Pine needle rust
Pine needle rust is a native fungal disease of Austrian (Pinus nigra), jack (P. banksiana), red (P. resinosa), ponderosa (P. ponderosa), mugo (P. mugo), and Scots (P. sylvestris) pine, caused by the fungus Coleosporium asterum. This rust causes little damage. However, browning and needle loss on lower branches may disfigure or retard growth of young pines. Wild or cultivated species of aster (Aster), goldenrod (Solidago), and several other members of the daisy or aster family (Compositae) are alternate hosts for this rust fungus in Minnesota.
Fig. 1. Infected pine needle. Insert shows detail of the split, exposing spores.
Pine needle rust requires one year to complete its life cycle. Reproductive structures (basidia) develop on aster or goldenrod leaves during late summer or early autumn. Spores produced from these structures are windblown to pine needles, where new infections begin.
The fungus survives winter in infected pine needles and in perennial alternate hosts. The following spring, small yellow spots appear on infected needles. Later that spring, white swellings (aecia, reproductive structures) develop, which split open, exposing orange spores (Fig. 1.). These spores are wind- dispersed and infect the underside of leaves on the alternate host. Orange reproductive structures (uredia) develop on the leaf tissues and produce spores that cause repeated infections of alternate hosts during early summer. During the same period, reproductive structures (telia) begin to develop among the uredia on infected leaves. Different reproductive structures (basidia) are produced from the telia. Spores released from the basidia in late summer to early fall infect pine needles, thus completing the rust life cycle.
Pine needle rust rarely causes severe damage to well established trees. Removal of alternative hosts from the immediate vicinity of the trees will reduce infections of pines. Cultural practices such as watering during dry periods, mulching, and fertilizing may increase the vigor of stressed trees.