Late leaf rust of raspberries—Pucciniastrum americanum =P. arcticum
Late leaf rust infects purple and red raspberries. It can cause fruit to become unpalatable.
Identification
Beginning in July many small rust spots develop on the older, lower leaves of raspberries. These spots are initially yellow then turn brown. Yellow, spore-forming bodies (uredinia) and powdery spores (urediniospores) form on the underside of infected leaves. In severe cases plants can be prematurely defoliated. In the fall different spore-forming bodies (telia) and teliospores appear as brown growth within existing uredinia. The rust can also infect fruit, causing uneven ripening and decay.
Note that cane and leaf rust, orange rust, and yellow rust also infect certain caneberries. These diseases can produce symptoms similar to those of late leaf rust. The season of rust appearance and which caneberry plant parts and varieties are affected helps to distinguish the pathogens. However, expert examination of spore-forming structures and spores may be required to confidently distinguish which fungal pathogen is causing the rust.
Life cycle
This pathogen normally overwinters on an alternative host, white spruce (Picea glauca) that is not common in California locations where most caneberries are grown. In most California locations the fungus appears to overwinter as mycelia in canes. In the spring urediniospores are produced and they infect new growth. Spores of P. americanum are spread by wind and infection of leaves is favored by high relative humidity and wet plant surfaces. The late leaf rust pathogen is not systemic; it does not move within the plant.
Damage
Pucciniastrum americanum can infect canes, fruit, leaves, and petioles at all stages of development. Small spots form on mature leaves and turn yellow and then brown, sometimes causing the leaves to die and prematurely drop. Pale yellow spores appear on the undersides of infected leaves. Highly susceptible cultivars may drop nearly all of their leaves, resulting in leafless canes. Spore-forming structures and powdery masses develop on the fruit, making them unappetizing. Infected fruit ripen unevenly and can decay from the fungus.
Solutions
Grow raspberries where they will receive full sun. Plant them on a raised berm or mound of soil to improve air circulation around the plants. To keep plants drier use drip irrigation instead of sprinklers. Remove any wild caneberries near those grown for fruit because they can be alternative hosts from which rust pathogens can spread. Note that where host plants occur in riparian areas there are regulations that limit vegetation removal. Before managing vegetation along waterways, contact the California Department of Fish and Wildlife to learn of any rules that may apply to your situation.
After harvest cultivate to bury debris or dispose of it away from caneberries because it is a source of this pathogen. Stripping leaves from primocanes (vegetative canes) in the fall will also reduce overwintering inoculum.
Any method of pruning and trellising that improves air circulation around caneberries helps to reduce rust because this allows flowers, fruit, and leaves to dry more quickly, reducing plant susceptibility to rust. The dry conditions in a macrotunnel (hoop house covered with clear plastic) greatly limit the development of rust if the tunnels are constructed around plants before July.
A light to moderate abundance of late leaf rust infections does not affect raspberry fruit yield. If the fungus was abundant the previous growing season, spraying Bordeaux mixture or another copper fungicide may be warranted during the delayed dormant season, which is when buds appear and swell before they open. Alternatively or in addition, a fixed copper, such as Bordeaux mixture, or myclobutanil can be sprayed during the first bloom.
Adapted from Raspberry Late Leaf Rust in Hawaii Caused by Pucciniastrum americanum (PDF), University of Hawaii and Pest Management Guidelines: Caneberries, University of California Statewide Integrated Pest Management Program (UC IPM). |
