Rust?  Fascinating?  Really?  Well, not the usual kind of rust, that unwelcome brown, crispy blistering on your beautiful old porch railing or your beloved old car or truck.  It’s the rust fungi that have lives that are truly fascinating.   Imagine spending your life in two completely different cultures.   A few people do that, but it’s pretty unusual.  It’s unusual for fungi too!  Most fungi that infect plants grow in and/or derive their nutrients from the plant host, produce spores of some type, and have various mechanisms of surviving undesirable conditions such as drought or freezing temperatures.   A number of plant pathogenic fungi can infect a wide array of different kinds of plant, but even these don’t require two completely different, unrelated plant hosts to complete their life cycle.   Some of the rust fungi do (and some don’t) and their life cycles are FASCINATING!  It makes one wonder at the turns taken in the course of evolution and why things have evolved as they have.  These fungi have the most complex life cycles in the Kingdom Fungi. 

One of the more common (and interesting) rusts in the northeast is cedar-apple rust caused by the rust fungus Gymnosporangium juniperi-virginianae.    This rust and its relatives are heteroecious, meaning that they require two unrelated hosts to complete their life cycle.  In this group, up to five different spore types may be produced.  The cedar-apple rust pathogen life cycle consists of four spore types, two on apple (Malus domestica) or crabapple (Malus sylvestris) and other related hosts,  and two on eastern red-cedar (Juniperus virginiana) or one of several other juniper species. 

Cedar-apple rust galls producing telial horns on Eastern red-cedar (May 2011).


Have you noticed showy orange structures on eastern red-cedar in the past couple of weeks during the rainy weather we’ve had?   A tree with multiple orange galls looks like it’s decorated.    If you look closely (you should!), you’ll find that the orange mass is composed of many gelatinous horns arising from a brown gall.   Each horn is covered with many teliospores (spore #1).  Teliospores have two cells each and each of these two cells germinates during wet weather to produce a club bearing four smaller spores called basidiospores (spore #2).   The basidiospores are spread by wind and driven rain to young apple leaves and sometimes fruit where new infections can begin if a film of water is present.  Apple leaves 4-8 days old are the most susceptible.   The fungus grows in the leaf tissue causing yellow to orange spots with yellow or red borders.  Pycniospores (spore #3) form in pycnia on the upper surface of the leaves 1-2 weeks after infection.   Pycniospores are exuded in a yellow liquid.  These spores are not infectious but may play a role in the production of the next spore type, the aeciospores.  Aeciospores (spore #4) are produced in aecia on the lower sides of the leaves or on infected fruit about 1-2 months after the appearance of pycnia.  In late summer aeciospores that are windblown to  young twigs of eastern red-cedar may start new infections there.   A gall will develop at the infection site through the next full growing season and in its second spring will produce telial horns, completing the life cycle of the fungus.  The life cycle  takes two years to complete and an individual gall of G. virginianae  will only produce telial horns for one season.   During that season, horns may be produced up to 8-10 times during rainy weather until the supply is exhausted. 

Telial horns of Gymnosporangium juniperi- virginianae on Eastern red-cedar.

This disease does not usually cause significant harm to its host trees except in the case of very susceptible apple cultivars.  On these hosts (apple), fruit may be disfigured and significant defoliation may occur. Resistant cultivars are the best way to minimize disease in areas where this pathogen and the alternate host occur.   For management of cedar-apple rust on susceptible apple trees, fungicides are used.

J Allen