EPWS 310 - PLANT PATHOLOGY
FUNGI LECTURE 5:
Diseases caused by fungallike organisms and the
lower Fungi
Contents
of Lecutre
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.
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.