Questions on: Apple Diseases

Ron Smith, Horticulturist, NDSU Extension Service


Q: I know you've had hundreds of questions concerning apple scab. I have two flowering crabs that are just starting to grow leaves. Should I spray with Captan now? Is it too late to apply lime sulfur? (e-mail reference)

A: If the leaves have begun opening, then it is too late for lime sulfur to be used. Go for the Captan or other labeled protective fungicides.


Q: I have recently moved to a home with a neglected apple tree that is at least 12-years-old. On one branch I noticed it was pealing almost like a snake shedding its skin. Do you know what that could be? Do you think it's possible for me to revive it? (E-mail reference)

A: Peeling bark is sometimes a characteristic of older trees. It sounds like it has been pruned very little over the years. I suggest tactful, focused pruning this spring before new growth begins, with the idea that you want to open the center of the canopy to allow for better air circulation and sunlight penetration. If you don't know what you are doing, get somebody else to do the pruning. Poor pruning has ruined many a good tree. No more than 25 to 30 percent of the crown should be removed in any given season.


Q: We have two apple trees in the backyard that are approximately 15 years old. The bark has peeled back as if burned and blistered on the trunks up to a point you would put on Tanglefoot Pest Barrier which we have been doing each spring. One of the trees was a very good producer of eating apples. This year it has dropped all of its apples. The other tree produces an abundance of pie apples which are also dropping, but to a lesser degree. Will the trees will recover or should we take them down? (Fargo, N.D.)

A: I would recommend waiting a year to see what happens. In the meantime, I suggest you clean up all fallen apples and leaf litter this fall, then wrap the trunk before the winter snow arrives. Early next spring before leaf-out, spray the trees with lime-sulfur to sanitize it. If the trees continue to decline after that, replace them.


Q: I was wondering the differences between fire blight and sodium damage in apple/pear trees. Are the black "scorched" leaf margins common in both? It has been very hot and very dry here (10 percent humidity and less). (Bowman, N.D.)

A: They are common to both, but the difference is that the salt scorched leaves are usually brittle and begin on the margin, with the branches having no "shepherd's crook" at the end, which fire blight damaged branches have.


Q: I have a question about my apple tree. It has always been healthy. It blossomed this spring, but now the bark is splitting away from the trunk on part of the west side and looking under that, there appears to be a couple of cracks in the wood. Three of the large branches now appear dead -- no leaves and blossoms are dried up. Do you know what the problem could be? (Langford, S.D.)

A: Sounds like a frost crack has developed (also called "sun scald"). Nothing you can do about it now. Hopefully the tree will heal over. Concerning the branches, it could be they have a girdling canker or damage from borers. In either case, I suggest removing the branches.


Q: I looked at an apple tree with a sooty black material on the top side. It rubs off but a little dark color stayed on the branch. The branch is about 1 1/2 in diameter, soot starts about 2 feet out on the branch-then goes for about 18 inches. Total branch length 7 feet. There are millers, millers everywhere. How do we get rid of them? (Harvey, N.D.)

A: The material on the apple branches is sooty mold, a saprophyte that lives on the surface of the branches and does not become directly parasitic. Left unchecked, it can interfere with the photosynthetic activity of the tree in some cases, weakening it to attack by parasitic organisms. It is easily controlled with insecticidal soap. Those millers or moths could be codling moths, among many other species. They are very vulnerable to pheromone traps, sticky traps, and to natural predators. The resulting larvae can be controlled with Bt applications, which is not toxic to the environment or non- destructive species.


Q: I hope that you can help me. In front of our house is an apple tree that was here when we moved in. It has been in great shape and produces a lot of apples, but we noticed over the past couple of weeks that now that the apples are dropping off like crazy, and all of the leaves have yellow spots on them that go completely through the leaf. On the underside of the leaf where the spot is, there is a wart-type thing with spiny spores about a quarter inch long. I can't think of any other way to describe them other than say that they look like hairy warts. If you touch these spores, they disintegrate. Also, some of the smaller branches are falling off now. It is breaking my heart and I do not know what to do. Any suggestions? (PS - We had a HORRIBLE drought here last year, and I am wondering if this strange fungus is because of the drought?) (E-mail reference, Pennsylvania)

A: It sounds like your apple tree has two very distinctly different problems: Cedar-apple rust (the leaf description you gave so well) and either codling moth or apple maggot larvae feeding on the developing fruit. Check some of the dropped apples by cutting them open and see if there is a grub or larvae (or evidence of one having been there) feeding within. Either pest will cause the fruit to drop prematurely. To protect the fruit from further damage, rake up and destroy all fallen apples, and spray the apples remaining on the tree with Sevin insecticide at 10 intervals until harvest. This fall, be sure to pick up any apples that have dropped. Next spring, spray the apples with the same material starting right after blossom drop. The rust fungus can only be controlled next spring. If you have any junipers in your yard, check those early next spring for a gummy, orange-colored glob with thread-like structures. Pick those off and destroy before they can "germinate" and spread to the apple tree. This is a disease that requires an alternate host to succeed; apple to juniper; juniper to apple, with the greatest destruction being borne by the apple. This disease too, will cause premature leaf drop. To control, spray next spring when the flower buds turn pink, with a fungicide that contains ferbam or zineb. Spray again when petal fall is about 75% complete, and once again 10 days later. If that tree has been around for a while, it is likely to survive this intrusive episode of disease and/or insect invasion.


Q. I am sending a clipping from our apple tree. We have sprayed it with Sevin several times, but don't see much improvement. Is it from the tough winter or are we losing our apple  tree?

A big flowering crab tree, probably about 30 years old, has leaves turning yellow and dropping off all summer long.

We have sprayed a viny lawn weed with Trimac and it seems all it does is tint the leaves a bit. If the solution is made stronger the grass will get a yellow tint, but the weeds still seem to exist. What do we do? Hankinson, ND)

A. I will start with the lawn weed. It is ground ivy, a very deep-rooted perennial. It should be controlled by Trimec. Make sure of two things:

1.You are using a formulation that contains Dicamba 

2.You are using a wetting agent to get better uptake by the plant.

Next, your flowering crab looks as if it could stand a shot of fertilizer. See the enclosed extension publication H-1035, "Fertilizing Trees," for guidelines for carrying this out.

Finally, stop spraying your apple tree with Sevin insecticide. You don't have insect problems. The new growth appears to be perfectly healthy. I think the old growth is   suffering from environmental setback the weather! Be patient.


Q. I am enclosing a couple leaves from our apple tree in the hope that they may be a clue to our problem.

Up to two years ago our apple tree (the only one in our yard and the nearest apple tree is a half block away) produced consistent crops of apples. Last year there were lots of blossoms that were very short lived and bore a single apple.

This year we had a fair amount of blossoms for a very short time to be followed by all the leaves curling and wilting with no sign of any fruit. Some branches have lost all of their leaves.

What is happening? The tree is probably 15 to 20 years old. Will appreciate your reply. (Maddock, N.D.)

A. Your leaf samples and your description indicate to me that one of two things, or possibly both, are happening to your tree:

1.Nectria canker this often occurs at the base of the tree, especially older ones like yours that have been wounded by repeated bumps from the lawnmower. It gradually girdles the tree.

2.Root rot again, often occurring on older trees.

It is not unusual for a fruit tree to bear heavily a year or two before it dies, which you indicated that yours did. Twenty years is about as good as one can hope for with apple trees.


Q: My apple tree leaves have all dried up and the bark is turning black. Enclosed are some leaves for you to examine. What kind of disease does my tree have, and is it still safe to harvest and eat the apples? I have other trees located near this one. Are they in danger also? (Lankin, N.D.)

A: Certainly go ahead and enjoy the apples! What I am seeing mostly on the leaf samples you sent are indications of root problems: a rise in the water table, root rot or a basal canker. With any of these, all you can do is wait for the tree to die.

The other trees are not necessarily in danger unless they are close enough for root grafting to take place. Sometimes trees afflicted like this take years to die, and other times they are history in one growing season.


Q: Can you tell me what we should do to treat our ash and apple trees with scale? (Breckenridge, Minn.)

A: Heavy infestations of scale need attention during the late winter or early spring while the trees are still dormant. Use dormant oil spray, and be sure to cover all branch surfaces.

You may have to repeat this a couple of years or more to get good control. But it is worth the effort to save ash trees. Dormant oil is effective and nondamaging to the environment. It also has essentially no mammalian toxicity when used properly.


Q: Can you tell me what is wrong with my apple tree? (Hankinson, N.D.)

A: Your tree is a plant pathologistís dream! I saw apple scab, fireblight, hail damage, and downy mildew. Your best bet is to clean up everything this fall -- leaves and apples. Then, next spring spray with dormant oil spray and lime sulfur. After the flower petals begin to drop, spray with Bordeaux mixture and repeat at least twice, in 10 to 12 days. If the tree doesnít improve after that, then cut it down.


Q: Enclosed are six leaves from my three apple trees. It appears they have a disease of some kind. My apples all developed well, but I always have another problem. I spray at least three times during the summer, yet about half the apples still have worms. We never had worms on our Bartlett pears until this year. I spray with regular fruit spray, with soap and with Malathion. someone suggested Sevin. Also, I planted a Sweet Sixteen about six years ago. This year it finally had two apples. Do they take that long to produce? (Mobridge, S.D.)

A: One of the best ways to control insect/disease problems is via good sanitation. Clean up fallen leaves and spray next spring with lime sulfur while the trees are still dormant. Yes, it does take six years to bear fruit.


Q: Can you tell me why my apple tree is drying up on the tips? What can I spray the tree with to make it stop? (Streeter, N.D.)

A: Your apple is getting hit with fireblight, a bacterium. You can spray it with Agristrep or Streptomycin to prevent further spread, along with cutting out affected branches. This is a common problem this year with the heat, rain and wind we've had across the region. So you have a lot of company!


Q: We have a plum tree where the fruit begins to grow then all of a sudden the fruit puffs up and falls off. What is the problem? We also have two apple trees that leafed out nicely, but now the leaves are shriveling up. We dusted with Sevin, but they are still sick looking. (Drake, N.D.)

A: The problem with the fruit on the plum is a fungal disease known as "plum pockets." Apply lime-sulfur in the fall after leaf drop or in the spring just before the buds break open.

The apple trees likely have a disease known as apple scab. Rake up all fallen leaves in the fall and spray next season with Captan or benomyl.


Q: Our two apples trees are dying. We thought it was wind damage, but now it looks more serious. Is there something we can spray them with, or is it too late? (Claire City, S.D.)

A: Your tree is being ravaged by fireblight bacterium. Prune and burn the diseased twigs this fall after several hard freezes. Spray next spring at blossom opening with Streptomycin antibiotic and again immediately after a damaging storm. In the meantime, try not to stimulate new growth as this is most susceptible to the disease.


Q: Could you please tell me what is wrong with my apples this year? They had these black spots on them that cover different areas of the apples. Is there any way to prevent this? (Hankinson, N.D.)

A: Your apples had plenty of company this last year with this common, but relatively harmless malady known as fly speck and sooty blotch—both typical fungi.

The fungi overwinter on twigs and buds, and during wet, mild spring weather, the fungi produce spores that are carried to apple fruit via wind or rain splash. If you were attempting to market your apples, you would have to hand rub each one vigorously to clean up the fruit for selling. For house consumption, the markings do not affect the eating quality.

Anything you can do to improve air circulation and sunlight penetration via selective pruning would help to control these disease symptoms. Spraying the tree with lime-sulfur next spring while dormant would do an excellent job of sanitizing the tree—acting as a fungicide, insecticide and miticide. This is all done with two plant elements, sulfur and calcium, with some wetting agents added.


Q: Our Haralred apples are speckled with gray-green mold or fungus. What causes this, and what can I do next year to prevent it? (Leola, S.D.)

A: Your apples have picked up some low-grade fungi known as sooty blotch and fly-speck. These are cosmetic fungi that may make the apple unappealing to look at, but neither affects the eating quality.

These fungi overwinter in the twigs and branches and develop on the fruit during mild, wet weather in the spring or fall. You'll find that the specks can be rubbed off.

Next spring, spray the tree with lime-sulfur before leaf-out. This usually acts as a good sanitizer of any overwintering diseases.


Q: I am sending you three of my apples that have spots on them, and I would like to know why? Most of the apples seem OK, but a few of them look like this. What can I do to stop this from happening next year? (Oakes, N.D.)

A: The apples you sent appear to be infected with the black rot fungus Botryosphaeria obtusa. This fungus can cause a "frogeye" spot on leaves, a dry rot of fruit and cankers that can kill entire limbs on trees. Remove all diseased twigs, branches, and clean up all fruit and leaf litter in the fall. Refer to the NDSU extension publication "Diseases of Apples and Other Pome Fruits" (PP-454).


Q: Can you tell me if there is anything I can do to save my apple tree? I have enclosed a picture of the tree. (Jamestown, N.D.)

A: Your apple tree is suffering from a fungus (Schizophyllum commune). The fungus probably entered the tree through the sun scald that seems apparent on your tree. Your tree may not completely recover, but you should not give up hope. If the wound on the tree completely closes, the fungus will not be able to spread, and your tree will survive. Encourage tree health by mulching, providing adequate water and protecting it from further injury.


Q: Can you tell me if there is anything I can do to save my apple tree? I have enclosed a picture of the tree. (Jamestown, N.D.)

A: Your apple tree is suffering from a fungus (Schizophyllum commune). The fungus probably entered the tree through the sun scald that seems apparent on your tree. Your tree may not completely recover, but you should not give up hope. If the wound on the tree completely closes, the fungus will not be able to spread, and your tree will survive. Encourage tree health by mulching, providing adequate water and protecting it from further injury.


Q: Enclosed are some leaves off a Radiant crab tree I planted about a year ago. I did not have this `blight' on it last year, and now it seems to be spreading to other trees. Help! (New Rockford, N.D.)

A: Your apple tree has a good dose of apple scab. Spray with Captan to prevent further spread of this fungus.

This fall, be sure to clean up all fallen leaves and apples. Next spring as leaves open spray again with Captan.


Q. I am enclosing a limb from an apple tree in our shelter belt. There are apples on one side of the tree but the other side looks dead. I am assuming it has a fungus and am wondering if you can identify it and tell me if there is a treatment for it, or if it has to be destroyed. The trees around it, including a crab apple tree, seem unaffected.

I enjoy your column. It has proven to be a source of invaluable information for me. (Tolna, N.D.)

A. The reason this fungus doesn't spread to other trees is because it is a saprophyte, not a parasite. Unlike a parasite, this organism lives off of decaying organic matter, the dead branch in this case, and does not move to parasitize the living branches.

The best treatment for this is to prune on an annual basis. This is carried out in the early spring, just prior to budbreak.

Thank you for the compliment about the column.


Q. Would you examine these samples of apple tree and tomato leaves and identify the problem they have?

I took a sample of the tree leaves to two nurseries. One said it was a blight and we bought the spray recommended and it didn't help. (Don't know what it was.) The other said it was a fungus and sold us Daconil. That didn't help either. What would you recommend? Do the tomato leaves have the same problem? I would appreciate your help, I would hate   to loose the trees. They are about 6 years old and bear apples every year.

I enjoy your articles. Thank you for your consideration. (Martin, N.D.)

A. The tomato plant has late blight, a relative of the blight that caused the potato famine in Ireland. Fungicides will not help at this stage. Try to plant in a different location next year and use resistant cultivars.

Your apple tree has scab and what appears to be a root problem. Application of fungicides must precede the appearance of the disease. I also suggest aeration around the dripline of the apple tree to possibly correct compaction, or other root anaerobic problems.

Enclosed is a copy of PP-659, "Disease Control in Home-Grown Tomatoes," and PP-454, "Diseases of Apples and Other Pome Fruits." Others may obtain copies of these publications at their local county extension offices or by contacting the NDSU Extension Distribution Center, Box 5655, Morrill 10, NDSU, Fargo, ND 58105-5655.


Q: What is the best treatment for apple scab? I have used Sevin to control apple maggots with good results. I believe apple scab is a fungus. Is that correct? Is a lime-sulfur mix about the best to control apple scab? When and how often should I spray the trees? Last year slugs ruined about 30 percent of my tomato crop. The slugs would remove the outer skin of the tomato and then a small black insect would then bore into the tomato. How does the slug live over winter? I cleaned the garden to be free of any material that would provide cover for the pests. I do have a strawberry bed at one end of the garden. I also had the garden tilled last fall. Is there any product that can be used before tilling the soil prior to planting? (Faulkton, S.D., e-mail)

A: Apple scab is a fungus that comes about with the humid weather of summer. There are some cultivars that are more susceptible than others, so selecting a resistant cultivar is the first step in limiting the incidence of this disease. Lime sulfur is a good preseason, or dormant, spray to sanitize the tree's surface. Do not apply after leaf-out. Once the leaves have emerged, use benomyl or captan fungicide, or something available locally that is Registered for scab control. Spray about every two weeks. This acts as a good preventer, but nothing really cures this disease. Slugs can be controlled just as you have done, through good sanitation. They can go into a "hibernation" that allows them to survive under leaf litter and other debris. Some have likely overwintered in your strawberry patch. There are plenty of slug control products on the market. Spreading egg shells or diatomaceous earth around each plant will also cut down on the damage. Hydrated lime will also work at controlling them.


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