Late Leaf Rust

Late leaf rust infects red and purple raspberry and is not a systemic pathogen. As the name late leaf rust suggests, later in the season (beginning in July) many small rust spots are found on the older, lower leaves of raspberries. These spots first turn yellow and then brown. In severe cases plants can be defoliated, and the rust can also infect flowers and fruit. Yellow uredinia and powdery urediniospores form on the bottom sides of leaves. In fall, telia and teliospores appear as brown growth within existing uredinia.

Because this pathogen's alternate host, white spruce (Picea glauca), is not common in coastal California where caneberries are cultivated, the fungus probably overwinters as mycelium within remaining canes and produces urediniospores in spring. These spring urediniospores then infect growing plants. Spores of P. americanum are spread by wind, and infection of leaves is favored by high relative humidity.

As with yellow rust, any method that improves air circulation in the raspberry hedgerow is helpful in mitigating the spread and development of this disease. Also, the removal of infected floricanes and primocanes is useful in that it removes an important source of inoculum. The dry conditions in a macrotunnel greatly limit the infestation of late leaf rust, provided the tunnels are constructed before ideal conditions for infestation begin. In the Monterey Bay area, this generally means mid- to late July.