EPWS 310 - PLANT PATHOLOGY
Diseases caused by Basidiomycetes
Rusts, Smuts, Root rots and Wood rots
Figure 11-129 - Symptoms
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.