''Gymnosporangium sabinae'' (also ''Gymnosporangium fuscum'' and other names) (pear rust, European pear rust, or pear trellis rust) is a

fungal A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

heteroecious A heteroecious parasite is one that requires at least two hosts. The ''primary host'' is the host in which the parasite spends its adult life; the other is the ''secondary host''. Both hosts are required for the parasite to complete its life cycle ...

plant pathogen Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...

with ''

Juniperus Junipers are coniferous trees and shrubs in the genus ''Juniperus'' () of the cypress family Cupressaceae. Depending on the taxonomy, between 50 and 67 species of junipers are widely distributed throughout the Northern Hemisphere, from the Arcti ...

'' (juniper trees and shrubs) as the primary ( telial) host and only ''

Pyrus Pears are fruits produced and consumed around the world, growing on a tree and harvested in the Northern Hemisphere in late summer into October. The pear tree and shrub are a species of genus ''Pyrus'' , in the family Rosaceae, bearing the po ...

'' (pear trees) as secondary (

aecial An aecium (plural aecia) is a specialised reproductive structure found in some plant pathogenic rust fungi that produce aeciospores. Aecia may also be referred to as "cluster cups". The term aecidium (plural aecidia) is used interchangeably but i ...

) hosts. It is one of many types of

rust fungi Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales (previously known as Uredinales). An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus ''Puccinia'', are currently a ...

affecting plants. Like many rust diseases, ''G. sabinae'' requires two different hosts to complete its life cycle from year to year. Juniper is the winter host and pear is the most common summer host. Spores (called

aeciospore Aeciospores are one of several different types of spores formed by Rusts. They each have two nuclei and are typically seen in chain-like formations in the aecium. References Fungal morphology and anatomy {{mycology-stub ...

s) are produced from the fungal lantern-shaped growths which protrude from the blisters on the underside of the pear leaf which become airborne and infect junipers. This fungus overwinters in swellings or galls on infected twigs and branches of susceptible juniper plants. In the spring after a rain or heavy dew, the galls on the juniper produce tiny dark horn-like growths that become covered with an orange-brown gelatinous mass called telia. The corresponding stage on the pear trees is known as aecia. The telia and aecia release wind borne resting or hibernating spores (called

teliospore Teliospore (sometimes called teleutospore) is the thick-walled resting spore of some fungi ( rusts and smuts), from which the basidium arises. Development They develop in '' telia'' (sing. ''telium'' or ''teliosorus''). The telial host is the p ...

s and aeciospores) capable of infecting susceptible pear leaves and Juniper respectively. Spores produced from the fungus-induced swellings on juniper stems can be infectious up to 6 km. The disease causes a yellow-orange spot that turns bright red on leaves of pear trees. The disease can be particularly damaging on pear, resulting in complete defoliation and crop loss if not treated. The fungus feeds on the living cells of the host plant and is not capable of surviving on dead plant material, and so must either alternate with a different host or produce resting spores to pass the dormant season. Pear rust is a regulated disease in some countries. Gymnosporangium telia macro IMGP0418.jpg, Telia emerging from juniper bark

Control

Pruning out any infected juniper twigs and branches in winter and early spring can help reduce the spread of ''G. sabinae''. The vulnerable point of the fungus lies in its usual inability once established on a tree to reinfect it. Generally, the fungus must cross over to the opposite tree host. The most direct method of control is to exterminate junipers near pear trees. If there is a chance of infection, spraying pear trees with a

fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

in spring and summer (typically a

systemic Systemic fundamental to a predominant social, economic, or political practice. This refers to: In medicine In medicine, ''systemic'' means affecting the whole body, or at least multiple organ systems. It is in contrast with ''topical'' or ''loc ...

one that is certified as capable of dealing with rust) may help, although this is often not considered worthwhile.

References

{{Taxonbar, from=Q449430 Fungi described in 1785 Fungi of Europe Fungi of Asia Fungi of Africa Fungi of North America Fungal tree pathogens and diseases Apple tree diseases Pear tree diseases Pucciniales Taxa named by James Dickson (botanist)