Location: Horticultural Crops Disease and Pest Management Research Unit
2021 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
Research in Sub-objective 1A identifies common pathogens causing severe damage in the nursery industry and evaluates their risk to rhododendron production and risk for movement among nurseries. ARS scientists in Corvallis, Oregon, completed and published research showing that four soilborne pathogens are responsible for causing the majority of damage to rhododendrons in nurseries and that two commonly used fungicides are often not effective in controlling the disease because of the presence of fungicide-resistant pathogens. Experiments were started to identify alternative fungicides to control rhododendron pathogens.
For Sub-objective 1B, the final samples were collected and the genomes of individual Xiphinema were sequenced. This approach was taken after it became apparent that cloning and sequencing mtDNA regions in this nematode was time consuming and often resulted in poor quality data. The data is being analyzed and preliminary results indicate that there are at least five species from this complex present in the Pacific Northwest. Also in support of Sub-objective 1B, the development of a droplet digital polymerase chain reaction (ddPCR) approach was initiated. Additionally, field soils were collected from across the region from areas where nematode-transmitted viruses were reported to be present. These soils were used in vector assays where cucumber plants were planted directly in soil containing nematodes. After two to three months, the virus status of the plants was determined using enzyme-linked immunosorbent assay (ELISA). These vector assays are ongoing.
Sub-objective 2A is now complete.
Field trials continue towards achieving Sub-objective 2B. The field trial in Washington evaluating rootstocks for nematode management is in its fifth year. In this trial, four rootstocks and own-rooted Chardonnay are being evaluated in areas of a vineyard initially with and without nematodes. A manuscript on the establishment years of this vineyard was published. Data showed the cumulative response of own-rooted vines to nematode parasitism with a negative relationship between nematode population densities and pruning weights. All of the rootstocks (Harmony, 101-14, 1103P, 5C) were hosts for X. americanum while they were poor hosts for M. hapla. A second trial in another commercial vineyard in Washington, where additional rootstocks and own-rooted vines are being evaluated in areas with and without nematodes, continues to be monitored. The upkeep of this trial is in collaboration with stakeholder viticulturists and colleagues at Washington State University. Data from a field trial where rootstocks or manipulation of irrigation were used to manage root-knot nematodes during the establishment phase of a vineyard was analyzed and submitted for publication. It was found that denying water to vines did not reduce nematode population densities, but that rootstocks (Matador, 101-14, 3309C) supported much lower population densities of root-knot nematode than own-rooted vines. Our almost decade-long effort to evaluate rootstocks for root-knot nematode damage in Washington winegrape vineyards has demonstrated that there are at least 10 rootstocks available to growers which can be utilized in replant situations to minimize nematode damage to establishing vines.
Progress was made towards Sub-objective 2C, to determine the effects of environment on the severity of Phytophthora root rot. ARS scientists in Corvallis, Oregon, completed and published research showing that a soilborne pathogen of red raspberry causes more damage at warmer temperatures than at cooler temperatures.
Accomplishments
1. Resistance to mefenoxam and phosphorous acid in some Phytophthora species. Disease control of root rot in plant nurseries is failing because many Phytophthora pathogens are resistant to fungicides. Fungicides are the most common way of controlling root rot in plant nurseries. However, growers often report that fungicides do not control the disease. ARS researchers at Corvallis, Oregon, determined that some Phytophthora species have developed resistance to the two most commonly used fungicides, mefenoxam and phosphorous acid. The fact that some Phytophthora species have developed resistance to mefenoxam and phosphorous acid explains why disease control has been poor and will be of benefit to growers who will need to use other fungicide chemistries to control root rot.
2. Pathogen kills raspberries at warmer temperatures than previously expected. Phytophthora rubi is a serious pathogen causing root rot that kills red raspberries in the Pacific Northwest. It was previously assumed that root rot would be worse in winter, when the soil was cool and wet. Thus, most disease control measures are implemented from late fall through early spring, when these conditions predominate. ARS researchers at Corvallis, Oregon, showed that the pathogen is actually more active during the warmer temperatures in late spring and summer when there is less rain. Therefore, disease control might improve if management tactics were implemented during the warmer weather occurring from late spring through summer.
Review Publications
East, K.E., Zasada, I.A., Tarara, J., Moyer, M.M. 2021. Field performance of wine grape rootstocks and fumigation during the establishment phase of a chardonnay vineyard in Washington. American Journal of Enology and Viticulture. 72:113-125. https://doi.org/10.5344/ajev.2020.20023.
Wasala, S.K., Howe, D.K., Dandurand, L.M., Zasada, I.A., Denver, D.R. 2021. Genomic analyses of globodera pallida, a quarantine agricultural pathogen in Idaho. Pathogens. 10(3):363. https://doi.org/10.3390/pathogens10030363.
Wram, C.L., Zasada, I.A. 2020. Differential response of Meloidogyne, Pratylenchus, Globodera, and Xiphenema species to the nematicide fluazaindolozine. Phytopathology. 110(12):2003-2009. https://doi.org/10.1094/PHYTO-05-20-0189-R.
Darling, E., Pu, J., Cole, E., Perrault, J., Christian, R., Hale, A., Warner, F., Zasada, I.A., Chung, H., Quintanilla, M. 2021. First report of the hop cyst nematode, heterodera humuli, in two counties of the Yakima Valley region, WA, USA. Plant Disease. 105(4):727-738. https://doi.org/10.1094/PDIS-08-20-1769-PDN.
Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Beck, B.R. 2020. Phytophthora species differ in response to phosphorous acid and mefenoxam for the management of phytophthora root rot in rhododendron. Plant Disease. 105:1505-1514. https://doi.org/10.1094/PDIS-09-20-1960-RE.
Sacher, G.O., Scagel, C.F., Davis, E.A., Beck, B.R., Weiland, G.E. 2021. Virulence of five phytophthora species causing rhododendron root rot in Oregon. Plant Disease. https://doi.org/10.1094/PDIS-09-20-1873-RE.
Graham, K.A., Beck, B.R., Zasada, I.A., Scagel, C.F., Weiland, G.E. 2021. Growth, sporulation, and pathogenicity of the raspberry pathogen phytophthora rubi under different temperature and moisture regimes. Plant Disease. https://doi.org/10.1094/PDIS-09-20-1916-RE.
Davis, E.A., Weiland, G.E., Scagel, C.F. 2021. Optimizing inoculum production methods for infesting soil with phytophthora species. Plant Disease. https://doi.org/10.1094/PDIS-12-20-2698-RE.
