Location: Horticultural Crops Disease and Pest Management Research Unit
2022 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
This is the final report for project 2072-22000-043-000D, “Development of Knowledge-based Approaches for Disease Management in Small Fruit and Nursery Crops,” which has been replaced by new project 2072-22000-046-000D, “Disease Management in Small Fruit and Nursery Crops Based on Knowledge of Pathogen Diversity, Biology, and Environmental Effects.” For additional information, see the new project report.
Substantial results were realized over the five years of the project to advance knowledge of key pathogens and sustainable management practices to improve the management of soilborne pathogens in small fruit and ornamental crops. For Sub-objective 1A, 11 species of Phytophthora (Ph.) and 20 species of Pythium (Py.) were identified from rhododendron with root rot during a disease survey at seven Pacific Northwest (PNW) nurseries. Forty to 80 years ago, Ph. cinnamomi was the most common pathogen causing root rot, but results from the survey showed that Ph. plurivora has since surpassed P. cinnamomi as the most common root rot pathogen of rhododendron in the region. Results also showed that the number of different Phytophthora species causing disease was least in propagation systems (1 species), intermediate in container systems (3 species), and the most diverse in field systems (10 species), but Pythium diversity remained the same across all three systems (9-12 species each). This increased Phytophthora diversity in field-grown plants led to substantial increases in root rot damage compared to propagation or container grown plants. Population analyses showed that Ph. plurivora is likely a relatively newly introduced pathogen with limited genetic diversity. The most common species of Phytophthora (Ph. cinnamomi, Ph. plurivora, Ph. pini, and Ph. pseudocryptogea) and Pythium (Py. cryptoirregulare) from the survey were then evaluated for their ability to cause disease. Results showed that Py. cryptoirregulare only caused minor root damage, while the four Phytophthora species all caused moderate to severe root rot and plant death. Based on this information, growers can focus disease control efforts on the Phytophthora species that cause the most damage in container and field systems.
Also related to Objective 1 (identifying pathogens constraining production of small fruits and nursery crops), a secondary, subordinate project was conducted to survey viruses, soilborne pathogens, and plant-parasitic nematodes in red raspberry fields in northern Washington. Results showed that a soilborne disease complex, consisting of Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans, were associated with late summer root disease symptoms, but Ph. rubi was the primary pathogen most strongly associated with severe root disease. Raspberry Bushy Dwarf Virus was not associated with the root disease symptoms. Later, growth chamber studies showed that Ph. rubi grew, sporulated, and caused root rot at warmer temperatures (15 to 20°C) than previous research had suggested (5-10°C). This means that the pathogen is more active in the summer than in the winter, and that root disease control measures should be applied during the growing season to increase their efficacy.
For Sub-objective 1B (characterization of X. americanum nematodes and ability to vector viruses), the project proved more difficult than expected. Data from nematodes collected across the PNW indicates that there are at least five species present in the region, if not more. After repeated attempts to amplify several mitochondrial regions, a whole genome amplification approach was pursued. This enabled the retrieval from sequence data of entire mitochondrial genomes as well as endosymbiotic bacterial genomes. The conserved evolution of the endosymbiont and the nematode may make it possible to focus on the easier to analyze and interpret bacterial genome for nematode identification compared to the use of nematode genomic data. Multiple experiments were conducted where field collected soil containing nematodes was brought to the lab and virus inoculated cucumbers were planted in pots containing the nematodes to evaluate differential transmission of virus. Dagger nematodes are difficult to work with due to their large size and inability to survive disturbance. The greenhouse approach did not result in consistent results; therefore, in-field bioassays with cucumbers directly planted into small fruit fields with nematodes and viruses were pursued. This approach was successful; however, due to limited number of locations in the PNW with the virus, only two populations of nematodes were evaluated.
For Sub-objective 2A, the effectiveness of removing old, dead red raspberry roots and nontarped (bare soil) fumigation with the chemical fumigant Telone C-35 (nontarped) were evaluated (alone or in combination) for their ability to reduce soilborne inoculum of plant-parasitic nematodes, and fungal and oomycete pathogens in red raspberry fields being prepared for replanting. Root removal was ineffective at reducing nematodes, fungi, and oomycetes, while Telone C-35 was mainly only effective at reducing nematode soil populations. However, this fumigant was not very effective at reducing soil populations of fungal or oomycete pathogens. These results help explain why Ph. rubi continues to be a long-term problem for red raspberry growers and why the longevity of raspberry plantings is declining over time. Subsequent fumigation trials showed that covering the fumigated area with plastic (tarping) increased control against Ph. rubi and nematodes, but control against Ph. rubi was still incomplete. Further refinement of fumigant chemistries is needed to more completely control Ph. rubi.
For Sub-objective 2B (Vitis rootstocks for nematode management), tremendous success was realized in providing growers with information about how to use rootstocks for the management of plant-parasitic nematodes in Washington. In greenhouse studies, it was determined that 10-12 of the most commonly planted rootstocks are not hosts for M. hapla. In the same study, the nematode increased in density up to 20-fold on own-rooted Vitis vinifera. A field trial was conducted with a grower cooperator to evaluate the use of rootstocks to manage nematodes in a newly planted vineyard. At this site there were two nematodes, Xiphinema sp. and M. hapla. Of the rootstocks evaluated, 101-14, Harmony, 1103 P, and Teleki 5C, all were hosts for Xiphinema sp.; therefore, none of these rootstocks are recommended for the management of this nematode. However, for M. hapla, all of the rootstocks delayed population increases of this nematode compared to own-rooted V. vinifera. Even five years after planting, population densities of M. hapla were significantly lower on all of the rootstocks compared to own-rooted V. vinifera. More importantly, there was no difference in vine growth of rootstocks with M. hapla compared to vines without the nematode. This was not the case for own-rooted V. vinifera. By year three of the study, pruning weights of V. vinifera vines with M. hapla were significantly lower than those of vines without nematodes, demonstrating for the first time that this nematode impacts vine productivity. Combined, these studies offer the Washington wine grape industry an economically viable option for managing M. hapla in replant vineyards.
For Sub-objective 2C, reduced irrigation treatments were evaluated for their ability to control Phytophthora root rot of rhododendron caused by Ph. cinnamomi and Ph. plurivora. Plants were inoculated with these pathogens and then irrigated to maintain moisture at 70% container capacity (full rate normally used by nurseries to maintain plant health), at half the volume of the full rate (half rate, drier than normal nursery irrigation rates), or irrigated on a schedule that alternated between full rate and half rate volumes. None of the treatments were effective at reducing root rot by either pathogen. Phytophthora isolates from Sub-objective 1A were also tested for their responses to the fungicides mefenoxam and phosphorous acid in a subordinate project related to Objective 2 (evaluating tools for management of diseases of small fruits and nursery crops). Although all 35 isolates of Ph. cinnamomi were sensitive to both fungicides, several isolates of Ph. plurivora and other Phytophthora species were resistant to both fungicides. This led to difficulties in controlling root rot caused by Ph. plurivora. For P. cinnamomi, soil drenches of mefenoxam or phosphorous acid were much more effective at controlling root rot than foliar sprays. This information helps growers improve control against Phytophthora root rot by showing that soil drenches are more effective. New fungicide chemistries are being evaluated to see if they will control fungicide-resistant isolates of Ph. plurivora.
Accomplishments
1. Reducing irrigation volume does little to reduce damage by Phytophthora root rot once infection has occurred. Phytophthora root rot is a serious disease of rhododendrons, which is a major crop of the Oregon nursery industry. Because the disease is favored by excess soil moisture, previous research has suggested that reduced irrigation treatments may be effective for managing the disease. ARS researchers at Corvallis, Oregon, showed that reducing the volume of water given to infected plants is ineffective at reducing damage. Rather, infection led to increased soil moisture because the infected roots were unable to translocate water out of the soil. This research suggests that a better way to manage the disease would be to preventatively reduce irrigation before infection has occurred.
2. Rootstocks for the management of nematodes in Washington wine grapes. Plant-parasitic nematodes, microscopic roundworms, are production-limiting pests in most grape growing regions of the world. In many places, rootstocks are used to reduce the impact of nematodes on grape production; however, this is not a common management practice in Washington, the second largest producer of wine in the United States. ARS researchers at Corvallis, Oregon, showed that rootstocks are poor hosts for the most common plant-parasitic nematode in the region. This research provides Washington wine grape growers with a sustainable means to manage plant-parasitic nematodes.
Review Publications
Mestas, A., Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Mitchell, J.N., Beck, B.R. 2022. Is disease induced by flooding representative of nursery conditions in rhododendrons infected with P. cinnamomi or P. plurivora? Plant Disease. 106(4):1157-1166. https://doi.org/10.1094/PDIS-06-21-1340-RE.
Weiland, G.E. 2022. First report of Macrophomina phaseolina causing charcoal root rot of Hebe (Veronica cupressoides, V. ochracea, and V. pinguifolia) in Oregon, U.S.A. Plant Disease. 106(7):1984. https://doi.org/10.1094/PDIS-09-21-2036-PDN.
