''Rhabdocline pseudotsugae'' is a fungal plant pathogen. The pathogen, along with '' Rhabdocline weirii'' causes Rhabdocline needlecast; ''R. weirii'' only affects

Douglas-fir The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is native to western North America and is also known as Douglas-fir, Douglas spruce, Oregon pine, and Columbian pine. There are three va ...

trees. The disease causes the needles of the tree to discolor and eventually fall from the tree. It was originally common to the Rocky Mountain states of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

but has since spread to

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...

. Infections usually start in the spring or early summer and can change the color of the foliage to a variety of hues. The

fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...

produces

apothecia An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mos ...

that are normally found on the underside of needles but they also occur on the topside as well.

Range

Originally common in the Rocky Mountain States, the fungi spread to the Northeastern United States on ornamental forms of fir trees planted there. It has spread to

on trees imported from western North America and it is causing problems with trees growing there also.Westcott, Cynthia, and R. Kenneth Horst. 1979. ''Westcott's Plant disease handbook''. New York: Van Nostrand Reinhold. Page 250.

Host and symptoms

''Rhabdocline pseudotsugae'', known as needle cast, is a fungal pathogen that affects Douglas Fir specifically in the Pacific Northwest and Intermountain provinces.Chastanger, Gary A. “Susceptibility of Intermountain Douglas-Fir to Rhabdocline Needle Cast When Grown in the Pacific Northwest.” ''IM Douglas-Fir and Rhabdocline Needle Cast in the PNW'', PMN International, 15 Oct. 2001, https://www.plantmanagementnetwork.org/pub/php/research/rhabdocline/ . Douglas Fir are used as Christmas trees and the aesthetic value can be decreased from R. pseudotsugae. The symptoms first appear as small yellow spots on current season needles in the fall/winter. In the spring the discolored areas on the needle expand and can spread to the whole needle, however bands of discoloration are often formed. The bands start yellow but turn brown/red and can become purple with spots on the upper or lower surface of the needle. Signs of Rhabdocline pseudotsugae include apothecia that mature below epidermis where swelling is visible. These fruiting bodies are present on the underside of needles. In spring a split develops and exposes the orange brown surface of fungal body.

Infection

New infections start in the spring or early summer and conditions that cause prolonged needle wetness promote heavier outbreaks.Hagle, Susan K., et al., ''A Field Guide to Diseases & Insect Pests of Northern & Central Rocky Mountain Conifers'',
Rhabdocline Needle Cast
"United States Forest Service, 2003, p. 112. Retrieved 7 September 2007. The first symptoms produced are small yellow spots on the needle ends that develop in late summer or fall. Normally, by the next spring, the needles have started to turn reddish-brown in color produced in mottled or banded patterns. If the infection is severe then needles will turn solid brownish-red and the entire tree will have a "scorched" look to it. The pathogen, along with '' Rhabdocline weirii'' causes Rhabdocline needlecast; ''R. weirii'' only affects

trees.How To Identify and Control Rhabdocline and Swiss Needlecasts of Douglas-Fir
" North Central Forest Experiment Station ''United States Forest Service'', 1983. Retrieved 7 September 2007. The disease causes the needles of the tree to discolor and eventually fall from the tree. The pathogen often makes Douglas-fir trees unsalable as Christmas trees and affects the Christmas tree farming industry.Rhabodocline Needlecast (''Rhabdocline weirii'')
" Department of Plant Pathology, ''Cornell University''. Retrieved 7 September 2007. The fungus ''Rhabdocline pseudotsugae'' have

that are normally first produced on the under side of the needles, but it's not uncommon to find them on the top surface too. They are first produced as rounded cushions and as the epidermal layer ruptures, a brown elongated disc is exposed. Needle drop occurs after the

ascospore An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or ...

s are discharged, normally resulting in only a one-year life span for the fungus.

Disease cycle

Fruiting bodies mature in May/June on one year old needles. In high humidity the mature pseudothecia on the underside of the needles open. The ascospores are windborne and require moisture to germinate. During the release of ascospores a longitudinal split reveal orange spore mass. Infection occurs when buds are opening and expose immature needles.ajotte, Ed. “Rhizosphaera Needle Cast.” ''Penn State Extension'', Penn State, 13 Oct. 2019, https://extension.psu.edu/rhizosphaera-needle-cast . The spores germinate on the immature needle when wet and penetrate the cuticle through an appressoria and haustoria.Stone, Jeffrey. (2011). Initiation and development of latent infections by Rhabdocline parkeri on Douglas-fir. Canadian Journal of Botany. 65. 2614-2621. 10.1139/b87-352. Once the pseudothecia turns dark brown, usually in June or July, spore production is done for the year. Needles that contain these fruiting bodies are cast from late spring to summer.

Management

The most ideal method of control/management is using resistant varieties. Resistant varieties include Shyswap and Pillar Lake, while Santa Fe, Silver Creek and Coville are only moderately resistant.Admin. “Rhabdocline Needle Cast.” ''Center for Agriculture, Food and the Environment'', UMassAmherst, 8 Mar. 2018, https://ag.umass.edu/landscape/fact-sheets/rhabdocline-needle-cast . Cultural control methods such as pruning unhealthy infected shoots and removing severely infected trees are common. It is important not to prune when foliage is wet and to sterilize tools by dipping in denatured alcohol for 3 minutes after shearing infected plantations. Trees should also be sheared in healthy plantations first to avoid contamination of these plantations by workers' clothing and equipment. Douglas-Fir also require full sun and if planted in a shaded area it can encourage needle blight development. Planting trees in dispersed rows, removing lowest canopy and avoiding cover planting reduces air flow which reduces high moisture conditions that promote infection.Forestry Service. ''How to Identify and Control Rhabdoclune and Swiss Needlecasts of'' ''Douglas-Fir''. USDA, 1983, https://www.fs.usda.gov/naspf/sites/default/files/publications/ht-59_how_to_identify_and_control_rhabdocline_and_swiss_needlecasts_of_douglas-fir.pdf . Chemical control such as fungicides is common, but coverage of all canopy parts is required. Fungicides that can be used include copper hydroxide, copper sulfate, copper salts of fatty and / or rosin acids, mancozeb and thiophanate-methyl. It is also recommended that the trees be sprayed with chlorothalonil at pounds per 100 gallons of water in hydraulic spray equipment or pounds per 100 gallons in high-pressure mist blower. The first application should be made as soon as the trees start to break bud. Treatment should be repeated two to three times a week for two to three week intervals.

References

{{Authority control Fungal conifer pathogens and diseases Helotiales Fungi described in 1922 Taxa named by Hans Sydow Fungi of North America Fungi of Europe Fungus species