases. For years, all fungi were considered part of the Plant Kingdom and frequently referred to as "lower plants". Mycology is the scientific discipline that studies fungi and has its roots in the broader discipline of botany. The "fungi" have undergone a significant reclassification since the early 1990's. A major driving force was the application of techniques of molecular genetics to study genetic relatedness among the phenotypically diverse organisms that composed the "fungi". The "fungi" have distributed to new Kingdoms.

Agrios (pp. 248-254) presents a detailed account of the new classification system. An attempt has been made in this study guide to summarize the account of Agrios into a summary that can be used by the introductory student and more germane to the goals of this course.

Characteristics of Plant Pathogenic Oomycetes - Agrios pp. 266-270

Morphology	Eukaryotic microbes that form filamentous structures called hyphae. In mass, the hyphae are referred to as a mycelium. Oomycetes reproduce sexually by oospores and asexually by zoospores formed in sporangia. Zoospores are motile one celled spores that are a product of mitosis and are analogous to conidia formed by fungi in form and function. Reproductive structures the basis of taxonomic classification of Oomycetes.
Reproduction	Other specialized asexual reproductive structures are formed. Oomycetes reside in spaces between plant cells or in plant cells.
Sites of pathogen survival	Dormant survival propagules form in colonized host debris, soil, vectors, seeds and as active pathogens in living plants.
Symptoms caused	Leafspots, blights, vascular wilts, root rots, cankers.
Signs of disease	Signs are reproductive structures of the pathogen.

Basic Components of Fungi and Fungi-like Organisms that are Potential Plant Pathogens

Vegetative Structures	- Filamentous hyphae with or without cell walls.
	- Nutrient and water uptake
	- Hyphae aggregate into mycelium
Reproductive Structures	- Hyphae specialize into spore bearing structures
	- spores are product of meiosis (sexual) or mitosis (asexual)
	- spores formed on hyphae or spore bearing structures (sporocarps)
	-mycelium may form specialized asexual structures

Classification of Fungi and Fungi-like Organisms that are Potential Plant Pathogens

Kingdom - Fungi

Members of kingdom form vegetative (mycelium and hyphae) and reproductive structures composed of chitin and glucans, but lack chlorophyl.

Classes with Kingdom of Fungi with relevance to plant diseases

1) Chytridiomycetes: vegetative structures are round or elongate mycelium and form zoospores with one flagellum.

2) Zygomycetes: vegetative structures are filamentous hyphae and reproduce by spores. Few are plant pathogens.

3) Ascomycetes: most form filamentous vegetative structures, but one-celled forms are found in the class. All form sexually derived (product of meiosis) ascospores and usually asexually derived (mitosis) spores usually referred to as conidia. Ascospores are usually formed within sporocarps, and provide the basis for further taxonomic classification.

4) Basidiomycetes: most form filamentous vegetative structures, but one-celled forms are found in the class. All form sexually derived (product of meiosis) basidiospores. Asexually derived (mitosis) spores are less common compared to Ascomycetes. Basidiospores are commonly formed within sporocarps, and provide the basis for further taxonomic classification. Some members of the Basidiomycetes have complex life cycles involving multiple spore stages of differing function.

5) Deuteromycetes (Fungi Imperfecti): most form filamentous vegetative structures, but one-celled forms are found in the class. All spores are asexually derived (mitosis) spores usually referred to as conidia. Conidia often are borne on hyphae but many form conidia sporocarps, and provide the basis for further taxonomic classification. There is a strong linkage between the Deuteromycetes and Ascomycetes and is based on spore and sporocarp structures present at a specific point in time. A specific species of fungus may be classified in both classes. This concept will be explored in more detail as specific diseases and their causes are discussed.

6) Sterile fungi: reproduce only by filamentous hyphae and mycellium. Spores of all types are absent.

Kingdom - Protozoa

Classes within Kingdom of Fungi with relevance to plant diseases

1) Myxomycetes (slime molds):

2) Plasmodiophoromycetes: non-filamentous vegetative structures that form in plant cells. Spores are formed within cells and give rise to zoospores with two flagella.

Kingdom - Chromista (Protista)

Class within Kingdom of Chromista (Protista) with relevance to plant diseases

1) Oomycetes (water molds): form filamentous vegetative structures, sexually derived (product of meiosis) oospores and usually asexually derived (mitosis) spores usually referred to as zoospores. Oospores are formed within oogonia and provide the basis for further taxonomic classification.

Oomycota

Characteristics of Pathogens and Diseases They Cause

Growth and reproductive structures

Asexual: coenocytic hyphae, sporangiophores, sporangia and zoospores

Sexual: Oogonia, antheridia and oospores

Hyphae:

Sporocarp:

Spore types and related structures

Genetics:

Common types of diseases caused by the oomycota

Seed rot, seedling blights (damping-off)

Root and stem rots

Leaf and stem blights, fruit and tuber rots

Downy mildew and white rust

Late Blight of Potato and Tomato

Disease Cycle and Epidemiology

Disease: Late blight

Pathogen: Phytophthora infestans

History:

Host Range: Potato and tomato

Sources of Inoculum:

Primary Inoculum:

Dissemination:

Environmental Factors:

Host Factors:

Symptoms:

Control:

Comments:

Review of specific diseases and pathogens

Seed rot, seedling blights (damping-off)

Pathogens: Pythium and Phytophthora species

Root and stem rots

Pythium: many hosts

Aphanomyces euteiches: legumes

Phytophthora medicaginis: alfalfa

Phytophthora sojae: soybean

Phytophthora cactorum: ginseng, woody ornamental, apples, shade trees

Leaf and stem blights and fruit (tuber) rots

Phytophthora infestans (late blight): potato and tomato

Phytophthora capici: peppers

Pythium: turf

Review of specific diseases and pathogens continued

Downy mildews and white rusts

Plasmopara viticola: grape

Peronospora: several species causing downy mildew of snapdragon, onion, pea, soybean, alfalfa, and tobacco

Bremia lactuae: lettuce

Albugo: white rust of crucifers