Hosts: Chihuahua pine, with live oaks as alternate hosts.
Figure 239. Swollen Chihuahua pine cone infected with C. conigenum and sporulating. An uninfected cone is on the right.
Symptoms/Signs: This disease produces swollen, deformed cones on Chihuahua
pine that, when ripe, are loaded with orange spores beneath a thick
papery outer surface. On oak, this disease produces several different
spore stages. The type most often observed has the appearance of
brown to black wool on the lower leaf surface.
Biology: This fungus produces spores on
swollen deformed cones (galls) of Chihuahua pine in July and August,
and other spores on the undersides of leaves of Mexican blue, Dunn,
Emory, gray, canyon live, netleaf, silverleaf, and Arizona white
oaks throughout the summer. Cones become infected during their first
year of development, swell into misshapen galls of various sizes,
produce no seeds, and do not open. Aecia develop 2 to 3 years after
infection. Galls die after aecial production but remain on the trees.
Large galls usually kill the branches that bear them.
Effects: During periods of outbreak brought
on by favorable weather, cone rust may kill more than 50 percent
of the cones on groups of trees. This disease also affects other
pine species in Mexico.