EPWS 310 - PLANT PATHOLOGY
LECTURE 14: ASCOMYCETES Vascular wilts
Fusarium, Verticillium
and Ceratocystis
Pages 522-534
VASCULAR
WILTS
These are extremely
serious diseases from the catastrophic Dutch elm disease to the garden variety Fusarium
and Verticillium wilts.
Vascular wilts
have many characteristics in common and can often be confused, there are only
five species responsible for all of this damage.
Fusarium
oxysporum
- very wide host range
Verticillium
dahliae
- very wide host range
Verticillium
albo-atrum
- very wide host range
Ophiostoma
ulmi -
Elm tree wilt
Ceratocystis
fagacearum
- Oak wilt
All wilt
diseases caused by Fusarium and Verticillium have very
similar symptoms. Initially a wilt of the herbaceous plant parts often
recovering overnight in the very early stages but quickly becoming a permanent
wilt leading to necrosis of the herbaceous parts and plant death.
The pathogens
enter into the xylem vessels and may move up the xylem by hyphal extension or
by microconidia. Toxins are involved in many diseases leading to symptoms in
advance of the pathogen and foliar symptoms such a vein chlorosis and general
cholorosis. Tyloses form and excessive xylem parenchyma proliferation occurs
all to block the water flow.
Leaf epinasty
and defoliation may also occur.
Diseases are
made far worse in the presence of root invading nematodes such as Meloidogyne.
Fusarium is a soil inhabitant and
Verticillium is a soil invader. They have very similar life cycles and
neither have a sexual stage (Teleomorph).
However both are
able to survive in soil for very long periods because they have specialized
structures for survival.
Fusarium
oxysporum
has chlamydospores:
Verticillium
dahliae
has microsclerotia:
Verticillium
albo-atrum has
thickened hyphae:
These are very
common structures in many soil fungi for survival.
LIFE CYCLE of
F.o. lycopersici FIG 11-105
Fusarium
oxysporum
occurs in about 300 formae specialis affecting hundreds of plants. These may be
further divided into races. All defined entirely by host genetics.
Worldwide fungus
- everywhere and most common as a saprophyte.
Verticillium these fungi are also
widespread but do not fall into pathological forms or races very neatly. They
tend to cause wilt at lower temperatures than Fusarium but there are
exceptions!
Verticillium is also more likely to
cause defoliation than Fusarium.
V. dahliae prefers higher
temperature than V. albo-atrum and is common in warmer areas. Serious
here on cotton and chile.
Control ??
Ophiostoma
ulmi follows a similar pattern.
LIFE CYCLE
FIGURE 11-109
Associated with
bark beetles, spreads through natural root grafts.
Control??