Our newest research project involves surveying boxwood nurseries in Oregon for the boxwood blight pathogen, Calonectria pseudonaviculata. Boxwood is a $126 million industry, of which almost 20% is produced in Oregon.
During our survey, we will be collecting data on which cultivars are affected, which production practices lead to severe outbreaks, and whether asymptomatic (latent) infection might be occurring.
We are also conducting research to determine how the Oregon environment affects disease expression. In many cases, boxwood blight symptoms in Oregon can be very mild and hard to detect. This is in contrast to the eastern United States, where symptoms can become much more severe during their warm and humid summers.
Boxwood blight in a flat of rooted cuttings of boxwood cultivar 'Green Velvet'.
The United States rhododendron nursery crop is worth $42 million annually. Of this, approximately 27% of the crop ($11 million) is produced in the Pacific Northwest states of Oregon and Washington.
Our ongoing research for this industry is starting to pay off. We conducted a disease survey from 2013 to 2017 and found that Phytophthora root rot still causes on average about 19% in losses ($8 million).
Historically, P. cinnamomi was long considered the most common pathogen causing this devastating disease. However, our survey showed that a different species, P. plurivora, is now more common than P. cinnamomi and is just as capable of causing severe root rot.
Furthermore, genetic analyses have shown us that P. plurivora is probably a relatively recently introduced pathogen species. For almost 40 years, disease control research focused almost exclusively on P. cinnamomi.
Are the methods developed to control Phytophthora root rot caused by P. cinnamomi just as effective against P. plurivora? Stay tuned - we will have some answers for you soon!
$25,000 loss due to Phytophthora root rot in a 0.5 acre field of rhododendron.
Forest nurseries of the PNW produce 250 million tree seedlings annually ($350 million industry). These seedlings are used to reforest land that has been logged or destroyed by fire.
We conducted field trials at several nurseries and demonstrated that lower rates of several fumigants were just as effective as conventional, full-rate methyl bromide applications in reducing soilborne pathogen and weed populations when applied under low-permeability plastic films. As a result of our research, at least one nursery adopted lower rates of methyl bromide as a standard operational treatment and almost all nurseries are now using impermeable films to reduce emissions.
We also described Pythium diversity in forest nurseries and showed that these damping-off pathogens, including fungicide resistant isolates, have been spread from nursery to nursery, either on infected nursery stock or contaminated equipment. We further demonstrated that Pythium species can vary in their sensitivity to fungicides and biocontrol agents, which helps explain why disease control can be inconsistent.
These results are important because they help growers reduce fumigant use and show the need for rotating fungicide chemistries to prevent the further development and spread of fungicide resistance.
Pythium damping off of Douglas-fir seedlings.