PNW Plant Disease Management

PNW Plant Disease Management This page supplements the PNW Plant Disease Management Handbook. Our mission is to disseminate information on plant disease management and improve plant disease control in the Pacific Northwest.
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The handbook is intended as a reference of control and management tactics for important plant diseases in the Pacific Northwest. Discussion on this site will be to that end.

Mission: Disseminate information on plant disease management. Improve plant disease control in the Pacific Northwest.

Operating as usual

Cold weather means plants are getting stuffed into greenhouses, hoop houses, cold frames, etc.... setting them up for Bo...
10/23/2020
Greenhouse Plants, Ornamental-Gray Mold

Cold weather means plants are getting stuffed into greenhouses, hoop houses, cold frames, etc.... setting them up for Botrytis. The problem is lots of moisture condensing out of the air when temperatures cool at night. Maintain a steady, relatively dry environment by keeping greenhouse humidity below 90%, increasing spacing between plants for good air circulation, and taking care not to splash water on foliage during watering. Heating the greenhouse at night (especially for zero or negative DIF) or venting around sunset may be necessary. Heating in the morning before sunup can also help prevent dew formation as air temperature increases faster than the temperature of plant parts. If using DROP or DIP for size control, do not let humidity rise above 90%. https://pnwhandbooks.org/plantdisease/host-disease/greenhouse-plants-ornamental-gray-mold

Cause Botrytis cinerea (sexual: Botryotinia f**keliana ) and other Botrytis spp., fungi that colonize dead, dying, and wounded plant parts. From these infections they...

Removing leaves off the lawn helps prevent a common problem called Microdochium Patch. Also known as pink snow mold, it ...
10/22/2020
Lawn and Turf-Microdochium Patch (Pink Snow Mold)

Removing leaves off the lawn helps prevent a common problem called Microdochium Patch. Also known as pink snow mold, it can get going in the cool wet environment under those leaves. More a problem on turf cut low and in shaded areas. High nitrogen fertility also favors disease development. Among other tactics, daily morning dew removal by rolling has been effective at reducing the disease in western Oregon. https://pnwhandbooks.org/plantdisease/host-disease/lawn-turf-microdochium-patch-pink-snow-mold

Hosts The disease is worse on annual bluegrass, tall fescue, and bentgrass. Fine fescues, bluegrass, and ryegrass (under high fertility) also can be infected...

One of the ways to manage scab on apples and pears is to apply nitrogen to the leaves now. This helps the leaves break d...
10/21/2020
Apple (Malus spp.)-Scab

One of the ways to manage scab on apples and pears is to apply nitrogen to the leaves now. This helps the leaves break down over the winter including making them more attractive to earthworms. Apply nitrogen (5% urea) plus an adjuvant to leaves (on or off the tree) in fall. The organic equivalent would be the application of Organic Wet Betty (yucca extract) or 30% yeast extract. Physically shredding the leaves will help as well. https://pnwhandbooks.org/plantdisease/host-disease/apple-malus-spp-scab

See: Crabapple (Malus spp.)-Scab Apple Variety Susceptibility Cause Venturia inaequalis , a fungal disease that is common and destructive west of the Cascade Range...

Master gardener presentation last night on "Very Viscous (plant) Viruses". Folks were intrigued by the Blueberry shock v...
10/20/2020
Blueberry (Vaccinium corymbosum)-Shock

Master gardener presentation last night on "Very Viscous (plant) Viruses". Folks were intrigued by the Blueberry shock virus example. Plants get the virus, show shock reactions but then recover. However, these infected plants always retain the virus, show few if any symptoms but are still able to transmit this pollen borne virus for the life of the plant. https://pnwhandbooks.org/plantdisease/host-disease/blueberry-vaccinium-corymbosum-shock

Cause The Blueberry shock virus (BlShV), which is pollenborne. Transmission occurs when pollinators, especially foraging honeybees, transfer infected pollen to flowers on healthy plants...

As big leaf maple leaves come down you might notice something we call the green island effect. Tar spot is a common leaf...
10/19/2020
Maple (Acer spp.)-Tar Spots

As big leaf maple leaves come down you might notice something we call the green island effect. Tar spot is a common leaf spotting fungal disease found on maples leaves. The green island effect is thought to be due to the production of cytokinins by the fungus. Infection occurred way back in spring. https://pnwhandbooks.org/plantdisease/host-disease/maple-acer-spp-tar-spots

Cause Rhytisma punctatum , a fungus. The disease is common on bigleaf maple on the Pacific Coast and also attacks many other species, particularly...

Funky Flora Friday - Filberts with Witches brooms are a rare sight. There are so few that it has not been investigated i...
10/16/2020
Hazelnut (Corylus avellana)-Witches' Broom

Funky Flora Friday - Filberts with Witches brooms are a rare sight. There are so few that it has not been investigated in the PNW. In Sovenia they think it could be due to a phytoplasma but the same pathogen in the UK only had yellowing leaf symptoms. In any case, they are easy to manage. One pruning cut well below the broom will do the job. https://pnwhandbooks.org/plantdisease/host-disease/hazelnut-corylus-avellana-witches-broom

Cause Unknown. Witches's brooms in hazelnut are an oddity that is seen every few years. Growers are curious about the problem but do not...

Use phosphites (not phosphate) this time of year to go after oomycete root rot problem such as Phytophthora. In other wo...
10/15/2020
Fungicidal Activity and Nutritional Value of Phosphorous Acid

Use phosphites (not phosphate) this time of year to go after oomycete root rot problem such as Phytophthora. In other words, don't use the fertilizer, use the fungicide. Phosphate and phosphite are not equivalent inside the plant. Phosphoric acid or phosphate cannot function like phosphorous acid or phosphite. Since phosphites are systemic and very stable in plants, they can work against oomycetes and also should not be applied too frequently. Confused? Read the article linked to this post. https://pnwhandbooks.org/plantdisease/pesticide-articles/fungicidal-activity-nutritional-value-phosphorous-acid

Asha M. Brunings, Guodong Liu, Eric H. Simonne, Shouan Zhang, Yuncong Li, and Lawrence E. Datnoff Adapted from original article: Brunings, A.M., Liu, G,...

Question: Our magnolia tree has developed white markings on just about all of it's trunk and branches. We think it might...
10/13/2020

Question: Our magnolia tree has developed white markings on just about all of it's trunk and branches. We think it might be lichen. This seemed to have happened in the past week during a very rainy time. Could you identify this and tell us what it is and if it is harmful to the tree, and if so, what could we treat it with. Thanks so much for your expertise and advice.

Time to put that holiday cactus away if you want flowers. Holiday cacti Schlumbergera truncata and S. bridgesii are "sho...
10/12/2020
Holiday cacti

Time to put that holiday cactus away if you want flowers. Holiday cacti Schlumbergera truncata and S. bridgesii are "short day plants." To produce flower buds, they require shorter days (fewer hours of light) and cool night temperatures. Easter cactus (S. gaertneri) is a long day plant and sets buds in late winter or early spring when days become longer. https://extension.umn.edu/houseplants/holiday-cacti#light-and-temperature-1431360

How to grow holiday cacti as houseplants.

If your chrysanthemum is wilting and you know it has enough water then it might have a root rot. Although not common in ...
10/09/2020
Chrysanthemum-Root and Crown Rots

If your chrysanthemum is wilting and you know it has enough water then it might have a root rot. Although not common in commercail production, several soilborne organisms have been associated with chrysanthemum root problems in the PNW including Fusarium sp. and Pythium sp., and sometimes Phytophthora sp. and Rhizoctonia sp. Avoid reusing pots or trays from a previous crop will help. Fertilize with nitrate rather than ammonium forms of nitrogen. Monitor EC levels closely so they do not get too high and do not let the media dry out completely. Ideal pH range is 5.4 to 5.8 for garden mums. https://pnwhandbooks.org/plantdisease/host-disease/chrysanthemum-root-crown-rots

See: Chrysanthemum-Verticillium Wilt Cause Although not common, the OSU Plant Clinic has found several soilborne organisms associated with chrysanthemum root problems including Fusarium sp...

Question: some Some “russeting” was seen on Pinot Noir earlier this year (July/Aug).  Rows run north and south and sympt...
10/08/2020

Question: some Some “russeting” was seen on Pinot Noir earlier this year (July/Aug). Rows run north and south and symptoms were seen predominantly on the morning sun side of the canopy and only on exposed fruit (and not on all exposed fruit). You can see in the picture that not every berry is affected and it appears that it’s worse on the berry shoulders where there would be more sun exposure. From the photo it looked like the start of sunburn but it didn’t make sense why it was predominately on the one side (that being the side not-exposed to late day exposure) and not consistent throughout the block, it seemed worse on one side than the other. Any help would be greatly appreciated.

Cytospora canker can be devastating in cherry, peach, and apricot orchards. Cankers have also been reported on apple in ...
10/07/2020
Peach (Prunus persica)-Cytospora Canker

Cytospora canker can be devastating in cherry, peach, and apricot orchards. Cankers have also been reported on apple in all Pacific Northwest regions, especially after severe winters. Various ornamental shrubs and trees are susceptible as well. These fungi are vigorous wound invaders including wounds from sunscald and winter injury. Painting the southwest side of tree trunks with white latex paint will help avoid winter injury. For stone fruits, prune as late as possible in the dormant season. Use of copper-based fungicides was not only ineffective on peach but resulted in some cases to be phytotoxic. The use of latex paint, Captan and/or Topsin was shown to be effective wound protectants prior to infection in Colorado. Latex paint, however, did not result in long term effective control. https://pnwhandbooks.org/plantdisease/host-disease/peach-prunus-persica-cytospora-canker

Cause Cytospora cincta (sexual: Leucostoma cinctum ), fungus, and the closely related C. leucostoma (sexual: Leucostoma persoonii ). These are vigorous wound invaders (including...

Sanitation of diseased material is an important method of reducing disease pressure. Many root rots and leaf spot pathog...
10/06/2020
Bamboo (Phyllostachys sp.)-Leaf Spots

Sanitation of diseased material is an important method of reducing disease pressure. Many root rots and leaf spot pathogens overwinter in the plant debris left at the end of the season. Cleaning it up through raking or blowing or encouraging it to break down quicker helps break the cycle. So when we need to come up with control tactics for a new leaf spot disease that has little research, it is an easy method to include. This bamboo leaf spot issue is case in point. https://pnwhandbooks.org/plantdisease/host-disease/bamboo-phyllostachys-sp-leaf-spots

Cause There have been odds and ends of leaf spots sent in to the OSU Plant Clinic over the years. A few fungal ones...

You may not be thinking of Poinsettias just yet as it is not even Halloween but many greenhouses are in the middle of pr...
10/05/2020
Poinsettia (Euphorbia pulcherrima)-Pythium Root Rot

You may not be thinking of Poinsettias just yet as it is not even Halloween but many greenhouses are in the middle of production. Root rots are a big problem especially if they get overwatered. That can happen if they dry out too much and then get flooded to bring them back. Pythium root rot will follow. This disease is difficult to control once it has begun. Focus on preventing the disease before it begins by eliminating the pathogen from the production system. Clean growing surfaces, clean water, and handling practices, along with soilless media are all helpful. Grow the plants at 69°F to 76°F and at a recommended pH range of 5.8 to 6.2 and evenly water. https://pnwhandbooks.org/plantdisease/host-disease/poinsettia-euphorbia-pulcherrima-pythium-root-rot

See: Greenhouse Plants, Ornamental-Pythium Seed Rot, Damping-off, and Root Rot Cause Pythium ultimum , P. aphanidermatum (found more often on roots in the Northeast...

10/02/2020

Got apples west of the Cascades then you have anthracnose. Time is now to get after this nasty canker disease. Prune out and burn affected twigs and branches, especially on highly susceptible cultivars. This is best done in dry weather before fall rains. On larger limbs and branches, use a sharp knife to remove cankers that are less than half the branch width. Cut out a pointed oval oriented along the axis of the branch. Apply materials like captan, ziram or copper-based materials before fall rains and again (about 1 month later) when leaves fall from trees. https://pnwhandbooks.org/plantdisease/host-disease/apple-malus-spp-anthracnose-bulls-eye-rot

"Spray in October before heavy rains return" is a frequent statement you will find for managing diseases that are active...
10/01/2020
Berckmann's Arborvitae (Platycladus orientalis)-Berckmann's Blight

"Spray in October before heavy rains return" is a frequent statement you will find for managing diseases that are active late fall and during the dormant season. Generally it goes along with multi-spectrum materials such as copper-based products. The application prevents the build up of bacterial as well as fungal pathogens. Berckmann's arborvitae-Platycladus orientalis L. var. conspicua 'Berckmannsii'-is particularly susceptible to fungal infection. The fungus produces spores in late September, which are washed and splashed by rains to infect new foliage within the plant. https://pnwhandbooks.org/plantdisease/host-disease/berckmanns-arborvitae-platycladus-orientalis-berckmanns-blight

Cause Seimatosporium berckmansii (formerly Coryneum berckmansii ), a fungus. The fungus produces spores in late September, which are washed and splashed by rains to...

Some of those really cool squash fruits that you might want for fall/Halloween decorations might just be due to a virus ...
09/30/2020
Squash (Cucurbita spp.)-Virus Diseases

Some of those really cool squash fruits that you might want for fall/Halloween decorations might just be due to a virus infection! These viral plant diseases first become noticeable in mid to late July, often following cutting of grass seed and alfalfa fields, which may displace aphid populations from these fields. Infections generally begin at the edges of fields and move inward. Don't let the names confuse you as summer squash can get Watermelon mosaic virus. Viruses can have wide host ranges. https://pnwhandbooks.org/plantdisease/host-disease/squash-cucurbita-spp-virus-diseases

See: Squash (Cucurbita spp.)-Curly Top Cause The three most common virus diseases in Oregon are zucchini yellows caused by the Zucchini yellows mosaic virus...

One way to tell if a conifer is under stress, especially from root diseases, is the production of an unusually heavy con...
09/29/2020
Fir, Douglas and True-Armillaria Root Rot

One way to tell if a conifer is under stress, especially from root diseases, is the production of an unusually heavy cone crop. Many conifers can be seen this year with a larger than normal amount of cones. Other indicators of root diseases include thin and/or chlorotic foliage, abundant resin flow or leaching of brown liquid at tree bases (basal resinosis) and, obviously, tree death. https://pnwhandbooks.org/plantdisease/host-disease/fir-douglas-true-armillaria-root-rot

Cause Armillaria ostoyae (may be referred to as A. solidipes in some publications) is the species associated with most conifer mortality in the Pacific...

There are many different post harvest rots of apples and pears. Since pears are not tree ripened they seem to get a lot ...
09/28/2020
Pear (Pyrus spp.)-Storage Rots

There are many different post harvest rots of apples and pears. Since pears are not tree ripened they seem to get a lot of problems. Harvesting at proper maturity will help as late harvest can result in increased decay rates. Harvest using the proper technique for removing fruit from the tree. Rather than grab and pull which results in bruising, gently rotate the fruit away from the stem. If it is ripe it will come off easily. https://pnwhandbooks.org/plantdisease/host-disease/pear-pyrus-spp-storage-rots

See: Apple (Malus spp.)-Storage Problems Cause There are many storage disorders caused by abiotic problems beyond Bartlett Decay described below. Many of these are...

An interesting problem with sweet box (Sarcoccoca) popped up on social media and other places. The plants are dropping l...
09/25/2020

An interesting problem with sweet box (Sarcoccoca) popped up on social media and other places. The plants are dropping leaves after the smoke event we had in western Oregon this past month. The image I show here has all leaves coming off. Other images I have seen show older leaves yellowing and falling off. Have you seen it this year and if so where are you located?

Look out tomatoes and potatoes - Rainy weather, fog, and dew are favorable for late blight development. No commercial to...
09/24/2020
Tomato (Solanum lycopersicum)-Late Blight

Look out tomatoes and potatoes - Rainy weather, fog, and dew are favorable for late blight development. No commercial tomato varieties have sufficient resistance to this disease to slow disease development, and once disease begins, cultural controls may fail to slow its development. Chemical control, initiated before disease begins, is the only method that can prevent complete crop destruction. (Other than hot dry weather!) https://pnwhandbooks.org/plantdisease/host-disease/tomato-solanum-lycopersicum-late-blight

See: Potato (Solanum tuberosum)-Late Blight Cause Phytophthora infestans , a fungus-like microorganism that also attacks potato (Irish, not sweet). It usually does not survive...

The College of Agricultural Sciences Celebrates The 2020 UN International Year of Plant Health. Articles on the Intellig...
09/23/2020
| College of Agricultural Sciences | Oregon State University

The College of Agricultural Sciences Celebrates The 2020 UN International Year of Plant Health. Articles on the Intelligent Sprayer, Potato disease diagnosis and management, Diagnosing Plant Problems, Monitoring and managing grape powdery mildew, and many more. https://agsci.oregonstate.edu/2020iyph

Here in the College of Agricultural Sciences at OSU, we have world class research, teaching, and outreach programs in plant health. If you would like us to feature your program please share your information with us.

Hazelnut kernel mold is defined as any visible growth of mold either on the outside or inside of the kernel. In practice...
09/22/2020
Hazelnut (Corylus avellana)-Kernel Molds

Hazelnut kernel mold is defined as any visible growth of mold either on the outside or inside of the kernel. In practice, it means any white, fuzzy mycelial growth is classified as 'mold.' Other kernel defects include: Insect injury which is generally due to filbert worm. Kernels are put in this category even if they also have obvious mold; Rancidity, which is evaluated solely by taste and not by visual characteristics; Shrivel which, is when kernels shrink and wrinkle due to moisture loss; Decay, which appear as a black (not brown) coloration that extends into the meat of the kernel; Internal discoloration of cut kernels shows a black coloration and not just brown which is considered normal. https://pnwhandbooks.org/plantdisease/host-disease/hazelnut-corylus-avellana-kernel-molds

Cause "Mold" is defined as any visible growth of mold either on the outside or inside of the kernel. In practice, it means any...

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A grower in southern OR has an issue in garlic. Bulbs appeared healthy at harvest but in storage, this happened. Looking for resources for an accurate diagnosis.
Crown gal in rhodo liners any suggestions on prevention and source of infection?
What is mode of action of fumigant fungicide against to soil borne plant pathogens and how does it kill the fungi?
What are the best methods to diagnose the pathogenic problems during in the field?
Started some peach trees from seeds this spring. Transplanted to large tubs about a month ago. All are doing well, except 2 of the Elberta's, the leaves are doing this. Any ideas what's causing this?
Found a couple of salal, Gautheria shallon plants with almost white leaves. Any suggestion what may cause these deficiencies?
I have an Ask an Expert question about a Japanese maple losing leaves mid-summer--and then putting on new foliage in some years. I think it needs water, but am puzzled by the splotchy leaves in one of these photos. Looks like a virus. Ideas? Thanks!
I have these dry brown streaks on the stems of hosta francee. There are at present no rot or fungi visible any suggestions about its cause and prevention?
Terrible year for fusarium. Anybody got a cure for my soil?
I think I have a virus on my tomatoes? Early girl La Grande Or.
I have a dogwood where half of the tree is normal and healthy, and half looks like this photo. I’ve scoured the internet, but am not sure what This is....these sickly branches also didn’t flower this year, the rest of the plant did....any advice would be appreciated!
This Rainier cherry tree has decided to live a little differently this year. Seemed to come to this really fast and I planted my new garden fairly close to one side and am worried it may be bad for the garden. It is usually healthy so this is unusual. Any ideas what is going on? Brown, dry leaves, all the undeveloped cherries are falling off, some with dark spots.