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Phytophthora root rot and wilt: a serious saboteur of raspberry production
from The Grower May 2023
by The Grower
DR. RISHI R. BURLAKOTI & DR. SANJIB SAPKOTA
Canadian raspberry growers, mostly situated in British Columbia, have been facing several challenges, particularly production decline due to unexpected weather patterns and emergence of diseases and pests.
Phytophthora root and wilt (PRRW), primarily caused by an oomycetes pathogen (water mould) Phytophthora rubi, is the most damaging one.
The disease is responsible for declining raspberry plant vigour and fruit yields. Moreover, the lifespan of the raspberry plantings in BC has declined from 10 to 12 years to five to seven years. BC raspberry growers consider their single largest non-labour expense to be the control of PRRW. The disease is also a major production challenge for raspberry growers in the Pacific Northwest regions of the United States as well as in
Chile, UK and several other European countries.
Although P. rubi is considered as a major causal agent of the disease, multiple species of Phytophthora such as such as P. gonapodyides, P. cryptogea, P. citricola, etc. were sporadically reported to infect raspberry from different countries. Phytophthora is a soil-borne pathogen and can survive in soil up to 10 years by producing thick-walled sexual spores, ‘oospores’. The pathogen also persists as mycelia in infected raspberry plant tissues. When the soil is wet either from precipitation or irrigation, the pathogen produces plenty of motile spores ‘zoospores’ which can swim in water and ultimately are able to move among plants and fields. The pathogen can spread within and among fields via infected propagating materials, contaminated soil materials stuck to farm machinery, tools and human wares.
Motile spores infect plants via root, colonize the root tissue and ultimately move to stem base and above-ground plant tissue. Multiple cycles of infection can happen in a single growing season. Root infections cause lesions and discolouration in roots, crown and stem base (Fig.1A). Once the disease progresses, scorching, chlorosis, and necrosis appears on leaves and ultimately causes wilting and dieback of stems and the entire plant (Fig.1B & C). Frequent and high amounts of rainfall and moderate temperature (15 to 22°C) favour root rot infection and disease progress. Disease is more problematic in heavy clay soil than the sandy loam or loam soils.
Management of PRRW is very challenging, requiring integration of several management methods during both pre- and post-raspberry planting. Most common raspberry cultivars are susceptible to the disease and have a very low level of resistance. Only a few cultivars of raspberry ‘Cascade Bounty’, ‘Cascade Harvest’, and ‘Squamish’ are moderately resistant to the disease, but their level of resistance in fields has been shown to be inconsistent.
Judicial use of cultural practices, such as selecting loam or sandy loam sites, use of raised beds and plastic mulch etc. can help in reducing the disease, but these practices are difficult to use and also are not sufficient to control disease.
Foliar fungicides (fosetyl-Al, Fungicide Resistance Action Committee, FRAC group 33) and soil application of metalaxyl (FRAC group 4), oxathiapiprolin (FRAC group 49), and cyazofamid (FRAC group 21) are registered in Canada for controlling root rot and wilt of raspberry. However, proper rotation, tank-mix and application timing is required for the fungicide application.
With funding support from the Canadian Agricultural
Partnership (CAP) AgriScience Program and Raspberry Industry Development Council and BC Blueberry Council, a five- year (2018-2023) research project led by Dr. Burlakoti was conducted in Agassiz Research and Development Centre, AAFC. The aims of the project were to: (i) survey the disease, identify the causal agents and understand the pathogen diversity; (ii) develop reliable disease screening tools to support the breeding program; and (iii) evaluate multiple fungicides groups and their application timing to reduce the disease. We revealed that PPRW is a serious problem in raspberry in BC, as Phytophthora infection was found in 55 per cent to 100 per cent of fields monitored during 2018 to 2020. Disease was found in most commonly grown cultivars: ‘Chemainus’, ‘Rudi’, and ‘Meeker’.
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