Location: Horticultural Crops Disease and Pest Management Research Unit
2018 Annual Report
Objectives
Objective 1: Identify species, populations, and genotypes of key pathogens constraining production of small fruit and woody nursery plant species in the Pacific Northwest region of the United States.
Subobjective 1.A: Evaluation of soilborne Phytophthora and Pythium communities and populations affecting rhododendron production.
Subobjective 1.B: Characterization of X. americanum-group nematodes and ability to vector viruses.
Objective 2: Identify and evaluate tools for management of economically-important diseases of small fruit and nursery crops.
Subobjective 2.A: Developing effective methods for soilborne pathogen management through removal of root Inoculum in continuous red raspberry production systems.
Subobjective 2.B: Identification and implementation of Vitis spp. rootstocks for the management of plant-parasitic nematodes of wine grapes.
Subobjective 2.C: Improved management of Phytophthora and Pythium of rhododendron through reduced irrigation regimes.
Approach
Determine the prevalence and characterize the population diversity of important soilborne pathogens affecting horticultural crops. Results from this research will identify specific pathogen populations that constrain production of horticultural crops. These populations can be targeted in the future to develop more effective, economical, and environmentally-acceptable disease management systems. Evaluate plant debris removal and irrigation practices for their ability to reduce disease in horticultural crops. Results of this research will identify specific cultural practices that reduce or suppress pathogen populations, thereby resulting in less disease. Evaluate germplasm of grape (Vitis species) rootstocks for resistance to dagger nematodes (Xiphinema americanum) and root knot nematodes (Meloidogyne hapla). Our research will identify grape genotypes that are resistant to these plant-parasitic nematodes, and can be deployed in horticultural systems in the future.
Progress Report
Towards Sub-objectives 1A1 and 1A2, diseased rhododendron plants from nurseries were sampled to obtain isolates of Phytophthora and Pythium. ARS scientists in Corvallis, Oregon, identified these isolates to species using morphology and DNA sequence analysis to determine the most common species associated with rhododendron root rot. They identified Pythium cryptoirregulare, Phytophthora plurivora, and Phytophthora cinnamomi as three of the most common species. Their next steps are to use isolates of these species to conduct pathogenicity experiments to determine which species cause the most damage (Sub-objective 1A1) and to conduct population genetics analyses to determine whether P. plurivora is being spread in the rhododendron nursery industry (Sub-objective 1A2). This meets both objectives for obtaining and identifying isolates of Pythium and Phytophthora species, and for P. plurivora specifically, from rhododendron nurseries.
Towards Sub-objectives 1B1 and 1B2, several populations of Xiphinema americanum have been collected from small fruit fields in the Pacific Northwest. In collaboration with an ARS scientist in Beltsville, Maryland. ARS researchers in Corvallis, Oregon, identified these populations as X. revesi, X. utahense, and X. pachticum, all part of the larger X. americanum complex. It is also possible that one of the collected populations is a new species.
For Sub-objective 2A, ARS scientists identified a commercial red raspberry field with substantial population densities of Pratylenchus penetrans that will be replanted next year (2019). In collaboration with the farmer and Washington State University, a field trial has been designed where fumigation and root removal treatments will be applied next August or September.
Greenhouse and field trials have been initiated towards achieving Sub-objective 2B. Ten rootstocks and an own-rooted Chardonnay control are in the process of being screened against Meloidogyne hapla in a greenhouse experiment. To assess rootstocks in a field environment, a vineyard in Washington slated for removal and infested with M. hapla and X. americanum has been identified as a test site. Fumigation treatments will be established in collaboration with the viticulturist and colleagues at Washington State University in the fall and the vineyard replanted (in 2019) with a replicated experiment evaluating four rootstocks and an own-rooted Chardonnay control for the management of plant-parasitic nematodes in wine grapes.
Towards Sub-objective 2C, ARS researchers are evaluating three irrigation treatments in containerized rhododendron. The three irrigation treatments are: 1) a 1X treatment to provide optimal moisture for rhododendron growth; 2) a 1/2X treatment providing drier conditions that may limit infection by the pathogen; and, 3) a cyclical treatment alternating between optimal moisture and dry conditions to simulate hand watering. Isolates of P. cryptoirregulare, P. cinnamomi, and P. plurivora were used to inoculate plants within each irrigation treatment and data on disease development (pathological) and plant health (physiological) is being collected.
Accomplishments
1. Root disease complex identified in red raspberry fields. Root diseases cause significant losses in red raspberry, but lack of knowledge about the identity and distribution of pathogens in production fields limits the development of effective disease control strategies. ARS researchers at Corvallis, Oregon, conducted surveys of Pacific Northwest red raspberry fields and identified that four different pathogens, an oomycete (Phytophthora rubi), a fungus (Verticillium dahliae), a nematode (Pratylenchus penetrans) and a virus (Raspberry bushy dwarf virus, RBDV) were widespread in fields, but root disease was more common in sites where the soilborne pathogens P. rubi, V. dahliae, and P. penetrans occurred together. This research indicates that all three soilborne pathogens must be targeted to achieve adequate disease control; however, current methods of disease control primarily target nematodes. Raspberry researchers and growers are now testing new methods of disease control to target all three soilborne pathogens.
2. Globodera ellingtonae, a new potato cyst nematode, rapidly invades potato roots. For the past decade, ARS researchers in Corvallis, Oregon, have striven to provide biological, developmental, and genomic data to Federal regulators to facilitate decision-making regarding the regulatory status of the potato cyst nematode Globodera (G.) ellingtonae. As part of this effort, ARS researchers characterized the developmental biology of G. ellingtonae on potato. It was found that this nematode invades the roots of potato very rapidly and is a very good parasite of potato. Additionally, it was observed that there was a 50% reduction in the number of eggs of this nematode in soil not planted with potato. These results are significant because they provide the first report on the developmental biology of G. ellingtonae in a field environment. This information was used by regulators while making decisions regarding the regulatory status of G. ellingtonae in the U.S.
3. Diagnostic test developed for alfalfa quarantine pathogen, Verticillium alfalfae. The presence of the quarantine pathogen, Verticillium alfalfae, in alfalfa products (hay or seed) prevents the export or import into states and countries where the pathogen has not been previously found, resulting in trade restrictions and significant economic losses. Previous tests for detecting V. alfalfae did not distinguish V. alfalfae from other Verticillium species or required specialized experience, expensive equipment, and significant time to use. ARS researchers at Corvallis, Oregon developed a rapid, easy-to-use, inexpensive test that can be used by growers, diagnostic labs, and regulatory/inspection agencies to detect V. alfalfae. This test reliably distinguished V. alfalfae from other known species of Verticillium, providing a rapid, reliable test for detecting the quarantine pathogen V. alfalfae in exported or imported alfalfa products.
4. Effective soilborne disease and weed management in forest nurseries. Seventy percent of the $350 million Pacific Northwest forest nursery industry still uses methyl bromide (MB) soil fumigation to manage soilborne diseases and weeds in tree seedling nursery beds. ARS researchers at Corvallis, Oregon, in collaboration with forest nursery industry partners, conducted field trials to evaluate other fumigants besides methyl bromide for disease and weed control. Totally impermeable films (TIF) were tested to reduce fumigant emissions from the soil, and fumigants were applied below label rates to reduce chemical inputs and meet buffer zone requirements for nurseries located next to suburban housing developments. Two alternative fumigants were identified (metam sodium + chloropicrin or 1,3-dichloropropene + chloropicrin) that were effective at controlling soilborne diseases and weeds when applied below label rates under TIF. These results provide effective alternative fumigants for soilborne disease and weed control, while reducing both chemical inputs, costs, and emissions. As a result of this research, forest nursery industry partners have adopted reduced-rate MB as a standard operational treatment and most nurseries are now using TIF to reduce emissions.
5. A new canker pathogen was discovered. Calocedrus decurrens, or incense-cedar, is a native tree species used to produce exterior lumber products and is planted in low-input landscapes as windbreaks and as an ornamental. ARS scientists at Corvallis, Oregon, with collaborators at Oregon State University and the U.S. Forest Service, found a new canker pathogen, Phaeobotryon cupressi, which kills branches throughout the crown of the tree. Landowners with native stands of incense-cedar or specimen trees consider the pathogen to be responsible for tree death. This is the first time this pathogen, originally described from Iran, has been found causing canker disease in the U.S.
6. Wheat provides a green bridge for root lesion nematode to infect raspberry. Cover crops are plants that are grown with, before, or after a cash crop with the intention of benefiting the cash crop and surrounding soil by lessening soil erosion, increasing water infiltration, and promoting pest management. In the northern part of the U.S. where most of the nation's processed raspberries are produced, a wheat cover crop is grown before a new raspberry planting is established. ARS researchers in Corvallis, Oregon, discovered that a wheat cover crop is a maintenance host for the root lesion nematode, a microscopic worm that damages the roots of raspberry. Methods used to reduce the number of root lesion nematodes in wheat were not successful. While a wheat cover crop is a host for root lesion nematode, the benefits of winter cover cropping, primarily by lessening soil erosion, out-weigh the contribution that wheat makes in contributing to the root lesion nematode population.
Review Publications
Weiland, G.E., Benedict, C., Zasada, I.A., Scagel, C.F., Beck, B.R., Davis, E.A., Graham, K., Peetz, A.B., Martin, R.R., Dung, J.K., Reyes Gaige, A., Thiessen, L.S. 2017. Late summer disease symptoms in western Washington red raspberry fields associated with co-occurrence of Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans, but not Raspberry bushy dwarf virus. Plant Disease. 102(5):938-947. https://doi.org/10.1094/PDIS-08-17-1293-RE.
Weiland, G.E., Littke, W.R., Browning, J.E., Edmonds, R.L., Davis, E.A., Beck, B.R., Miller, T.W. 2016. Efficacy of reduced rate fumigant alternatives and methyl bromide against soilborne pathogens and weeds in western forest nurseries. Crop Protection. 85:57-64. https://doi.org/10.1016/j.cropro.2016.03.016.
Weiland, G.E., Sniezko, R., Wiseman, M., Serdani, M., Putnam, M. 2016. First report of Phaeobotryon cupressi causing canker of Calocedrus decurrens (incense-cedar) in Oregon. Plant Disease. 100(8):1793. https://doi.org/10.1094/PDIS-03-16-0313-PDN.
Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Beck, B.R. 2018. Variation in disease severity caused by Phytophthora cinnamomi, P. plurivora, and Pythium cryptoirregulare on two rhododendron cultivars. Plant Disease. https://doi.org/10.1094/PDIS-04-18-0666-RE.
Press, C.M., Rolfe, K.J., Weiland, G.E., Grunwald, N.J. 2017. Efficacy of management tools for control of Phytophthora plurivora leaf spot of Rhododendron, 2014. Plant Disease Management Reports. 11:OT035.
Zasada, I.A., Peetz, A.B., Forge, T.A. 2017. Pratylenchus species associated with blueberry (Vaccinium spp.) and weed species in the Pacific North-west of North America. Canadian Journal of Plant Pathology. 39(4):497-502. https://doi.org/10.1080/07060661.2017.1367724.
Bird, G., Zasada, I.A., Tylka, G.L. 2018. Role of population dynamics and damage thresholds in cyst nematode management. In: Perry, R.N, Moens, M., Jones, J.T., editors. Cyst Nematodes. Wallingford, UK: CAB International. p. 101-127.
Tabima, J.F., Coffey, M.D., Zasada, I.A., Grunwald, N.J. 2018. Populations of Phytophthora rubi show little differentiation and high rates of migration among states in the Western United States. Molecular Plant-Microbe Interactions. 31(6):614-622. https://doi.org/10.1094/MPMI-10-17-0258-R.
Rudolph, R.E., Walters, T.W., Devetter, L.W., Zasada, I.A. 2018. Contribution of a winter wheat cover crop to the maintenance of root lesion nematode populations in the red raspberry production system. HortTechnology. 28(2):182-188. https://doi.org/10.21273/HORTTECH03971-18.
Reyes Gaige, A., Dung, J.K., Weiland, G.E. 2018. A rapid, sensitive and field-deployable isothermal assay for the detection of Verticillium alfalfae. Canadian Journal of Plant Pathology. https://doi.org/10.1080/07060661.2018.1474262.
