Phytophthora crown and root rot can cause tree loss in walnut. The disease is caused by species of Phytophthora, a soil-inhabiting fungus-like organism. More than 10 species of Phytophthora attack walnut, some invading the root crown or trunk base and some invading the roots, which can cause tree decline and death.

“Sometimes it causes a canker, which is crown rot where you see the dead tissue moving up the trunk. And sometimes it kills fine roots,” said Jaime Ott, UCCE orchard systems advisor for Tehama, Shasta, Butte and Glenn counties.

There are a lot of organisms that cause cankers the same way there’s a lot of viruses that give you a runny nose, Ott said.

Flooding

Flooding in walnut orchards can cause problems with both waterlogging and Phytophthora. Flooding while trees are dormant is less of a concern, but there is an increased risk for Phytophthora disease with flooding, Ott said. If the trees are out of dormancy, the risk of Phytophthora infection is higher.

“If the trees are starting to come out of dormancy, flooding can be really bad,” Ott continued.

If the trees are flooded with moving water while they’re dormant, the roots are less likely to die from lack of oxygen (waterlogging) because moving water has more oxygen in it. But if the water is stagnant, and the trees are sitting in stagnant water, that’s the more likely scenario that the roots will die from lack of oxygen regardless of whether there is Phytophthora present, Ott said.

Management to Avoid Infection

There are preventative measures that growers can take. In the fall, Ott encourages growers to check their drainage systems to make sure there aren’t weeds or other things blocking them that would back up and flood the orchard.

“Make sure that your culverts and your drainage ditches are working the way they’re supposed to,” Ott said.

Another cultural control measure is to plant the trees on berms so if there is flooding, the water won’t be against the trunks.

“When you’re planting, this is worth thinking about. If I’m going to get a flood, even if it’s once every five years, or once every 10 years, over the course of an orchard’s lifespan that happens a lot. It’s a lot of time you might be losing trees, so it’s worth thinking about, ‘How is this field going to drain? Where should that water go? How can I make that happen better?’” Ott said.

Early rain in November 2024 concerned Ott because the trees weren’t completely dormant. “They’re shutting down, but they’re not shut down, so they might be more susceptible to disease with this flooding,” she said.

Ground cover is another preventative measure. Water tends to infiltrate better in orchards that have a cover crop or native vegetation; there is also less runoff and less water standing on the surface of the soil. The extra organic matter also provides orchard access faster because the water soaks in rather than pooling on the surface, Ott said.

Preventative Measures

• Plant walnuts on well-drained soils that aren’t prone to flooding, long periods of saturation, pooling or high water tables.

• Encourage good soil drainage with proper soil preparation, such as deep ripping, slip plowing or backhoeing; grading and leveling. Avoid soil compaction.

• Plant trees on berms or mounds to avoid water pooling around the root crown.

• Avoid standing water for more than 18 hours because spore production increases after this. Avoid irrigation water hitting the limbs or trunk.

• Choose a resistant rootstock. The clonal paradox rootstock RX1 is resistant to Phytophthora.

• Plant a cover crop for less standing water on the soil surface.

Symptoms

Symptoms may not appear until May when temperatures increase, then the trees will begin to collapse. “That’s because they’ve actually been infected in the fall and all winter long, but it’s not until the weather warms up and they’re really trying to move water to the leaves that you notice there’s something wrong,” Ott said.

Symptoms could include:

• Smaller leaves and sparse foliage.

• The tree is off-color.

• The tree goes dormant faster; the leaves are changing color or it’s defoliating.

• Bleeding canker with liquid dark streaks running down the trunk.

These are usually signs something is wrong with the tree, Ott said, emphasizing these symptoms are not specific to Phytophthora and to look deeper to find out what is going on.

“If I was looking down a line of trees and I saw one tree was going dormant now, or a week earlier than everybody else, or in the spring if I’m looking down my row of trees and I see a couple trees that are off-color or starting to collapse, that’s when I would start to look really closely at the trunk and see if there is a bleeding canker,” Ott said.

Cutting into the canker will reveal a distinct region of black, dead tissue. “The diseased tissue is black. It’s really, really dark, almost completely black, and this is under the bark,” Ott said, adding this could potentially be Phytophthora crown rot, but there are other things that cause cankers.

This is when Ott advises calling out a farm advisor or PCA and sending a sample in for testing to know exactly what it is because Phytophthora crown rot can be confused with paradox canker disease, or “lethal paradox canker,” that can also cause trunk bleeding and a canker coming up from the ground. These cankers have a little bit of a different look. The canker grows as concentric lobes, or kind of rounded lobes, while Phytophthora tends to be a slightly spikier canker, Ott said, adding it is hard to tell them apart even with a lot of experience until the lab results are in.

In terms of root rot, the symptoms are a lot harder to see,” Ott continued, adding the tree must be dug up to look for a lack of fine roots.

“But again, it’s not really something you can know without digging up a big chunk of roots. Maybe you don’t have to dig the whole tree up, but you definitely have to expose some of the roots so you can see whether those fine roots are present or not,” Ott said.

There are multiple things that could cause a lack of fine roots, Ott continued. “Nematodes feeding on the roots can cause a lack of fine roots. Phytophthora can certainly rot the roots off. But also, if the trees are just sitting in water, even if you don’t have Phytophthora, the roots can suffocate from sitting in water, and you will lose fine roots from sitting in saturated soil.”

Flooding in general isn’t conducive to good tree health, Ott said. She encourages growers to take a sample and have it tested so they don’t go to the expense of using a material to treat for Phytophthora when the problem might have been something else.

Treatment

Phosphite treatments have been shown to reduce the severity of canker development in walnut. Phosphite treatments for walnut have been most effective when applied as a foliar spray and provide a few months of protection.

“Fall phosphite sprays are not as expensive as other treatments,” Ott said, and it will protect the trees from Phytophthora through spring. “It’s kind of an insurance spray.”

If a fall spray of phosphite isn’t done, there is Orondis, but it’s more expensive. Orondis is effective against Phytophthora and has a curative as well as preventative action whereas phosphite is only preventative.

“Your tree can get infected. It can have a canker, and if you treat it with Orondis, that canker will stop growing and it will kill the Phytophthora in the tree tissue,” Ott said.

Other treatments would include removing an infected tree. Ott advised removing the infected trees to avoid spreading the soil around.

“If you use a backhoe to get the tree up, you don’t want to sprinkle that soil from the root system around the other trees. However, chances are good that if your orchard is flooded and you get the infection in a couple trees, the Phytophthora is in the orchard already,” Ott said.

Moving the soil as part of the tree’s removal increases the risk of contaminating other trees with Phytophthora.

“If it’s in the soil, it’s not easy to treat, and even fumigating I wouldn’t expect to kill 100% of the Phytophthora in the soil,” Ott said.

If replanting after the tree is removed, Ott recommended using RX1 rootstock, which is tolerant to Phytophthora.

Keep Records

If there is flooding in the orchard, document where the flooding happened with photos, so in May or in the fall, if there are issues with the trees, it can be related back to the flooding.

“I think documenting flooding in your orchard is important so you remember what the problem is, but also so in six months when your trees start to die, it’s not such a mystery. ‘Oh yes, I have pictures, and weirdly, the flooding corresponds really closely to where the trees are dying,’” Ott said.