Carolyn Scagel, Nikalus Grunwald, Anne Davis, Bryan Beck and Jesse Mitchell

Oregon is a major player in the nursery industry, recently ranking third in the U.S. for nursery stock production. However, only 26% of this stock stays in-state, with 69% going to other states and 5% sold internationally. Rhododendrons are a key crop in the Pacific Northwest nursery industry, valued at over $11.6 million annually (USDA 2020). Unfortunately, rhododendron production is often severely impacted by Phytophthora root rot, which can wipe out entire crops (Weiland et al. 2020).

Several species of Phytophthora can cause root rot, including P. cinnamomi and P. plurivora, which were identified as the most common in Oregon nurseries affecting rhododendrons (Weiland et al. 2020). While most research has focused on P. cinnamomi, recent work highlights the potential threat posed by P. plurivora, a species that was only described about 15 years ago. This pathogen has been shown to aggressively attack important native tree species in North America and Europe. It also has the ability to infect leaves and stems where it causes leaf spots and cankers, as well as roots where it causes root rot. This has led it to become more common on rhododendron than P. cinnamomi, which only infects roots

Phytophthora thrives in moist conditions, as the pathogen’s spores rely on water to spread and infect plant roots. Therefore, root rot tends to be worse when soil moisture is high. Reducing soil moisture can lessen infection but also risks stressing plants, which may make them more vulnerable to the disease (Scagel et al. 2011, 2014). Additionally, alternating periods of wet and dry conditions can increase the severity of root rot, as stressed plants become more susceptible to infection.

Many rhododendron growers inadvertently create cycles of stress by waiting until plants show signs of water stress, such as wilting, and then watering heavily to recover. This cycle can exacerbate root rot. Finding irrigation methods that maintain growth while minimizing excessive moisture could be a key strategy for managing Phytophthora root rot.

Recent studies have shown differences in how P. cinnamomi and P. plurivora respond to irrigation practices and fungicide treatments. By understanding how these pathogens react to different water levels, we can develop better management practices for rhododendron growers that incorporate both biological insights and practical disease control methods.

The goal of this experiment was to find out whether reducing the amount and frequency of watering could impact the occurrence and severity of Phytophthora root rot in container-grown rhododendron plants. We tested two types of Phytophthora—P. cinnamomi and P. plurivora—at both low and high levels of infection.

The experiment was set up using a randomized design with 10 groups of plants, each group containing one plant from every treatment. We used five different pathogen treatments: a non-inoculated control group and plants inoculated with P. cinnamomi or P. plurivora at low and high levels. We then irrigated according to three different watering schedules (normal watering, half the normal amount, and a “boom/bust” cycle where plants are alternately watered and allowed to dry out).

We performed the inoculations in three separate trials during the summer. The different watering schedules started two days after inoculation. The trials lasted for around six months each.

Effects of Phytophthora on plant health

In general, non-inoculated plants stayed healthy and green while those inoculated with either P. cinnamomi or P. plurivora developed root rot symptoms including stunting, chlorosis, wilting, rotted roots, and plant death. At higher levels of Phytophthora exposure, the disease was more severe and progressed more quickly. But even plants exposed to low levels of Phytophthora showed more symptoms and took up fewer nutrients than the control plants. This meant that they were generally smaller and more yellowish-green (less chlorophyll) than the healthy control plants even when they didn’t show overt symptoms of wilting and death.

There were slight differences between the two species of Phytophthora, with plants infected by P. cinnamomi sometimes showing more severe symptoms than those infected by P. plurivora. However, in the third trial, P. cinnamomi caused little disease, which was likely due to issues with the inoculation process and extreme soil temperatures during that trial. Some of the other studies we have conducted have also shown that P. plurivora can cause less disease and infect more slowly than P. cinnamomi.

Effects of irrigation on disease

Different watering treatments didn’t have much impact on the development of disease for either type of Phytophthora, although there was a slight indication that one of the watering methods (alternating between dry and wet conditions) could encourage disease caused by P. plurivora more than the other methods. Overall, the number of plants that developed disease was similar across the different watering schedules, regardless of how much or how little water they received.

Once the plants became infected, their ability to take up water was quickly compromised, which led to higher moisture levels in the soil. This created favorable conditions for more root rot to develop. As a result, the amount of water provided through different irrigation schedules made little difference in stopping the disease.

Previous studies have shown that reducing soil moisture can help prevent infection if done before the disease starts, but once plants are infected, reducing moisture is less effective. Additionally, keeping the soil too dry could harm plant growth. In this study, the plants were able to grow reasonably well under all three irrigation treatments, although in one trial, plants under the driest treatment showed signs of water stress when temperatures were higher.

Conclusion

Reducing watering frequency and volume didn’t seem to have much effect on controlling root rot after the plants were already infected. Instead, the focus should be on preventing the initial infection by avoiding overwatering and ensuring that the soil has good drainage so plants don’t sit in water for long periods. While this is easier to control in container production systems, it can be more challenging in outdoor field settings where heavy rain can cause soil saturation. In those situations, avoiding low-lying areas and improving drainage with techniques like installing drain tiles could help reduce the risk of disease.

Grower takeaways and tips

Phytophthora species cause root rot, which damages plants and leads to significant economic losses for the nursery industry. Rhododendron production, in particular, is often impacted by this disease, and there are limited ways to prevent or manage it. In recent years, P. plurivora has been identified as a common issue for rhododendrons in nurseries across the Pacific Northwest. However, little is known about how this pathogen compares to the more studied P. cinnamomi, especially in different growing conditions.

While we know that Phytophthora thrives in wet soil, it has been unclear whether changing watering practices can help reduce root rot in container-grown rhododendrons. Our study provides key insights:

Low levels of either pathogen cause mild disease, but higher levels result in more severe damage.

P. plurivora can be just as damaging to rhododendrons as P. cinnamomi.

Even plants that don’t show obvious symptoms can harbor Phytophthora, making it easier for the pathogen to spread and affect plant health.

Rhododendrons grown in Phytophthora-free soil can handle different watering schedules better than plants already infected with the pathogen.

Reducing watering after plants are infected is not an effective way to control the spread of root rot.

These findings provide new information on how water management may (or may not) help control Phytophthora infections in container nurseries, offering insights for nursery growers to better manage plant health. For more information, please see the detailed study titled “Irrigation Frequency and Volume has Little Influence on Phytophthora Root Rot in Container-grown Rhododendron,” accessible for free online at the Journal of Environmental Horticulture (J. Environ. Hort.) 40(2):67–78, June 2022.

Jerry Weiland is research plant pathologist with the USDA Agricultural Research Service in Corvallis, Oregon. Contact him at [email protected]. Caroln Scagel is a research leader with the USDA Agricultural Research Service in Corvallis. Contact her at [email protected]. Niklaus Grunwald is research plant pathologist with the USDA Agricultural Research Service in Corvalis. Contact him at [email protected]. Bryan Beck is biological science technician with the USDA Agricultural Research Service in Corvallis. Reach him at [email protected]. Jesse Mitchell is biological science technician with the USDA Agricultural Research Service in Corvallis. Reach him at [email protected]. Anne Davis is research technician with the USDA Agricultural Research Service in Corvallis.