EPWS 310 - PLANT PATHOLOGY
Diseases caused
by Basidiomycetes
Rusts, Smuts,
Root rots and Wood rots
Figure 11-129 -
Symptoms
Pages 562-582
Date from Roman
times even had a god "Robigus" who probably stood for warding off
rust outbreaks on grain crops.
All rusts are in
the order Urediniales of the Basidiomycetes.
Extremely
destructive diseases in monoculture.
They attack
leaves and stems and occasionally flowers and fruit.
There are about
4000 species in about 100 genera affecting nearly all plants. There are about
11 genera affecting economic plants.
Puccinia - The infamous cereal
rust also affecting other monocotyledonous (e.g. sugarcane) cotton and various
vegetables.
Gynmnosporangium - Cedar-apple rust.
Hemileia - Historically
significant coffee rust.
Phragmidium - Rose and raspberry
rust
Uromyces - rust of legumes e.g.
beans.
Cronartium - White pine blister
rust and rust of various forest trees.
Melampsora - rust of flax (famous)
Coleosporium - Blister rust of pines
Phakopsora - Rust of soybeans
FIGURE 11- 127:
Fruiting bodies of rusts
Rusts are highly
specialized pathogens like the Fusarium oxysporum and each species and
strain usually attacks only one host and often only one or a few cultivars.
They are divided into f. sp. and races.
Breeding for
resistance is the ONLY economic form of rust control.
Rusts are
biotrophs: i.e. obligate pathogens.
The life cycle
of rusts is unique producing five different spore stages in a definite
sequence. These relate to the generalized life cycle like this:
Figure 11-130 :
Ploidy and life cycle of rusts:
Macrocyclic
rusts are those with all five spore stages and microcylic rusts are those with only
teliospores and basidiospores.
The
overwintering spore is the teliospore which germinates to form basidiospores.
Basidospores germinate to form haploid primary mycelium which produces
spermogonia (gametangia) bearing spermatia and receptive hyphae. The spermatia
are male gametes which can only fertilize receptive hyphae of correct mating
type. They cannot infect plants. This plasmogamy forming dikaryotic mycelium.
The next spore stage is aeciospores in aecia which infect usually the alternate
host to form more dikaryotic mycelium which forms uredia (the summer cycling
spore). The uredia then develop into telia bearing teliospores at the end of
the season.
Rusts may be
heteroecious or autoecious. Alternate hosts.
Spread by wind
and rain: they may be carried literally thousands of miles in jet stream
airflow etc.
The classic
example is stem rust of wheat:
DISEASE CYCLE
Figure 11-134.
Hypertrophy on
barberry plant. Control: resistance and eradication of barberry in wheat
growing areas. Avoid heavy use of N fertilizer and dense planting. Monoculture
is a prime cause of rust outbreaks.
DISEASE CYCLE
Figure 11-142.
Spread from Asia
to Europe to North America. Very serious disease new in New Mexico. Wipe out
white pine very effectively.
Macrocyclic rust
with White pine being the aecial host and Ribes spp. (wild and
cultivated currant and gooseberry bushes) being the telial host.
Can kill pines
in one or a few years depending upon the age of the tree. Ribes spp.
are just defoliated and usually not killed.
The first
symptoms on white pine are spindle shaped swellings and cankers develop forming
a cushion of spermogonia which mature and then a crop of aecia develop which
push through the bark as white sacs contain yellow-orange spores.
The aecoispores
infect Ribes spp. and continue the cycle, but unlike rust of annual
plants canker lesions of this rust can live for several years until the branch
or tree is dead.
Over winters as
mycelia and aeciospores on white pine and as telia on Ribes spp. Note
that only basidiospores from Ribes can infect pine!
Control: Control
Ribes spp. mechanically and with herbicides. The hyperparasite Tuberculina
is also a possibility. Resistance is on its way.