Over troubled water
Consumers want to purchase the perfect Christmas tree. Growers want to grow perfect trees. Unfortunately, a problematic disease threatens many Christmas tree nurseries and fields.
Dr. Gary Chastagner of Washington State University recently discussed research gaps in managing Phytophthora root rot (PRR) in Christmas trees.
“Phytophthora diseases can be a significant problem in all phases of the production of Christmas tree conifers, nursery stock and young transplant beds and larger trees,” Chastagner said. “Research requires funding, and the Real Christmas Tree Board has provided funding.” Additional Phytophthora research funding is through the Gift Seed Project, Specialty Crop Block Grants and USDA-NIFA.
Chastagner outlined some of the changes in the Christmas tree industry over the years, some of which created challenges such as PRR. The industry began with the harvest of wildlings from logged-over areas in forests, then trees were taken to major market areas. Today, the industry is much more advanced, particularly in seedling production.
“There’s a lot of bare-root production, an increasing amount of container-grown material and much more extensive plantings in plantations,” Chastagner said. “There’s also quite a bit of wreath and bough production.”
Another major change is species-related, with the popular Fraser fir currently one of the most widely grown species. There’s also more Noble and Douglas fir production due to their desirable needle and moisture retention. However, the popularity of these species has become a driver for PRR.
The Latin name for Phytophthora translates to “plant destroyer” and that’s precisely what it does. Phytophthora is a water mold, not a true fungus, that produces zoospores to reproduce.
Chastagner outlined the growth stages of Phytophthora: Mycelium grows internally within the infected tissue; sporangia are produced in soil or on plants. Zoospores are the primary spore stage that result in infection.
“The spores swim in films of water and are attracted to root tips where chemicals are being released,” Chastagner explained. “The zoospores swim toward and infect the root tip. Once the root tip is infected, Phytophthora grow, colonize the root and ultimately kill the root.”
Symptoms of PRR vary significantly. Mortality can occur in nursery stock, where it shows up as reddish-brown decay on the root system. Disease may be visible in a field in a small pocket where water stands.
“In some cases, you don’t see off-color before branches start to flag,” Chastagner said. “The disease is developing rapidly in those situations. This pathogen can be spread with irrigation water, so if you over-irrigate, you create situations that increase the risk of Phytophthora. If you’re using open-source water from a pond or stream for irrigation, you might be spreading the pathogen in irrigation water.”
Above-ground symptoms such as browning or death of shoots are not necessarily due to Phytophthora. Flagging or dead trees may also be due to shoot blight, canker disease or drought stress. Laboratory tests with newer, molecular diagnosis tools can help identify Phytophthora. Metabarcoding, a technique that allows identification of multiple species within a single mixed sample, has also increased the ability to detect and identify Phytophthora species.
Connecticut researcher Dr. Richard Cowles found that molecular techniques allowed identification of new Phytophthora species associated with Christmas tree infection, including Phytophthora abietivora. Canadian researchers looking at Christmas trees in the Quebec area found P. abietivora associated with dead and dying Fraser and Balsam fir. North Carolina researchers studied the origin of infected seedlings.
“Phytophthora was causing problems in the 1960s,” Chastagner said. “It continues to be a problem with major impact in certain areas with poorly drained soils and favorable conditions. There are more diverse Phytophthora species found than previously detected in North Carolina.”
Water molds thrive in poorly drained soil and during periods of soil saturation. This can be challenging in areas where rain events dump large quantities of water in a short time. Plantings in low areas increase the risk for Phytophthora.
Container seedling production is on the rise, and Chastagner said this production method should be low risk if best management practices are followed. Containers should be elevated off the ground or on a paved surface to avoid inoculum movement.
“Nursery stock plays a role in spreading Phytophthora,” he said. “It’s important for nurseries to use BMPs to minimize the risk of introducing Phytophthora into Christmas tree plantings as well as forest sites.”
Growers should remain aware of the disease cycle that includes host, pathogen, time and the environment, all of which are necessary for disease development. Even in favorable conditions, it may take weeks for symptoms to develop. Managing any of the variables, such as improving drainage, can reduce disease development. Changing the host to a more tolerant species can reduce or eliminate disease.
Once it’s in a field, Phytophthora is difficult to manage. Chastagner urges growers to plant healthy seedlings and limit the movement of contaminated soil and water. Integrated management for PRR in nurseries includes starting with healthy seedlings and sanitizing containers and potting media (typically with steaming). Also helpful is baiting water to determine inoculum presence and using only pathogen-free water for irrigation.
“Avoid planting swales and bait open sources of water to see if you have Phytophthora,” Chastagner said. “Manage irrigation so you aren’t over-irrigating. Understand what your infiltration rates are in media and in the field so you don’t create situations where you apply water faster than the infiltration rate of the soil.”
Regarding irrigation, Chastagner said there’s a risk of Phytophthora with open-source water but little risk with well water. Fumigation reduces the risk in bare-root nursery production. However, if tractors are driven throughout the nursery then to an infected field, Phytophthora can be introduced back into the nursery.
Chastagner described research to introduce resistance to P. cinnamomi in Fraser fir via grafting to a resistant species such as Momi (Japanese) fir. Other ongoing PRR projects include screening seed sources for susceptibility to Phytophthora species and fungicides to control PRR in both Noble and Fraser fir. Increased IPM research includes raised beds, fungicide and gypsum application to inhibit Phytophthora sporulation and seasonal variation in infection periods.
Researchers are looking at several FRAC groups for efficacy and residual activity of root dips, drench applications of fungicides to protect seedlings, post-infection drench application of fungicides to suppress PRR development and efficacy of autumn and spring drench fungicide treatments. Chastagner emphasizes the importance of following labeled instructions on any treatments.
The IR-4 Project support specialty crop growers in gaining access to registered pest control tools. The project conducts extensive environmental horticulture trials on conifers to evaluate herbicides, fungicides and insecticides for efficacy and phytotoxicity to protect commercial Christmas tree, nursery and forestry crops. Since 2003, the IR-4 program has tested 74 products against 11 Phytophthora species on numerous ornamentals.
Future research includes monitoring existing national IR-4 conifer Phytophthora field trials and conducting greenhouse and field fungicide residual trials.
by Sally Colby
