Research shows Phythopthora root rot, leaf spot require different approaches for management

It’s been 10 years since our research team started receiving funds from the Floriculture and Nursery Research Initiative (FNRI) to study soilborne diseases for the nursery industry. Based on Oregon Association of Nursery’s research priorities, we chose to study Phytophthora root rot because it is one of the most common diseases of nursery plants nationwide.

We also focused on the disease as it affects Rhododendron because the Pacific Northwest is the top producer of this valuable ornamental shrub ($13 million/year). Since 2013, we have collaborated with other scientists to better understand this troublesome disease, including Johanna Del Castillo-Munera (University of California-Davis), Carla Garzon (Delaware Valley University), Nik Grunwald (United States Department of Agriculture-Agricultural Research Service), Jay Pscheidt (Oregon State University), and Luisa Santamaria (OSU). Here are some of our most important findings.

Main causes of Phytophthora root rot of rhododendron

Our first task was to determine which Phytophthora species were causing root rot of rhododendron in the Pacific Northwest.

A 1974 survey indicated that P. cinnamomi was the main culprit in our region, with losses averaging 10–15%. However, a lot has changed since then, and nurseries may be dealing with new species that weren’t around 50 years ago. Indeed, more recent surveys show that there are about 20 different Phytophthora species causing leaf spots, cankers, and root rot on many of Oregon’s most important nursery crops.

Nevertheless, it still wasn’t clear which species were causing root rot on Rhododendron, and we also wanted to know if disease control had improved since the 1970s, when few Phytophthora-specific fungicides were available.

To answer these questions, we evaluated root rot at seven Pacific Northwest rhododendron nurseries and collected samples to identify the Phytophthora species involved.

Our survey found that root rot is still a serious problem for the rhododendron industry, causing up to 41% in inventory losses per year (average 15%), with damage generally being least on cuttings in propagation facilities, intermediate in container-grown plants, and worst in the field. The increase in root rot corresponds to an increasing number of Phytophthora species as plants move through the production cycle (see chart above). This is because propagation facilities generally start with clean materials, while plants in container and field systems are increasingly exposed to Phytophthora from contaminated pots, gravel, irrigation water, potting media, and field soil.

Our survey also found that P. plurivora and P. cinnamomi are now the top two species causing root rot in most rhododendron nurseries, with P. plurivora (44% of samples) replacing P. cinnamomi (29% of samples) as the #1 species in our region.

Phytophthora plurivora is a relatively newly-discovered species that may have been introduced into the U.S. as early as the 1960s. It is unique in that it produces inoculum that spreads both above and belowground. It causes leaf spots, cankers, and root rot.

On the other hand, P. cinnamomi is restricted almost completely belowground where it mainly causes root rot. This difference in biology allows P. plurivora to spread more easily than P. cinnamomi, and has led to it becoming one of the most important Phytophthora species in nurseries worldwide.

Fungicide resistance hampers control

While conducting the survey, growers mentioned that Phytophthora root rot was increasingly difficult to control, and that fungicides weren’t always effective at preventing the disease.

Based on this information, we tested Phytophthora isolates from the survey for resistance to two popular fungicides, mefenoxam (e.g., Subdue MAXX®) and phosphorous acid (e.g., Alude™).

While all P. cinnamomi isolates and many P. plurivora isolates were sensitive to both fungicides, isolates of P. plurivora at four nurseries had developed resistance to one or both products. In fact, 90% of the P. plurivora isolates at one nursery were resistant to phosphorous acid, which explained why that grower had difficulty controlling root rot.

So, although both fungicides are effective for P. cinnamomi, they may not work at nurseries where P. plurivora has developed resistance (see Figure 3, below). This situation illustrates why it is important to not rely on fungicides alone for disease control and why growers should alternate with more than one or two different fungicide chemistries over time.

Drenches vs. foliar sprays

We also tested whether disease control depends upon where the fungicides are applied to the plant.

Both mefenoxam and phosphorus acid are systemic, and some growers apply these fungicides as a foliar spray to protect against leaf- and stem-infecting Phytophthora species such as P. syringae and P. ramorum, hoping that the systemic activity will also protect against root rot (see Figure 1, above). But does this actually work? Unfortunately, no.

Our research showed that soil drenches are much more effective at reducing root rot than foliar sprays. It is therefore better to put fungicides where the plant needs the most protection: on the leaves for Phytophthora leaf spots and on the roots for Phytophthora root rot.

The impact of nitrogen fertilization

Fertilization is often used to produce larger, quality plants more quickly. We therefore wanted to see how nitrogen (N) application influences root rot in a study evaluating three levels of ammonium sulfate (none, low, and high).

Generally, increasing the amount of N applications caused root rot to get worse. Although lower levels of N reduced the number of visibly diseased plants, it also led to plants becoming stunted and chlorotic, and could increase the risk for selling apparently healthy plants with low levels of infection. Growers should therefore focus on a balanced approach, being cautious not to overfertilize, as this makes plants more susceptible to several diseases, not just Phytophthora root rot.

Climate change affects Phytophthora biology

Climate change is having an effect on nursery production in the Willamette Valley, with hotter and drier summers and more unpredictably cold or wet winters. We therefore conducted experiments to determine how temperature and soil moisture influence Phytophthora biology and control.

Our studies showed that P. cinnamomi and P. plurivora are adapted to a broad range of temperatures, growing best at temperatures near 80 F. Phytophthora plurivora, however, sporulates better at cooler temperatures (50-70 F) than P. cinnamomi (60-80 F), which may give it an advantage for causing disease in spring and fall. We also found that temperature influences how well fungicides inhibit the growth of Phytophthora isolates in the laboratory. But additional experiments are needed to determine how this relates to disease control.

Because Phytophthora root rot is frequently associated with overwatering and wet soil, we tested whether we could reduce disease by growing newly infected plants under drier conditions.

Unfortunately, plants grown with less water developed just as much disease as those grown in consistently moist soil. Counterintuitively, root rot actually became severe under dry conditions because the newly infected roots slowly lost their ability to take up water.

This, in turn, caused soil moisture to rise and resulted in a slightly wetter environment that was more conducive for root rot. So, once infection has occurred, there is little that can be done to manage the disease by reducing irrigation. Instead, focus disease control efforts on preventing infection from occurring in the first place by avoiding situations where plants are overwatered or sit in puddles for any length of time.

Future directions

In summary, our research has shown that there are two main species of Phytophthora causing root rot of rhododendron, P. plurivora and P. cinnamomi. This is important, because these two species differ in their biology and adaptability to different environments (affecting their ability to cause disease) as well as their sensitivity to fungicides and other disease control measures (affecting disease control). Disease control strategies that were developed for P. cinnamomi back in the 1970s–1990s may not work as well for other Phytophthora species such as P. plurivora.

Our next step will be to evaluate new fungicide chemistries to see if they are effective against mefenoxam- and phosphorous acid-resistant Phytophthora isolates. We will also be expanding our Phytophthora root rot survey to include other plants important to the nursery industry. This will allow us to identify other Phytophthora species constraining nursery production and will help us develop disease control methods that are effective for a broader range of nursery plant species.

Growing Knowledge, from the November 2023 issue of Digger magazine | Download PDF of article