Elisabeth C. Miller Library logo Miller Library Home UW Botanic Gardens Home UW Botanic Gardens Home book graphic

3501 NE 41st Street, Seattle, WA 98195 | (206) 543 0415 | Open: | Library Schedule

Gardening Answers Knowledgebase

Plant Answer Line Question

Keywords: Prunus, Fruit--Diseases and pests

I have a flowering purple plum tree. For the last two years it has had black knobby growths on the limbs. The number of these growths are increasing and there is no sign of any type of bug involved. The tree is healthy in all other respects and the growths remain on the limbs all year. I cannot find anyone who knows what these are and if I need to do anything to stop and/or remove these growths. Obviously they are ugly but probably not fatal and do not spread to any other trees. Can you give me a clue?


We can only guess from your description, and in order to get an accurate diagnosis you will need to take a sample (including both healthy and affected parts if possible) to a Master Gardener clinic. Click on the appropriate link for your local clinic through Master Gardeners / Washington State University Extension.

Meanwhile, for information about common diseases of plums in the Pacific Northwest, try searching Washington State University's HortSense website.
The symptoms you describe are similar to 1. Crown gall, 2. Black canker and 3. Black knot. Click on those diseases for descriptions, photos, and control methods.

You can also take a look at pages like this one, on black knot of ornamental cherry and plum, from Morton Arboretum. See if the images resemble what you are seeing on your tree.

Date 2017-05-17
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Sawflies, Ribes, Disease-resistant plants, Master gardeners, Fruit--Diseases and pests

We moved into a new house which has a large currant or gooseberry bush. Now that it has leafed out there are numerous caterpillars eating the leaves. I know they are not tent caterpillars, but I cannot identify them. They are whitish-green with yellow bands across the top and bottom, with many black dots or bumps. The head and first six legs are black. It would be nice to learn more about them.


I cannot make a conclusive pest identification remotely, but there is a possibility these caterpillars are currant sawfly, or imported currantworm. Here is some information about this pest from Colorado State University Extension.

If this pest is the culprit, the book, The Organic Gardener's Handbook of Natural Insect and Disease Control edited by Barbara Ellis (Rodale Press, 1996) recommends using Pyrethrin spray, spraying into the center of the bush. Bees are severely affected by this pesticide, so follow the guidelines shown on this University of California, Davis >a href="http://ipm.ucanr.edu/TOOLS/PNAI/pnaishow.php?id=62">Integrated Pest Management page. Also see this guide to reducing bee poisoning from pesticides.

For a definitive pest identification, you may want to bring a sample of the pest and its damage to a Master Gardener Clinic. Using the following link, you can locate a Master Gardener Clinic in your part of Washington State.

Date 2017-05-26
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Rubus, Fruit--Diseases and pests

It appears that my raspberries may have a disease. I noticed some fruiting canes getting discolored, then curling leaves, then dying completely. I ripped out ones that were dying or dead, but others seem to be showing beginning symptoms.


There are many potential culprits that could be causing the problems you are observing with your raspberries. It could be a pest, or it could be a fungal disease. Remember that summer bearing raspberry canes die after bearing fruit. The canes start to look bedraggled even as fruit is ripening. Once all fruit has been picked these canes should be cut to the ground. Next year's fruiting canes will look healthy and should not be cut down to the ground.

Have you looked inside any of the affected canes after cutting them? If you have cane borers, you may find that white grubs have burrowed toward the base of the cane. Crown borers also cause wilting of new growth in the spring followed by dieback of the cane. Here are links to information and images so you can compare what you are seeing with your plants:

Insects and Diseases of Raspberries from University of Illinois Extension.

Pests and Disorders of Blackberries and Raspberries from University of California.

Washington State University's Hortsense page on Raspberries (see sidebar on left for diseases and insects)

Raspberry root rot information is available in the Pacific Northwest Disease Management Handbook.

Washington State University Extension's information for home gardeners says:

"Foliage symptoms of root rots. Root rot is usually noticed when leaves begin to wilt, turn yellow or brown, and die. Symptoms commonly occur during warm spring or summer weather and may develop in a few days or take longer. If longer, leaves are generally yellowish and stunted before they die.

"Root symptoms of general root rot. Root systems are small, dark brown or black, and rotted. Since healthy roots may or may not have dark surfaces, determine root condition by cutting or scraping them. All of the inside of a healthy root is whitish, but the inside of a rotted root is partly or entirely brownish or blackish. Wash the cutting tool in soapy water and swab in rubbing alcohol after cutting."

You may want to bring samples plus photos of the whole plant to a Master Gardener clinic for diagnosis. There is a link to the current clinic schedule on their website.

Once you have identified the source, you can try to address the problems and resume growing happy raspberry plants. Oregon State University has a guide to growing raspberries which may be helpful.

Date 2017-07-18
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Prunus, Fruit--Diseases and pests

For three years, my plum tree has had leaves that curl and shrivel somewhat. I have heard of aphids causing leaf curl in plums, but I don't see many aphids.

I also have a peach tree that has "Peach Leaf Curl" or Taphrina deformans and the symptoms on the plum leaves look similar to that. Here is what I wonder:

  • The peach and plum are at least 100 yards apart. Is it really possible that the peach infected the plum?
  • With the peach tree the fruits are also affected but with the plum fruits do not appear to be affected.
  • Are peach and plum affected by the same diseases?


Both peach and plum trees are in the genus Prunus. Your plum tree's problem sounds like plum pockets and peach leaf curl, which are caused by fungus (usually Taphrina).

From Iowa State University Plant Pathology:
Have you noticed lately that your peach leaves appear curled or puckered? Do leaves appear to be lighter than normal, flushed with red, blistered, distorted, and curled? Chances are your tree has peach leaf curl, a fungal disease caused by Taphrina deformans. Although peach leaf curl is primarily a disease of peach, nectarines are also affected. Peach leaf curl is first noticed in spring when young leaves start to emerge. The entire leaf or a portion of it may appear crinkled and curled with flushes of red or purple . Later on in the season, the fungus begins to produce spores and leaves appear silvery or powdery gray. Infected leaves turn yellow and brown and fall off the tree and are replaced by a new set of foliage. Flowers, young fruits and stems may also be infected. Affected fruits are distorted with wrinkled, discolored areas on the surface. Extensive defoliation may affect fruit yield the following year and may also predispose the tree to winter injury and other diseases.

Plum pocket is a disease in plums caused by Taphrina communis. Leaf symptoms are similar with peach leaf curl and the plums appear to be distorted, wrinkled, and puffy. This disease is not considered a serious problem in most commercially cultivated plum varieties.

Here is Oregon State University's online guide to plant diseases. This is Washington State University's comparable site.

I don't know if your plum could have gotten the same species of Taphrina fungus that is affecting your peach (i.e., Taphrina deformans), but the conditions in our climate are probably ideal for this type of fungal disease. University of California, Davis says that the pathogen which causes peach leaf curl survives on tree surfaces and buds, and is enhanced by wet spring weather.

From University of Nebraska Plant Pathology:

Plum Pockets is very similar to the well-known disease peach leaf curl. It reached epidemic proportions on plum in the 1880's and sand cherry in the 1940's. The disease is still common today but rarely has an economic impact on stone fruit production. However, its unique symptoms always seem to peak the interest of individuals who are seeing it for the first time. The disease is caused by two species of Taphrina. Taphrina communis (Sadelbeck) Giesenh. has a worldwide distribution. Its hosts include plum (Prunus angustifolia) and several wild Prunus spp. found in America. Taphrina pruni primarily infects European plums and is rarely found in America. The disease cycle of Taphrina communis is similar to that of Taphrina deformans (peach leaf curl). The fungus overwinters as conidia on twigs and bud scales. Infection generally begins at bud break when these spores are rain splashed to susceptible green tissue. Leaves, shoots, and fruit are all susceptible but symptom development is most common on fruit. The fungus invades host tissue directly through epidermal cells. Once the fungus is established, a specialized mat of fungal cells (hymeneal layer) containing asci and ascospores forms. The asci are not protected by a specialized ascocarp. Ascospores are released, germinate and begin budding, much as a yeast does. Conidia (bud conidia) serve as secondary inoculum in the spread of the disease. Initiation of the disease cycle is favored by cool wet weather.

You might consider bringing in samples of the affected leaves to a Master Gardener Clinic for a definitive diagnosis. They may also have more information on whether the disease can pass from peach to plum, or whether your two types of trees simply have two different strains of the pathogen.

Date 2017-07-18
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Fragaria, Cutworms, Fruit--Diseases and pests

My small patch of strawberry plants has this year suffered from green fruit dropping off, forming neat piles under each plant. Each fallen fruit has a short bit of stem still attached. A few fruit are still attached. No sign of slug or squirrel damage, no signs of fungus of insect attack. The weather has been colder and wetter than average. Owing to natural layering, the plants are closer together than when first planted. This happened a couple of years ago, but we had a good crop last year. Any idea what's going wrong?


I wonder if this might be the work of cutworms. You can take a close look just under the soil surface, along the stems, and inside curled or folded leaves during the day, or take a flashlight at night, which is when they feed, and see if that may be why your strawberries are being cut away from the plant. If you find them, cut them with garden pruners.

Washington State University's pest and disease site does list the cutworm as a known pest of strawberries. Excerpt:

"Cutworms and armyworms are the larvae of noctuid moths. These common moths are medium-sized with fairly dull coloration. The greenish, grayish, or tan caterpillars are hairless, nocturnal, and generally spotted, striped, or otherwise marked. They may be 1/4" to 1" in length and tend to curl up when disturbed. They may climb into the plant and feed on foliage, buds, flowers, or fruit. Armyworm behavior is similar to that of cutworms, but armyworms feed in large groups instead of individually. They tend to be voracious feeders. The caterpillars typically spend the day just beneath the soil surface or under debris near the host. Weeds are a primary food source for both cutworms and armyworms."

I looked at Pests of the Garden and Small Farm by Mary Louise Flint (University of California, 1990), but could not find any strawberry disease resembling what you have observed in your garden, which leads me to believe it is a pest problem. Here is a link to U.C. Davis's Integrated Pest Management page on strawberries.

Date 2017-01-13
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Plant diseases--Control, Prunus, Fruit--Diseases and pests

Have you any advice about how to combat peachtree leaf curl using natural methods at this stage in the season? I've just read about the use of thyme or oregano oil, but no advice on amount used. I would be glad of any help!


The information I was able to find about thyme oil as a treatment for Taphrina came from an application to the U.S. Patents Office, so I cannot speak for its efficacy. I did find information from Connecticut Agricultural Experiment Station which mentions thyme oil as an organic-acceptable insecticide.

The U.S. Department of Agriculture's Agricultural Research Service offers research on the use of plant essential oils in postharvest disease control,too.My impression is that the efficacy of these plant-based oils is still being studied and evaluated.

I also found information on managing peach leaf curl (Taphrina deformans)[formerly available online] from the National Sustainable Agriculture Information Service which suggests using lime sulfur, acceptable by U.S. organic standards, though European standards may differ. Below is a more substantial excerpt:

The life-cycle diagram above in Figure 2 shows that the infection period for leaf curl is when new leaves start emerging from buds in the spring. Spraying after the buds have opened is ineffective, because infection takes place as the young leaves emerge, and the fungus develops inside the leaf.

Accordingly, sprays must be applied during the trees' dormant period after the leaves have fallen and before the first budswell in the spring. Many orchardists spray just prior to budswell during the months of February and March. Orchards with a history of severe peach leaf curl benefit from a double application: in the autumn at leaf fall and again in late winter or early spring just before budswell.

Fortunately for the organic grower, lime sulfur is one of the most effective fungicides for control of peach leaf curl and is allowed in certified organic production . Bordeaux and copper fungicides are also approved for certified organic programs and are effective as well, but not as effective as lime-sulfur.

Pscheidt and Wittig (6), performed trials comparing Kocide, lime-sulfur, several synthetic fungicides, and Maxi-Crop seaweed for leaf curl control. Lime-sulfur and one of the synthetics (ziram) were best, roughly twice as effective as Kocide. Seaweed sprays, despite positive anecdotal reports, were completely ineffective.

Severe leaf curl infection can cause the tree to shed many of its leaves and to replace them with a second flush of growth. At this time the tree will benefit from a light feeding with a quickly-available soluble fertilizer such as compost tea or fish emulsion to help it recover.

There are various levels of resistance to leaf curl among varieties; however, because of the relative ease of controlling the disease, breeding for resistance has not been a priority. Redhaven, Candor, Clayton, and Frost are some of the cultivars with resistance to leaf curl, though none is immune. In contrast, Redskin and cultivars derived from it are susceptible.


The City of Seattle's Integrated Pest Management Solutions pages for landscaping professionals also suggests methods of prevention and control. Damage may be reduced by sheltering the tree from winter and early spring wet. If only a few leaves are affected, they may be removed by hand. Peach leaf curl does not usually kill the tree, though fruit yield will be reduced. This resource also mentions using copper fungicides and lime sulfur when the tree is dormant.

Date 2017-01-06
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Ribes, Fruit--Diseases and pests

We purchased a gooseberry last year. It got powdery mildew and lost its leaves. This spring it came back with lush leaf growth and has not had the mildew, but it has brown spots that just developed. Some leaves are turning completely brown. Any suggestions? On the east coast in NY we had huge prolific berried bushes that didn't seem to have any of the problems we are having here in Snohomish County.


Powdery mildew is certainly a known problem for gooseberries here, but there are other diseases that show up as spotting on the leaves. The Royal Horticultural Society describes mildew on gooseberries. Cornell University's Department of Horticulture has a guide to currants and gooseberries which describes several other problems affecting the leaves of these plants.

We can't diagnose the problem remotely, but compare your plant's symptoms with this information about anthracnose from Washington State University's HortSense website:
"Anthracnose is a fungal disease affecting the leaves of currants and gooseberries. Leaves show small, round or irregularly-shaped spots on the upper or lower surfaces. The spots are usually dark brown in color and may develop tiny, gray fungal structures in the centers. Severely affected leaves may turn yellow and drop prematurely. The leaf loss can weaken plants and reduce yields. Currant fruit may also show spotting. On fruit, the spots are tiny and resemble flyspecks. Severely infected berries crack open and drop. The fungus is spread from infected to healthy leaves by splashing water and overwinters in fallen leaves. Disease development is favored by wet spring weather.
Avoid overhead watering.
Rake fallen leaves from beneath plants. Destroy or discard (do not compost) diseased plant materials. Cultivation to bury diseased leaves may also be effective.
Space plantings and prune to provide good air circulation and reduce humidity."

It would be worth having a your local county extension agent test the affected foliage before you attempt to treat the problem.

Just as an aside, gooseberries do well in areas that have good winter chilling and humid summers, which sounds more like parts of the East Coast than our winter wet/summer dry Northwest. The website of California Rare Fruit Growers describes the native ranges of gooseberries:
"Gooseberries are derived mostly from two species: the European gooseberry (Ribes grossularia), native to the Caucasus Mountains and North Africa; and the American gooseberry (R. hirtellum), native to northeastern and north-central United States and adjacent parts of Canada. So-called European cultivars are pure species, but virtually all so-call American cultivars also have European genes.

Date 2017-04-19
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Vaccinium, Berries--Diseases and pests, Rubus, Fragaria, Insect pests, Prunus, Fruit--Diseases and pests, Berries

Could you tell me more about a new type of fruit fly that is supposedly infesting fruit here in the Pacific Northwest? Which fruit are affected?


The fruit fly is called the Spotted Wing Drosophila. It is known to affect strawberry, raspberry, blueberry, plum, peach, cherry, and grape. Oregon State University has created an information clearinghouse about this pest. Here is their information for home fruit growers. Washington State University has also devoted several web pages to this fly. Here is their Integrated Pest Management information, excerpted below (SWD stands for Spotted Wing Drosophila):
"Monitor for SWD using traps. [...] These vinegar traps are for monitoring purposes only and will not provide control of SWD. Remember, chemical control is not necessary if SWD is not present.
Composting fruit will likely not be effective at destroying maggots and pupae.
Remove infested and fallen fruit. Destroy or dispose of infested fruit in a sealed container.
Management recommendations are currently being developed for this pest. For the time being, good sanitation practices should be used."

Whatcom County Extension has clear, basic information for home gardeners as well. Since this insect is a relatively recent invader in the Northwest, information is constantly being adjusted and research is ongoing.

Date 2017-08-15
Link to this record only (permalink)

Didn't find an answer to your question? Ask us directly!

Browse keywords

Search Again:

August 01 2017 12:36:01