Conifer Needle Rusts, Vol 1, Issue 3

James L. Green Ornamentals Specialist ORNAMENTALS December 1975 Horticulture Department, OSU Volume 1, Issue 3 NORTHWEST Bernie Moore Page 4 - 5 ARCHIVES Pathologist Extension Plant Clinic, OSU

CONIFER NEEDLE RUSTS

Needle rusts have become a serious problem in some areas of the state. The disease appears to be associated with the nursery or sapling stage of conifers. The intensive cultural practices of Christmas tree production provide an ideal situation for needle disease attacks.

The disease and life cycle of the six fungal organisms that cause needle rusts are very similar regardless of the coniferous host (Table 1). The rust fungi have several spore forms, which appear in a definite succession (Figure 1). The pycniospore (1) and aeciospore (2) are usually produced on the conifer needle. The urediospore (3), teliospore (4), and basidiospore (5) are produced on the alternate host. The color of the spores vary from white, yellow, orange, or orange-red. The fungal fruiting structures may be on the lower or upper needle surface. The life cycle of the organism is completed in one year, and two distinct plant hosts are required, the conifer and an alternate host.

Figure 1: Disease Cycle of needle rusts. Pycniospores (1) produced on the conifer in sp Aeciospores (2) produced on the conifer and w borne to the alternate host where infection occ Urediospores (3) produced on the alternate ho Teliospores (4) produced on the alternate host Basidiospores (5) wind-borne from the alterna host in fall or early spring to the conifer where infection occurs with re- sultant production of Pycniospores (1) and completion of the annu disease cycle. Orange or yellow slightly raised spots appear on the needle in the late spring or summer. The fruiting structure of the fungus is below the cuticle or epidermis. This tissue ruptures and the colorless pycniospores are extruded in a sticky liquid. It is thought that these are non-functional spores. The aeciospores are produced next and can only infect the alternate host. These are found in white or yellow blisters (flattened- covering from host tissue "Caeoma" or raised-covering from fungal tissue "Peridermium") with either white or yellow spores (Figure 2). The aeciospores are wind-borne to the alternate host on which they germinate. The fungus penetrates the leaf tissue, and the urediospores develop beneath the epidermis in white or orange blisters.

These pustules erupt to discharge white or yellow urediospores. The urediospores can only reinfect the alternate host on which it was produced. Later in the fall, dark, waxy cushions are produced on the alternate host leaf. These contain the teliospores. The teliospore may germinate in the fall producing basidiospores that are wind-borne back to the conifer needle in the fall. Or, the fungus may overwinter in the alternate host leaf tissue as the "overwintering teliospore"; the following spring the resulting basidiospores may be wind-borne to the conifer and result in spring infection.

Figure 2: White needle rust (Uredinopsis pteridis) on Grand Fir.

Identification of the rust fungus will depend on the conifer host species. The general term, either white or yellow rust, is usually adequate identification. The fungus can be further characterized by the shape of the fruiting structure that the aeciospores are produced in. When the covering is made up of host tissue and is flattened, it is called the "Caeoma." The "Peridermium" types are membraneous structures rising above the host tissue. Cedar apple rust is of an intermediate type - "Rosestelia" (Table 1).

Table 1: The most common needle rusts found on conifers in Oregon.

Fungus Host Structure Type Color of Alternate Host Aeciospore Melampsora Doug Fir Caeoma Yellow Western Poplar occidentalis Melampsora Doug Fir Caeoma Yellow Western Poplar labertensis Uredinopsis pteridis True Fir* Peridermium White Sword & Braken Fern Chrysomyxa Spruce Peridermium Yellow Rhododendron piperiana Gymnosporangium Cedar Rosestelia Yellow Apple juniperi-virginianae Pucciniastrum True Fir* Peridermium Yellow Blueberry- geopertianum Huckleberry * White, Grand, Silver, Noble Fir

Heavy foliage with the "normal" species color pattern is very important on Christmas trees. Needle rusts cause defoliation and off-color; severe infections will result in a non-salable tree.

The only known control for the various conifer rust fungi is the destruction of or isolation from the alternate host for at least 1000 feet. The fungicides ferbam, maneb and polyram (1 ½ lbs. /100 gal. water) have been effective for controlling rusts on other hosts. Pesticide applications should be made according to the label.

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