Puccinia (genus)

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stripe rust, caused by Puccinia striiformis, on wheat
Source: PD-USGOV-USDA-ARS - Wikimedia Commons

Puccinia P. Micheli (1729)

Puccinia is a very import genus of rust fungi and species of this genus often cause severe losses to crops, in particular to cereals and other gramineous crops. The spores spread through the air and infect the above-ground parts of the host plant. Often an obligatory passage through a secondary host is required to complete the life cycle (heteroecious species). Control typically involves the use of resistant cultivars or the elimination of the alternate host. For species with an obligatory passage through an alternate host, the absence of this secondary host can break the cycle and stop transmission of the pathogen. However, alternate hosts and crops would need to be separated by many kilometers to make transmission by the airborne spores impossible. In addition, the crop may be present all year round, for example as volunteer wheat or winter wheat. This allows the disease to persist in the absence of the secondary host.

Puccinia psidii - scanning EM of urediniospore (click on image to enlarge it)
Authors: J.R. Liberato, S.F. Silveira, D.T. Junghans, J.A. Rocabado, C.C. Aparecido and R.G. Shivas PaDIL
Source: PaDIL

Like other rust fungi, species of Puccinia often have a number of different stages and spore forms, depending on the species, the environment and the season. In general, the fungus forms asexual urediniospores on the crop during the summer which disperse and can infect neighbouring plants of the same crop. The surface of urediniospores often has characteristic projections or spines (echinulate). Near the end of the season, thick-walled, asexual teliospores are normally produced which persist on plant debris during the winter. During the spring, the teliospores germinate and form haploid basidiospores. The basidiospores disperse again and infect the secondary or alternate host. For example, several species of Puccinia have barberry (Berberis) as secondary hosts.

Life cycle of stem rust
Source: USDA-ARS - Wikimedia Commons

On the alternate host, the fungus forms small structures on the leaf surface, called pycnia. These pcynia develop pycniospores and receptive hyphae, both are haploid. The pcynia also produce a sticky honeydew which attracts insects and promotes the dispersal of pycniospores. If the pycniospores reach receptive hyphae of another picnium, sexual reproduction is initiated, provided spores and hyphae are sexually compatible. Rust fungi have plus and minus mating strains and opposite strains are sexually compatible. This means the pycniospore from a plus strain has to land on and fuse with the receptive hypha of a minus strain or vice versa. If sexual reproduction is successfully initiated, another structure called an aecium is formed, typically on the other leaf surface of the secondary host. The aecia produce aeciospores which again disperse by wind and infect the primary host.

Those species of Puccinia which follow the above life cycle with 5 spore types are termed macrocyclic. However, some species of Puccinia are demicyclic and do not form urediniospores. Still others are microcyclic. These do not have an alternate host (autoecious species) and, therefore, lack both pycniospores and aeciospores in their life cycle.

Puccinia species infect a large variety of plants, including weeds (see Puccinia as weed pathogens). However, the species causing rusts on cereals are economically most important. These include:

For other species see the list below.

Type species: Puccinia graminis

For a taxonomic review of the genus see Marin-Felix et al. (2017) which contains additional illustrations.

The literature database currently contains 2197 publications for Puccinia (genus). This includes articles on the genus as well as the different species (see below) which, however, may be incomplete. See the box above on the left for queries from the literature database.

Currently, the following species of Puccinia have been entered into the system: