Phytophthora Root Rot of Raspberry


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Phytophthora Root Rot of Raspberry

Phytophthora root rot could be considered one of the up and coming diseases of raspberry although it was first reported in Scotland in1937 and the U.S. in 1958. The first widespread incidence of the disease in North America came in the mid-1980. Phytophthora root rot is now considered a major factor in the decline of established plantings as well as an element that limits the establishment of new ones. The disease may have been established much earlier than 1958 but was not identified as an infectious condition because its symptoms resembled those of winter injury or asphyxiation and the causal organism was difficult to isolate. Researches currently identify the causal organism as one or more of eight species of Phytophthora. The species found in the United States include: P. fragariae C. J. Hickman (not the same isolate found on strawberry), P. erythroseptica Pethybr., P. megasperma Drechs., P. cactorum (Lebert & Cohn)J. Schrot., P. citricola, and P. cryptogea Pethybr. & Lafferty. Red raspberry is the primary host but some black raspberry cultivars may also be affected. The epidemiology of the disease is variable depending upon the Phytophthora species present in a given area. One constant in the development of the disease is its dependence on excessive soil moisture. The pathogen may be endemic in a given area or introduced on infected plant material. It may also be introduced on infested soil or contaminated water, either by runoff or reuse of collected surface water. The Phytophthora spp. over seasons winter as mycelium in infected plant material or as oospores in the soil. Zoospores are formed in sporangia arising from mycelium or oospores. They are liberated when the soil becomes saturated. The duration of saturation necessary for zoospore release is also variable depending on the species. Once liberated, zoospores swim to susceptible root tissue and invade the host. The fungus becomes established in the host and more sporangia and zoospores are produced. These spores serve as inoculum for secondary spread of the disease. Oospores are produced when the environmental requirements for an individual Phytophthora species have been met.

Symptoms

Gross symptoms of Phytophthora root rot in large plantings appear as localized areas where the stand is thin. These areas commonly correspond to low, poorly drained sites. The disease progresses from these "hot spots" uphill. Individual plants have a reduced number of primocanes (new vegetative stems) in the spring. Dark, water-soaked lesions appear at the base of these canes. As the season progresses, primocanes become chlorotic then wither and die. In some cases the canes may appear normal during the first year of infection and die the next year. Floricanes (second year canes bearing fruit) may also become infected. Leaves that emerge from the lateral shoots of these canes may be chlorotic, wilted or scorched along the margin or between the veins. The variability of these symptoms is again related to the Phytophthora spp. present as well as the cultivar type and environmental conditions. In severe cases the floricanes die prior to harvest. Infected roots have a reddish brown discoloration in the cortical tissue. This tissue becomes necrotic and a definite line of demarcation may be seen between healthy and infected root tissue. The crown may also be infected and exhibit the same type of necrosis.

Plant Health Management

Site Selection.

Choose a well-drained site. A raised bed may be substituted if a good site is not available. Drainage tile may also be used to improve the site.

Resistant Cultivars.

Many popular cultivars of red raspberry are susceptible to Phytophthora root rot. One widely used cultivar that is resistant to P. fragariae, the most common species in the eastern U.S., is Latham.

Exclusion.

Use only disease free material when establishing a planting. It is also important to avoid contaminating an area with infested soil or water.

Fungicide Application.

Metalaxyl (Ridomil) is labeled for the control of Phytophthora root rot. It is important to note that its effectiveness may be limited when susceptible raspberries are grown in poorly drained soil.

References

Ellis, M. A., Converse, R. H., Williams, R. N. and B. Williamson, eds. 1991. Compendium of raspberry and blackberry diseases and insects. APS Press, St. Paul, MN. 100 pp.

Useful Links

Texas A&M University

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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