EPWS 310 - PLANT PATHOLOGY

FUNGI LECTURE 5: Diseases caused by fungallike organisms and the lower Fungi

Contents of Lecutre

A. Myxomycetes

B. Chytridiomycetes - Chyritridiales

C. Plasmodiophorales

Readings Page 404-409, 433

A. Specific Diseases: Myxomycetes - Physarales

Not serious pathogens...cause plant damage only by growing over low lying plants such as turf, strawberries, vegetable and ornamentals.

Common after rain in warm weather.

Look like the BLOB and most common cause of panic calls to extension people. Often brightly colored plasmodium is yellow or orange and moves like a slug.

The plasmodium is a mass of membrane bound protoplasm: it is coenocytic and contain many nuclei.

LIFE CYCLE Fig. 11-9

They have a cell wall on resting spores. Spores germinate to form zoospores and then the myxamoebae which fuse to form the plasmodium. This creeps around and then fruits forming sporangia containing lots of walled resting spores.

No control is usually required other than hosing, mowing or raking. They are sensitive to fungicides if problem is serious.

B. Chytridiomycetes - Chyritridiales

Favored by Wet conditions

see Page 433

Only four plant pathogenic/parasitic genera in this order:

They are: Olpidium, Physoderma, Synchytrium, Urophlyctis

These are very simple fungi with only a small simple thallus, they survive as resting spores in the soil and produce motile zoospores. Secondary disease cycles by zoospores. These are obligate parasites/pathogens.

The diseases caused by these fungi are locally serious and usually root rots or diseases below the soil line.

Olpidium - Root PARASITE of many kinds of plants including fields crops and vegetables. Transmits at least six viruses including tobacco necrosis v. and lettuce big vein v. (see around here).

Physoderma - Brown spot of maize first found in India, locally important in parts of south east USA. Only disease of above ground parts.

Urophlyctis - Crown wart of Alfalfa, first found in South America now widespread in western USA, not serious.

Synchytrium endobioticum - Black Wart of Potato - serious in Europe but after early occurrence in USA quarantine and certified seed pieces has kept it to very low levels in USA.

Control is by quarantine, seed certification and resistance.

C. Plasmodiophorales

Three serious pathogens in this group:

Plasmodiophora brassicae - clubroot of crucifers

Polymyxa graminis - root rot of cereals, virus vector

Spongospora subterranea powdery scab of potato, virus vector

LIFE CYCLE Figure 11-13 Plasmodiophora brassicae

These fungi overwinter as resting spores and germinate with one zoospore which infect a root hair and forms a plasmodium which develops into a zoosporangium and releases lots of zoospores as secondary inoculum and fuse to dikaryotic zygote. Infection continues until the plasmodium develops into resting spores (within cells) which are released into the soil.

These pathogens are obligate pathogens (biotrophs) but can reside in the soil for many years.

Cell enlargement appears to be a way to increase nutrient supply for the pathogens... the plasmodium does NOT kill the host cells!

In Plasmodiophora diseases there is a wilt and chlorosis, indication of a dysfunctional root system (very common symptoms for many root diseases). Recover at night. Young plant may be killed. Typical knobby club like roots appear after several weeks and roots collapse and die. Not always the whole root. Infection is direct and through wounds, only about 30% of the diseased tissue actually contains plasmodia.

Polymyxa graminis vector of soilborne wheat mosaic virus, barley yellow mosaic, oat mosaic, wheat spindle steak and peanut clump virus.

 

Spongospora subterranea is a vector for the potato mop-top virus

CONTROL - Control of these diseases depends on hygiene, use of clean planting material avoiding contaminated soil with susceptible crops which is possible because these fungi have limited host ranges. Favored by wet conditions, provide good drainage, keep pH above 7.0 (around 7.2) because spores do not germinate in alkaline soil. Fumigation is possible but rarely economic. Some resistance is available.

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