Apple Tree Root Rot – Reasons For Root Rot In Apple Trees
We love our apples and growing your own is a joy but not without its challenges. All species of stone and pome fruit may be afflicted by fruit tree root rot, usually when the trees are in their prime fruit bearing years between 3-8 years of age. What are the signs of root rot in apple trees and is there a Phytophthora treatment for apple trees?
Apple Tree Root Rot Symptoms
The apple tree root diseases called crown rot is caused by Phytophthora cactorum, which also attacks pears. Some rootstocks are more susceptible to the disease than others, with dwarf rootstocks being the most vulnerable. It is often seen in low lying areas of poorly draining soil.
Symptoms of root rot in apples trees appear in the spring and are heralded by a delay in bud break, discolored leaves, and twig dieback. The most observable indicator of apple tree root rot is a girdling of the trunk wherein the bark browns and when wet becomes slimy. If the roots were to be examined, water soaked necrotic tissue at the base of the root would be evident. This necrotic area usually extends up into the graft union.
Phytophthora Apple Tree Root Rot Disease Cycle
Fruit tree root rot caused by this fungal disease can survive in the soil for many years as spores. These spores are resistant to drought and to a lesser extent, chemicals. Fungal growth explodes with cool temperatures (around 56 degrees F. or 13 C.) and ample rainfall. Hence, the highest incidence of fruit tree rot is during blossom time in April and during dormancy onset in September.
Collar rot, crown rot and root rot are all other names for Phytophthora disease and each refers to specific regions of infection. Collar rot refers to infection above the tree union, crown rot to infection of the root base and lower trunk, and root rot references infection of the root system.
Phytophthora Treatment in Apples
This disease is difficult to control and once infection is discovered, it is usually too late to treat, so choose rootstock with care. While no one rootstock is completely resistant to crown rot, avoid dwarf apple rootstocks, which are particularly susceptible. Of the standard sized apple trees, the following have good or moderate resistance to the disease:
- Grimes Golden and Duchess
- Golden Delicious
- Rome Beauty
- Red Delicious
Also important to combat fruit tree root rot is site selection. Plant trees in raised beds, if possible, or at the very least, channel water away from the trunk. Don’t plant the tree with the graft union below the soil line or plant in areas of heavy, poorly draining soil.
Stake or otherwise support young trees. Windy weather can cause them to rock back and forth, resulting in a well opening up around the tree that can then collect water, leading to cold injury and collar rot.
If the tree is already infected, there are limited measures to be taken. That said, you can remove the soil at the base of infected trees to expose the cankered area. Leave this area exposed to air to allow it to dry. Drying may prevent further infection. Also, spray the lower trunk with fixed copper fungicide using 2-3 tablespoons (60 to 90 mL.) of fungicide per one gallon (3.8 L.) of water. Once the trunk has dried out, refill the area around the trunk with fresh soil late in the autumn.
Lastly, reduce the frequency and length of irrigation, especially if the soil seems to be saturated for long periods of time which is an invitation to Phytophthora fungal disease when temperatures are mild, between 60-70 degrees F. (15-21 C.).
Sudden Death of Apple Trees
Growing apples (Malus spp.) in your home orchard brings an abundance of rewards, including billows of fragrant spring flowers and late-summer treats such as fresh apple juice and home-baked apple pies. The downside of growing apples is that you may wake up one morning to discover your trees have suddenly died. Fungal diseases working beneath the soil stealthily attack apple trees, with their above-ground symptoms often delayed until their root systems have irreparable damage.
Also called papery bark, sappy bark is caused by the fungal pathogen Trametes versicolor that attacks older apple trees. The fungal pathogen infects the apple tree through pruning cuts on large branches and causes the infected bark to decay, discolor and develop a spongy texture. The infected bark peels away from the tree, which exposes the decayed tissue underneath. Damp weather causes the infected bark to appear spongy, while the bark has a papery appearance in dry conditions. The diseased bark can seep sap, and cankers begin to form. These cankers may girdle the branches and -- if located on the trunk -- threaten the apple tree’s life. According to the University of California Agriculture and Natural Resources, these cankers grow only a few inches a year, so they can be removed from the apple tree before severe damage has occurred. To prevent sappy bark disease, keep the apple tree vigorous, and never leave stubs when pruning. Remove and destroy any diseased limbs and bark from the tree, and prune only during dry conditions.
How to Fight Phytophthora Crown, Collar, and Root Rot in Trees
Have you just started a garden in your new home only to learn to your dismay that the yard has been plagued with Phytophthora in the past? If so, or if you’re just looking for the best ways to identify, treat, and prevent Phytophthora crown, collar, and root rot in your trees, keep reading to learn exactly what you should do to keep this potentially fatal disease out of your soil and off of your plants.
The list of host plants for Phytophthora root and crown rot includes the following: andromedas (Pieris), apple tree (Malus domestica), apricot tree (Prunus armeniaca), azaleas (Rhododendron spp.), beech (Fagus), cherry tree (Prunus), dogwood (Cornus), holly (Ilex), juniper (Juniperus), peach tree (Prunus persica), true fir (Abies) and yew (Taxus baccata).
Identifying the Symptoms of Phytophthora Rot in Trees
The rot diseases caused by Phytophthora affect different areas of the plant depending on which disease the plant has contracted. Trees can be affected by root rot and crown rot simultaneously.
- Phytophthora collar rot results in girdling of the scions, which are the new growth shoots and twigs. Infected trees can develop a sunken canker that is gray, purple, or dark brown on the bark near where it was joined in grafting.
- Phytophthora crown rot affects the roots that lie right underneath ground level as well as the part of the tree trunk that is close to the ground.
- Phytophthora root rot infects the smaller fine roots, resulting in necrotic tissue and eventually, if the disease goes untreated, causing the death of the plant.
The phytophthora rots are especially troublesome in gardens that have heavy soil and a climate with abundant rain. Soil that stays damp for too long after irrigation or rain, as well as any condition in the garden that contributes to an overly wet environment, increases the risk of phytophthora rot. Infections become more likely the longer the soil remains too damp.
The organisms that cause Phytophthora rots are called oomyctes, which are similar to fungi. They are common in the soil of existing orchards. Phytophthora may also enter an orchard or garden by hitching a ride in soil, water, or on a newly introduced plant.
Trees suffering from Phytophthora crown or root rot won’t necessarily exhibit symptoms specific to their illness, instead seeming to become vaguely unhealthy. Most trees will have a long period of illness before they succumb to a fatal infection, but susceptible plants tend to die more quickly if they become infected with Phytophthora rots. Trees that do show more precisely identifiable signs of the disease normally have symptoms that resemble fire blight, malnutrition, wet feet (which occurs when standing water remains around the plant’s root system too long), or cold damage.
If you believe a tree in your garden may be experiencing a Phytophthora rot disease, you’ll need to perform a thorough visual inspection of the tree to confirm your suspicions. Examine every part of the tree carefully, starting with the roots at its base. You can move away some of the soil around the tree’s roots to get a better look. One way to check for Phytophthora rot is to gently peel back the outer layer of bark so you can see the color of the cadmium underneath.
On a healthy tree, the cadmium will be green, but in trees with Phytophthora, it ranges from orange to brown. Infected trees may have brown or orange roots that may shed and eventually fall away from the tree’s primary root. Slicing away the outer layer of bark on an infected tree will reveal discolored, diseased tissue, which may also be orange or brown.
The diseased tissue will be distinctly different from the healthy tissue, with the line between the two clear and easy to see. On the other hand, if the tree is suffering from rot due to overwatering instead of the Phytophthora organism, its roots will be brown and you will notice a distinct smell of decay.
Preventing and Treating Phytophthora Crown, Collar, and Root Rot
Now that you know how to identify Phytophthora rots in your garden, you need to know how to address an outbreak of these diseases if they do occur. Even if your property doesn’t have a history of Phytophthora crown, collar, or root or (at least not to your knowledge), a prevention routine is a best practice that every gardener should implement. Listed here, you’ll find techniques to avoid dealing with Phytophthora in your garden or to treat the diseases if the need ever arises.
Inspect vulnerable plants regularly so you can catch Phytophthora early if it strikes.
Now that you know exactly what Phytophthora crown, collar, and root rot would look like, create a routine so you’ll remember to inspect your trees for symptoms on a consistent basis. Especially during rainy seasons, keeping track of how your trees are doing will make it possible for you to diagnose a Phytophthora problem early. Catching these diseases before they have the chance to spread to other trees in your garden means less likelihood of severe or widespread infections.
Resolve any situation that causes excess moisture in the soil and lessen Phytophthora’s opportunity to spread.
When the soil in your garden is too waterlogged, Phytophthora rot diseases have the chance to pass from plant to plant. This is the only time you’ll notice new cases of Phytophthora. That‘s because one of Phytophthora’s forms is a zoospore, for which swimming is the only method of traveling from one place to another. The longer the soil remains in this slightly liquid state, the longer the zoospore has to find more plants in your garden to infect.
To lessen the risk of Phytophthora in your garden, keep soil at only the needed level of moisture, and reduce the time the soil remains wet after a rainfall however you can. One of the best ways you can do this is by choosing a place for plants susceptible to Phytophthora to grow that offers plenty of drainage.
Boost your soil’s drainage with an amendment that will improve its texture.
There are so many amendments on the market that you can mix into your soil to improve its drainage. Choose one of these treatments to give your soil a looser texture, which provides room for air to circulate and to allow water to flow through from the surface.
Organic materials like shredded dead leaves, well rotted compost, or clippings from mowing the lawn will do the trick just as well as a commercial product made to improve drainage in garden soil. Just spread your organic materials out in a layer two to three inches thick, then mix the amendment down into the top six to 12 inches of soil.
When you’re choosing new plants for your garden, go for Phytophthora-resistant varieties.
When you’re shopping for seeds or selecting some new plants for your garden this season, look for those marked as Phytophthora-resistant. You can also do a bit of research online to find which plants are recommended for gardeners in your USDA Hardiness Zone working with properties where Phytophthora has been a problem before. There are so many to choose from that you won’t feel too limited in your options by narrowing the field this way. Check the list we provide at the end of this article for resistant varieties to consider for your garden.
Do your gardening in the soil instead of in containers.
Container gardens are customizable or portable, so you can stow plants away in winter, and appropriate for pretty much anything you want to grow. But unfortunately, Phytophthora is most likely to strike plants when they grow in pots.
Whenever you can, choose direct planting instead of planting in a container. But if you must, make sure that the soil medium you use will facilitate drainage and that the container has holes in the bottom to allow excess water to escape. Nurseries and garden centers tend to use containers for their baby plants, so make sure to check any new plants over for signs of the disease before you bring them home.
Target the base of your plants when you water the garden.
It’s easy to water from above and let the liquid splash all over your plants and the ground around them. However, the moisture sitting on their leaves can lead to sunscald, which is caused when the sun burns the plant’s foliage. Another side effect of splashing water around instead of carefully controlling its stream from the garden hose is that you’re creating just the type of moist conditions where Phytophthora thrives. All you need to do is correct your aim. Point the water hose lower, where the plant meets the soil, and you’ll both get more hydration to its roots and help prevent Phytophthora in your garden.
Before winter sets in, make sure to clear all plant debris from the ground, or Phytophthora can hide there until next season.
It’s a good idea regardless of your garden’s situation to gather up all the plant debris you can and discard it or add it to the compost heap before cold weather sets in. (If you have any reason to suspect that a plant in your collection might have a disease or be infested with insects, don’t compost the debris from your garden, or you risk passing the problem on to your future self next spring. )
Take cuttings from parts of the tree that aren’t in contact with the soil.
Whenever you propagate a tree by taking cuttings from it, you have a chance to start the new cutting off right and reduce the chance it will carry Phytophthora diseases. Just take your cutting from a part of the tree that is growing high enough that it is free from contact with the soil. Because many Phytophthora organisms hide in the soil when conditions don’t allow them to travel, you’ll be avoiding a lot of the potential for disease in your new tree.
Minimize a bout of Phytophthora when you apply a nitrogen fertilizer.
The rots associated with the Phytophthora organism mainly impact a plant’s most recently grown, most tender shoots as the garden wakes up and starts putting out new foliage each spring. A low- to medium-sized dose of a fertilizer containing nitrogen will stave off Phytophthora. Nitrogen fertilizers have this effect because they stop plants from producing new, delicate shoots, so the Phytophthora has nothing to latch on to.
Throw out your used soil after each growing season.
Phytophthora doesn’t only hide in plant debris on the surface on the soil. The organisms also spend their winters in any planting medium they can find, often hiding out in potting soil. Although some gardeners may tell you it’s OK to recycle soil from one year and use it again the next, if you want to keep your garden free of Phytophthora, the savings just aren’t worth the risk. Start each new spring in your garden with a fresh new batch of soil.
Keep gardening tools clean and sanitized.
Before and after each gardening session that requires use of your tools, you should clean and sanitize them to avoid spreading diseases like Phytophthora. In fact, the best way to prevent diseases traveling from plant to plant as you work is to wipe your shears, pruners, and other tools down with rubbing alcohol between each plant you work on. It may seem like a pain, but this preventive measure really doesn’t take much time at all, and it goes a long way toward keeping your garden clear of Phytophthora and other plant diseases.
Plants Resistant to Phytophthora Crown, Collar, and Root Rots
As we’ve already mentioned, one of the easiest and most effective ways to avoid doing battle with Phytophthora rots in your garden is to choose resistant plants. That way, you’ll bypass the need for preventive treatments entirely. And although it can be nearly impossible to remove the organisms that cause Phytophthora rots from your garden, you’ll be able to relax knowing that your plants are safe from this potentially fatal disease.
However, keep in mind that even resistant or tolerant plants can be infected with Phytophthora when conditions are too wet or soil doesn’t drain well enough, so it’s still important to manage your garden’s moisture level.
Here’s a list of many of the resistant plants available to help guide you as you plan your garden for next season.
- Alyssum (Lobularia)
- Brazilian Vervain (Verbena bonariensis): Also called purpletop vervain and tall verbena
- Burford Holly (Ilex cornuta ‘Burfordii’)
- Butterfly Bush (Lantana)
- Chinese Holly (Ilex chinensis): Also called Kashi holly, Oriental holly, and purple holly
- Dawn Redwood (Metasequoia glyptostroboides)
- Floss Flower (Ageratum)
- Geranium (Pelargonium): Also called storkbill
- Glossy Abelia (Abelia x grandiflora): Also categorized as Linnaea x grandiflora
- Marigold (Tagetes)
- Mexican Sunflower (Tithonia rotundifolia)
- Morning Glory (Ipomoea purpurea)
- Moss Rose (Portulaca): Also called little hogweed and purslane
- Ornamental Cabbage (Brassica oleracea var. acephala)
- Ornamental Kale (Brassica oleracea)
- River Birch (Betula nigra): Also called black birch, red birch, and water birch
- Summer Snapdragon (Angelonia)
- Sweet Potato Vine (Ipomoea batatas)
- Sweetshrub (Calycanthus): Also called spicebush
- Tree marigold (Tithonia diversifolia): Also called Japanese sunflower, Mexican sunflower, Mexican tournesol, Nitobe chrysanthemum
- Winterberry (Ilex verticillata)
- Wintersweet: Also called Japanese allspice (Abelia)
- Yaupon Holly (Ilex vomitoria)
When you make it a priority to keep diseases like Phytophthora out of your garden, you’ll need to start a preventive control regimen. Luckily, most of what you need to do to keep plant diseases and infestations at bay is a best practice in general, even if your plants are the healthiest on the block.
You’ll find that management techniques like these are actually easy to put into place and keep up with. So pick the ones you think will fit best into your gardening style and start building a healthier environment where your plants can really thrive now.
(Listed in Order of Importance)
Importance as a Pest onВ Apple:В high
Other Fruit Hosts: crabapple and pear
General Info:В Codling moths are the adults of the common вЂњwormsвЂќ that infest apples. These moths emerge from overwintering sites in the spring and lay their eggs on and near developing fruits. There are up to 3 generations per season, from spring to late summer.
- frass (sawdust-like excrement) on the outside of the fruit
- small holes in fruit and if still present, white larva in fruit
- fruit drop
- fruit rot that is associated with entry/exit holes
Management:В The key to successful management by the backyard orchardist is a combination of cultural practices and accurately timed insecticide sprays. Cultural practices include fruit thinning, removing infested fruits, and bagging fruit. Codling moth activity depends on temperature, and the time to treat varies from year to year. To find out when codling moth is active in your area of the state and for spray recommendations, contact your local county Extension agent and subscribe to the USU IPM Tree Fruit Advisory. Insecticides include carbaryl, malathion, gamma-cyhalothrin, and spinosad.В
Importance as a Pest onВ Apple: moderate
Other Fruit Hosts:В all fruits
General Info:В Mites are very small arthropods that are more closely related to ticks than insects. Spider mites overwinter as adults at the base of trees, or in ground cover, and may become a problem during hot, dry conditions in mid and late summer when they reproduce rapidly. They remove sap and chlorophyll from leaves causing a stippling appearance.
- stippled leaves
- scorched leaves due to heavy feeding (shown at right)
- fine silk webbing that becomes apparent when populations are high
Management:В Predatory mites that feed on spider mites can provide effective biological control if they arenвЂ™t harmed by pesticides. Low populations of spider mites can be ignored and are often kept in check by the predatory mites. Spider mite outbreaks often follow pesticide applications that upset the predator-prey balance. Options to reduce spider mite problems include washing down trees or plants with a stiff spray of water or applying insecticidal soap or 1% horticultural oil every 5-7 days. Avoid applying soaps or oils during the hot part of the day as some leaf burn may result.
Woolly Apple Aphid
Importance as a Pest onВ Apple: moderate
Other Fruit Hosts:В none
General Info:В The woolly apple aphid overwinters on roots as well as in protected areas of the canopy. As the weather warms, they move up from the roots, and colonies in the canopy start multiplying. They first become noticeable in late June as sticky, cottony colonies (shown at top right) at the base of leaves, on old pruning cuts, or on cracks and crevices. Their feeding on twigs and roots causes galls (shown lower right). Extreme cold winters will kill most aphids overwintering in the tree canopy.
- galls on roots and twigs that sometimes crack
- cottony masses on twigs and wounds
- curled leaves
Management:В Woolly apple aphids are controlled with careful management. The key is to inspect apple bark carefully for the formation of new aphid colonies, and when they are found, treat them immediately with spot sprays. Conventional options are a pyrethroid (products ending in вЂњthrinвЂќ) or carbaryl. An organic option is 1% horticultural oil mixed with the labeled rate of insecticidal soap.
Aphids(Rosy Apple Aphid, Green Apple Aphid)
Importance as a Pest onВ Apple: low-moderate
Other Fruit Hosts:В none
General Info: Aphids suck sap from the phloem vessels and reduce tree vigor. They exude sticky honeydew as they feed. Adults feed on leaves as well as apple fruits causing deformities. They overwinter as eggs on apple trees. Rosy apple aphid migrates to alternate weed hosts for the summer.
- deformed fruit (caused by rosy apple aphid colony shown at right)
- curled and sticky leaves and black sooty mold
Management:В The home orchardist can usually ignore aphid infestations unless the populations are extremely high, growth of young trees is being stunted, or black sooty mold is staining the fruit. Numerous beneficial insects (e.g., lady beetles, lacewings, and syrphid flies) help suppress aphid populations, so conserve and protect these natural enemies. A 2% oil application when buds start to leaf out will smother eggs. In summer, insecticidal soap or 1% horticultural oil-works well.
Flatheaded Borers(Flatheaded Apple-tree Borer and Pacific Flatheaded Borer)
Importance as a Pest onВ Apple:В low-moderate
Other Fruit Hosts: cherry and plum
General Info: Flatheaded borers are beetles (shown at right), usually only a problem on apples stressed by drought conditions, or when pest populations are high. The beetle larvae girdle trunks and can kill limbs and trees. The adult beetles are active in June and July.
- sawdust-like frass (insect excrement) on bark
- loose, flaking bark
- large oval exit holes on large limbs or trunk (right)
- dead limbs
Management:В Apply protective trunk sprays to prevent larvae from entering trees in June and July. Carbaryl, zeta-cypermethrin, or gammacyhalothrin are options. Keep trees healthy with optimal watering, fertilization, pruning, and removal of infested limbs and trees.
San Jose Scale
Importance as a Pest onВ Apple: low-moderate
Other Fruit Hosts: stone fruit trees
General Info:В San Jose scale is the most common scale insect to attack apple. Scales will feed on bark and apple fruit, creating small red halos with white centers. The soft body of the insect is hidden underneath an armored shield. Females produce young that crawl from under the mother scale before settling to feed. The вЂњcrawlersвЂќ are active in the late spring. If heavy infestations are not controlled, a tree can be killed.
- peppery-looking flakes on apple fruit (shown right) which are the scale bodies
- limbs encrusted with small, circular, black and gray armored scales
- small red halos with white centers on apple
Management:В A 2% oil application at half-inch green stage will kill overwintering immature scales (but not adults). Adults are difficult to kill. To kill crawlers, apply a pyrethroid (products ingredients ending in вЂњthrinвЂќ) or carbaryl in early June. (Adults die after crawlers hatch.) For non-chemical control, scrub branches with soapy water during the dormant season.
Apple-Leaf Blister Mites
Importance as a Pest on Apple:В low
Other Fruit Hosts: crabapple
General Info:В Blister mites are microscopic mites in the eriophyid group. They burrow under the lower surface of leaves and feed within small galls. Adults overwinter under leaf bud scales and emerge with new leaf growth in the spring. Very high populations can reduce photosynthesis and tree vigor. Trees easily tolerate lower populations.
- вЂњblistersвЂќ that start green and turn brown as they age (shown at right)
Management:В Treat large infestations in early fall, before leaves drop. At this time, mites are migrating from leaves to buds and are exposed. Insecticide options include carbaryl, sulfur, or 1.5% horticultural oil.
Importance as a Pest on Apple:В low
Other Fruit Hosts: pear and stone fruits
General Info: Lygus bug, stink bug, and boxelder bug sometimes feed on young fruits with their piercing-sucking mouthparts and cause depressions in the fruits by killing plant cells. These dead cells result in indentations and distortions in fruit shape as the fruits mature.
- scarring that resembles a catвЂ™s face with puckered cheeks (shown at right)
- pits and sunken areas on fruits
- reduced fruit quality and storability
Management:В Usually cat-facing injury is not severe enough in backyard trees to warrant treatment.
Speckled Green Fruitworm
Importance as a Pest on Apple:В low
Other Fruit Hosts:В cherry, pear, and plum
General Info:В In Utah, the speckled green fruitworm is sometimes a pest of fruit trees. Young larvae hatch in spring and begin feeding on new leaves, flowers, and young fruit. They are active for approximately 6 weeks in spring and there is just one generation per year. The larvae can be detected by shaking branches over a tray.
- chewed leaves
- early fruit drop
- round, deep holes in fruit (shown at right) that heal as a scar or sunken area
Management: A single application of a reduced-risk insecticide after fruit starts forming, such as Bt (Bacillus thuringiensis) or spinosad, is very effective. There are many brands of insecticides that contain these active ingredients.
White Apple Leafhopper
Importance as a Pest on Apple:В low
Other Fruit Hosts: cherry and crabapple
General Info:В The white apple leafhopper sucks sap from leaves. The white flying adults can also be a nuisance when picking fruit. The adults are wedge-shaped with wings meeting in a sharp peak over the back. Nymphs are white and flightless
- stippled leaves (yellow specks)
- black spotting on fruit (вЂњtar spotsвЂќ)
Management:В Best control is achieved when leafhoppers are still in the immature stage (nymphs) in spring. Nymphs feed on the undersides of leaves, so good coverage of lower leaves with insecticides is necessary. Options include carbaryl, spinosad, or insecticidal soap(organic).
Apple (Malus spp.)-Crown and Collar Rot
A shallow cut with a pocket knife shows healthy white to greenish tissue above and diseased dry brown necrotic tissue below.
OSU Plant Clinic Image, 2016.
Even without cutting into the tissue one can see the slightly sunken necrotic area near the base of this tree.
Discoloration of the vascular cambium of this 'Honeycrisp' apple above the graft line would be referred to as collar rot.
Soil has been removed to expose a dead, infected, discolored root.
Photo by Jay Pscheidt, 1991.
Crown rot can also occur on young trees. The outer bark has been cut away to expose the vascular cambium where healthy tissue above is a normal light green while the diseased tissue is a dark cinnamon brown below.
Cause Phytophthora cactorum and other species, a soilborne fungus-like microorganism. Crown rot is a disease of the rootstock portion (or root crown area) of the tree collar rot is a disease of the scion portion. Both are serious diseases of apple and other orchard trees in British Columbia, Washington, and Idaho, and have become a problem in Oregon orchards with clonal rootstocks, principally Malling Merton (MM) 106. Crabapple can also be infected. Pear scions are more susceptible than apple but popular pear rootstocks are much more resistant.
The fungus survives primarily as oospores in soil, organic debris, or infected tissues. Oospores produce a swimming spore stage (zoospores) when soils are at or near saturation. Zoospores swim to and infect roots. Movement within roots to the root crown is greatest between pink bud and shoot elongation. Saturated soils due to high rainfall, excessive irrigation, and/or poorly drained soils favor infection and disease development.
The fungus Athelia rolfsii (formerly) Sclerotium rolfsii has been isolated from trees with collar rot symptoms in eastern Washington. Overwatering was among the various issues in these orchards.
Symptoms In early fall, an affected tree shows bronzing, purpling, or yellowing foliage of one or more limbs, accompanied by bark reddening. There is a reduction in the size of leaves and terminal growth. Examination at the root crown or collar after scraping away the soil reveals dead bark. The cambium will be orange-brown to red-brown, eventually becoming dark brown instead of white. A distinct margin may separate healthy from infected tissues. In many cases, the tree may be completely girdled before its condition is noticed. Fire blight symptoms may be similar when confined to the rootstock.
Moderately resistant: MM111, M2, M7, M26, 'Golden Delicious', 'Delicious', and 'Rome Beauty'. Susceptible: MM104 and MM106.
Cultural control Water management that minimizes excessive water around the root crown is critical for management of this disease.
- Locate orchards on well-drained slopes if possible.
- Tile orchards or sites with poor drainage.
- Plant on a raised bed to help keep water away from trunks.
- Use resistant rootstocks. 'Antonovka', 'McIntosh', and 'Wealthy' apple seedlings, and M9 clonal rootstocks, as well as B9 and advanced selections in the Geneva series have shown high resistance to collar rot.
- Avoid the application of too much irrigation water.
- Do not let irrigation water repeatedly hit trunks.
- During summer, examine root crowns of trees for collar rot and scrape off diseased tissues. Leave root crowns exposed to the air until late fall.
- Avoid wounding root crowns. If a wound is made, keep it uncovered and open to the air for the rest of the season.
Chemical control Apply before Phytophthora symptoms appear, especially in orchards favorable for disease development. No chemical will revitalize trees showing moderate to severe crown rot symptoms. Although resistance has not been reported, alternate materials so resistant fungi do not develop quickly.
- Agri-Fos at 1.25 to 2.5 quarts/A. Do not combine with a copper-spray program for control of other diseases. Group P7 fungicide. 4-hr reentry.
- Aliette WDG at 2.5 to 5 lb/A. Spray foliage to run off. Follow manufacturer's directions for timing of spray. Do not apply within 14 days of harvest. Do not combine with a copper spray program for control of other diseases. Phytotoxicity may result if applied within 1 week of a copper spray. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Fixed-copper products. Use 4 gal solution as a drench on the lower trunk of each tree in early spring or after harvest. Do not use if soil pH is below 5.5. Not considered organic since application is to the soil. Group M1 fungicides.
- Champ WG at 4 lb/100 gal water. 48-hr reentry.
- Copper-Count-N at 4 quarts/100 gal water. 48-hr reentry.
- Cuprofix Ultra 40 Disperss at 2.5 lb/100 gal water. 48-hr reentry.
- Kocide 3000 at 1.75 lb/100 gal water. 48-hr reentry.
- Nordox 75 WG at 2.5 lb/100 gal water. 12-hr reentry.
- Nu-Cop 50 DF at 1 lb/25 gal water. 48-hr reentry.
- Fosphite at 1 to 3 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. May also be injected into trunk. Group P7 fungicide. 4-hr reentry.
- MetaStar 2E at 1 quart/100 gal water. Apply diluted mixture (based on trunk size measured at 12 inches above the soil line) around each tree trunk. Group 4 fungicide. 48-hr reentry.
- Organocide Plant Doctor at 2.5 to 5 teaspoons/gal water as a foliar spray or at 16 fl oz/16 fl oz water plus Pentra-Bark as a basal trunk spray. Group P7 fungicide. H
- OxiPhos at 1.3 to 5 quarts/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phospho-Jet is registered as a basal bark spray at 62.4 fl oz in 62.4 fl oz water plus 3 fl oz of Pentra-Bark. Spray first 5 feet of trunk including scaffold limbs until runoff. Group P7 fungicide. 4-hr reentry.
- Phostrol at 2.5 to 5 pints/A. Group P7 fungicide. 4-hr reentry.
- Rampart at 1 to 3 quarts/100 gal water/A. Do not use copper products within 20 days of treatment . Can also be trunk injected. Group P7 fungicide. 4-hr reentry.
- ReLoad at 2.5 to 5 pints/A. Do not use copper products. May also be injected into trunk. Group P7 fungicide. 12-hr reentry.
- Ridomil Gold SL at 0.5 pint/100 gal water. Apply diluted mixture (based on trunk size measured at 12 inches above the soil line) around each tree trunk. Apply once at planting or in spring before growth starts. Apply again in fall after harvest. Group 4 fungicide. 48-hr reentry.
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:
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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 childhood discovery 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.