While the name Phytophthora might not mean much to you, these organisms are a menace to agriculture and forests alike. The name is derived from Greek and tells you what you need to know about them: plant destroyers.
Once thought to be fungi, these water molds are now considered a separate type of organism. However, they are as aggressive and destructive as any fungus.
More than 100 species of Phytophthora cause a complex of diseases on hundreds of kinds of plants. Several different species can infect temperate fruit trees such apple and crabapple, pear, peach, apricot, plum, and cherry and also tropical fruit trees such as oranges, limes, and lemons.
Difficult to diagnose, these organisms can kill the fine roots (root rot), damage the roots right below the soil surface (crown rot), and rot the tree above the union (collar rot).
Read on to learn how to distinguish Phytophthora infections from other fruit tree pathogens and steps you can take to prevent these debilitating diseases.
What You Will Learn
- Collar Rot Symptoms
- Crown Rot Symptoms
- Phytophthora Can Infest Soils for Years
- How to Prevent Crown and Collar Rot
- What to do If Your Tree Comes Down with a Phytophthora Rot
- Phytophthora is a Menace to Fruit Production
Collar Rot Symptoms
You may not even realize your trees are infected until you discover that the scion has been totally girdled, and your trees are on death’s doorstep.
Look at the lower part of the scion, and you may be able to see a depressed canker that can be purple, grey, or dark brown. A dead giveaway can be a gummy exudate under the dead bark.
Another symptom can be early ripening of the fruit, which may be small and highly colored.
Armillaria root rot (the honey mushroom) is another devastating disease that can cause similar symptoms. Armillaria infections start at the roots and move upwards, while Phytophthora collar rot starts at the crown and spreads down towards the roots.
Another way to distinguish these two infections is to look at the decayed tissue. The honey mushroom produces fan-shaped mycelia.
However, it may take a lab test to prove that the rot is due to Phytophthora. This is worth having done, since the future of your trees may depend upon this knowledge!
This disease can be intermittent, too. It may not be a problem for several years. However, with a prolonged period of cool, wet weather in the spring before blooming – boom, your trees can suddenly be infected.
Crown Rot Symptoms
Diagnosing crown rot can be a judgment call, since the trees show very similar symptoms if the crown or the fine roots have been subject to rot.
Your tree may have an obvious decline that could be due to many factors, including fire blight and winter injury.
Remove the soil from around the trunk and look at the crown. Peel back the bark from the trunk. If it is infected with Phytophthora, the bark will be orange or brown instead of green.
Phytophthora Can Infest Soils for Years
Once this pathogen is in your soil, it will stay there. Phytophthora produces two types of spores that can lurk in the soil for years. Both oospores and chlamydospores have thick walls and can resist weather like drought or freezing.
Unfortunately, these spores are very resistant to treatment with chemicals. As soon as spring arrives, the oospores can germinate in crevices in the bark.
Another problem with these organisms is that they can spend the winter in trunk cankers. The mycelia can overwinter there. As soon as the temperatures start warming up, they are poised and ready to infect the trunk or root collar.
And it gets worse! When conditions get favorable, a few oospores can turn into large numbers of spores that can swim in films of water. These spores, called zoospores, can infect large numbers of trees in a short period of time.
How to Prevent Crown and Collar Rot
While there are some steps you can take if your tree isn’t too badly damaged, your best bet is to try and avoid the problem. Here are some tips on ways to do this:
1. Grow Resistant Cultivars and Rootstocks
While resistant rootstocks can help prevent collar and crown rot, no one rootstock is resistant to all the species of Phytophthora that can infect various temperate fruit trees.
Knowing what kind of Phytophthora is in your soil can help to determine which rootstock to choose. You should consult with experts in your area to determine which rootstocks and cultivars are likely to resist infection.
2. Minimize Wet Soil
Try not to plant in soil that drains poorly, has a lot of clay, and is damp and low. Planting on a slope will provide the most drainage.
Channel water away from the trunk – do not allow a saucer to form in the soil around it. And make sure that you don’t flood your trees if you irrigate them.
3. Improve Your Soil Structure
You can limit the chances of your soil being excessively wet if it has a lot of organic matter and has low compaction. Work on building organic matter over time by incorporating compost, grass clippings, and other materials.
4. Use Biopesticides
Believe it or not, the soil is a vicious place. Microbes aggressively compete with each other by producing chemicals (like antibiotics) or even directly feeding on each other.
You can harness this competition to your advantage by inoculating the soil with beneficial microbes. A number of soil microbes can act as biopesticides, including the fungus Trichoderma.
You can buy special microbes to inoculate the soil. And you can also increase their natural populations by adding a lot of organic matter to your soil.
Some recent research suggests that you can apply these types of organisms as a dip before planting or on newly planted trees using the dripline.
5. Support Your Trees
We don’t mean emotional support. Young trees that rock back and forth in the wind can develop an open area around the base of the trunk that invites infection.
Make sure to use a trellis or stake, so your tree is properly supported.
6. Minimize Winter Injury
Trees that have been damaged in the winter are vulnerable to Phytophthora infection, so take steps to minimize winter injury.
The way to do this is to avoid new growth in the mid or late fall. For example, avoid fertilizing in the late summer or early fall.
7. Plant Your Tree Shallow
If you plant your fruit tree more deeply than it was in the nursery, it can cause two problems. One is that it will be more susceptible to collar rot. The other is that you can lose the dwarfing when the cultivar strikes roots.
What to Do if Your Tree Comes Down with a Phytophthora Rot
If your tree has a severe infection, you should remove it and declare that area a loss for fruit prodction. However, if the infection is in its early stages, there are some steps you can try.
Expose the entire cankered area of the tree and let it dry. You can do this by cutting away all the diseased tissue and leaving the trunk area open. Then put fresh soil back in the late fall.
You can also try grafting a one-year-old resistant cultivar whip into the trunk. Be sure and put it much higher than the canker. This will not work if 25-50% or more of the trunk is infected.
If the infection isn’t too severe, fungicides may be another option. Since water molds aren’t fungi, your choice of fungicides will be restricted.
You can treat the root zone and other infected areas with metalaxyl or mefenoxam. Be sure to treat the entire root zone or area of infection. Or you can spray the trunk and root zone with phosphorous acid or potassium salts or fosetyl. Do not mix these fungicides with a copper spray.
Phytophthora is a Menace to Fruit Production
With hundreds of species of Phytophthora infesting soils all over the world, it can be difficult to avoid these insidious organisms.
You can take steps to prevent infection by not planting your trees in wet soil and keeping the area around them from flooding.
However, if you have severe Phytophthora rots in the collar, crown, or roots of your trees, the disease will persist in the soil. You may have to forego planting apples and crabapples, pears, peaches, plums, apricots, cherries, or citrus fruits in that location.
And be sure to check out some of these guides for other tips and tricks in preventing or treating various fruit tree diseases or physiological conditions:
- What’s the Difference Between Tree Burr Knots (Burl) and Crown Galls?
- How to Identify, Prevent, and Treat Gummosis on Fruit Trees
- How to Manage Root Rot in Fruit, Nut, and Landscape Trees and Shrubs
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About Helga George, PhD
One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the knowledge that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.