These days, digital photos of diseased plants are arriving fast and furious in the Plant Disease Diagnostics Clinic (PDDC) email inbox. While clients have been having problems with many different types of plants, I have been receiving a large number of photos of stone fruits. Ornamental and fruit-bearing varieties of cherries, plums and peaches seem to be having a rather tough year this year. Several of the diseases that adversely affect stone fruits are fairly straightforward to diagnose by photo. Below are the common diseases of cherries, plums, and peaches that I have been seeing thus far this season.
The name of this fungal disease is quite descriptive. Infected peach leaves become curled and puckered, and often have a combination of a green, creamy-white and fuchsia color. Peach leaf curl seems relatively cosmetic, but repeated occurrences of the disease over time can reduce the quantity and quality of fruit. Typically with leaf diseases, I recommend good fall clean up of leaves for management. Unfortunately, this strategy does not work for pearch leaf curl, as the pathogen (Taphrina deformans) overwinters on peach branches. Management of the disease relies of use of fungicides (e.g., copper-containing products) applied either after leaf drop in the fall or prior to bud swell in the spring.
This disease, which is specific to plants in the genus Prunus (e.g., cherries and plums), is what I affectionately refer to as “poop-on-a-stick”. It really does look as though some pesky animal has defecated on the branches of affected trees and shrubs. The fungal pathogen involved (Apiosporina morbosa) induces formation of black, gnarly swollen areas (called galls or knots) on infected branches.
Unfortunately, once the knots form, the only method of management is to remove the growths by pruning. For fungal diseases, I typically recommend pruning roughly four to six inches below the diseased area. When pruning, be sure to decontaminate tools between cuts by treating them for at least 30 seconds with 70% alcohol (e.g., straight rubbing alcohol), a commercial disinfectant or 10% bleach. Spray disinfectants work as well (as long as they contain roughly 70% active ingredient). Just spray tools until they drip and then allow them to air dry. When using bleach, be sure to rinse tools completely after pruning and then oil them to prevent rusting. Dispose of black knot galls by burning (where allowed) or burying them. In some situations, there will be so many galls in a tree that my recommendation is what I call “basal pruning” or “a single pruning cut at the ground level”. You remove the affected trees and replace it with non-susceptible plants.
Probably the most serious of the diseases that I have seen on stone fruits this year is this one. The pathogens involved (two variants, called pathovars, of the bacterium Pseudomonas syrinage) infect branches causing branch dieback. From infected areas, sap emerges and gelatinizes on branch surfaces. For bacterial canker, timely pruning of diseased branches is critical for management, as the pathogens can rapidly colonize infected branches and move into the main trunks of trees where they can girdle the trunks, killing the trees.
Prune at least 12 inches below visible dieback on affected branches and again dispose of branches by burning (where allowed) or burying them. Decontaminate tools as described above for black knot. When bacterial canker occurs in main trunks, tree removal and replacement is the only real option.
If you need help diagnosing plant diseases, feel free to contact the PDDC. For the PDDC’s current policy on sample submission, including submission of digital photos, check out the following link. As always, be sure to check out the PDDC website for timely information on plant diseases. Also, feel free to follow the clinic on Twitter or Facebook (@UWPDDC) to receive timely PDDC updates. Or alternately, put in a request to subscribe to the clinic’s new listserv (UWPDDCLearn) by emailing firstname.lastname@example.org.
Hang in there, be safe, and stay healthy everyone!
Most days, I really love my job. I am well-known for my love of plant disesaes and I tend to get giddy when plant samples arrive at the PDDC. There is always the possibility with each new package that I will become reacquainted with an old disease friend (e.g., cedar-apple rust) or that I will be introduced to new disease friend that I’ve been wanting to meet for years (e.g., zonate leaf spot).
Others days, I open a package and my shoulders sag, and I let out sigh. This most often occurs when the sample potentially has a disease/pathogen that is regulated by either the state or federal government. These diseases are often fascinating in and of themselves, but the paperwork involved with their diagnosis can be soul crushing. Right now in Wisconsin, there are three diseases on my radar that fall into this dreaded category. This month’s web article is devoted to these medal-winning diseases that keep me up at night.
Bronze Medal – Boxwood Blight
In the scheme of things, boxwood blight is not bad as regulated diseases go. Boxwood blight was introduced into Wisconsin in 2018 through contaminated nursery stock and is regulated at the state level. The Wisconsin Department of Agriculture, Trade and Consumer Protection (WI DATCP) monitors boxwood blight’s spread and is currently attempting to eradicate the disease as it rears its ugly head, particularly in nurseries. I first encountered boxwood blight last summer when a landscape maintenance professional submitted a sample from a boxwood shrub planted at a Madison area residence. Once I made my diagnosis, I immediately contacted WI DATCP so that they could follow up with the homeowner regarding containment and eradication.
Boxwood blight typically first shows up as distinct spots appearing on leaves in the lower canopy of boxwood shrubs. Most boxwood varieties are very susceptible to the disease and rapidly defoliate and die. Pachysandra, a common ground cover, is also susceptible. If you want to see how devastating this disease can be, do an internet search on “boxwood blight” and your favorite state along the eastern seaboard (e.g., North Carolina, Virginia, Maryland). You will find photos of landscapes where every boxwood has been wiped out. For additional details on this disease, check out our boxwood blight pest alert.
Silver Medal – Sudden Oak Death/Ramorum Blight
Sudden oak death (I prefer the name Ramorum blight) was first described in California in the 1990’s and has killed millions of oaks in that state. Because of its destructive potential, the disease/pathogen is regulated at the federal level by the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS). There has been movement of the disease/pathogen over the years into other states (again through movement of nursery materials), and in 2019, WI DATCP inspectors found the disease on an azalea in a nursery in Wausau. Unfortunately, azaleas from the same supplier were distributed to nurseries around Wisconsin, and many were sold to homeowners before WI DATCP became aware of potential problems. Another possible introduction of the disease/pathogen on red ‘Double Knockout’ roses also occurred in 2019. I have not yet had this disease arrive in my lab, but I did prescreen several samples for the disease last summer. Based on my preliminary testing, I forwarded two suspicious samples to a second lab for another round of testing. If those samples had tested positive at this second lab (luckily they didn’t), they would have been sent to yet another lab (a USDA APHIS facility) for a final round of testing.
Unfortunately the symptoms of sudden oak death/Ramorum blight are not readily distinguishable from other diseases. Branch dieback, nondescript leaf browning and eventually plant death can be typical symptoms. See our sudden oak death pest alert for additional details on this disease.
Gold Medal – Ralstonia Wilt
This is the granddaddy of regulated diseases that I have encountered over the years. One variant of the bacterium that causes this disease (Ralstonia solanacearum race 3, biovar 2) causes a devastating disease of potatoes (called brown rot) and was classified in the early 2000’s as a select agent by the federal government. This means that the pathogen is recognized as having the potential to be weaponized and used in bioterrorism attacks against US agriculture. Ralstonia wilt was first detected on geraniums in Wisconsin (on a plant submitted to the PDDC) in 1999 with additional introductions on this crop through 2004. In March of 2020, the disease/pathogen was detected after a 16 year absence, this time on Fantasia® ‘Pink Flare’ geraniums in Michigan. This variety of geranium was also distributed to greenhouses in 38 other states including Wisconsin. USDA APHIS is currently leading efforts to eradicate potentially contaminated plants and to decontaminate affected greenhouses. The PDDC has the capacity to detect the bacterial species involved in the disease (but not the specific race and biovar) using the plant disease equivalent of a home pregnancy test. Suspect samples must be forwarded to USDA APHIS labs for a final confirmation of race/biovar.
A major problem with Ralstonia wilt is that plants can be contaminated with the bacterium without showing symptoms. Eventually, in susceptible hosts like geranium, the bacterium colonizes the plant’s water-conducting tissue and blocks water movement, leading to leaf wilting and yellowing. Sometimes, only part of the plant will wilt at first, but eventually the disease is lethal. For more on this disease, check out our Ralstonia wilt pest alert.
Think you have seen any of the diseases described in this article?
Please contact me IMMEDIATELY at (608) 262-2863 or email@example.com. We will need to make arrangements for appropriate testing. And also, as always, feel free to follow me on Twitter or Facebook (@UWPDDC) to receive updates on these and other diseases.
Hang in there, be safe, and stay healthy everyone!
San José scale (Diaspidiotus perniciosus) is a fruit tree pest that can be found in most fruit growing regions of the United States. Native to China, this insect was introduced into the United States in the late 1800s. In well-managed orchards, populations of San José scale are generally too low to cause economic damage. In poorly managed orchards however, populations can become high enough in one to two growing seasons to cause tree and fruit injury. Once established, San José scale can be difficult and expensive to control. San José scale is of historical interest because, in the early 1900’s, it was the first insect observed to develop resistance to an insecticide.
Appearance: San José scale females are yellow, wingless and legless, have a soft, globular shape and are approximately 1/12 inch long. Male scales are 1/25 inch long, are yellowish-tan with a dark band across the back and have wings and long antennae. Immature San José scales (called nymphs) go through three stages (crawler, white cap, and black cap). Crawlers are roughly the diameter of the tip of a pin, are yellow, and have six legs and antennae. Crawlers develop into the white cap stage as they become immobile and secrete hard, white, waxy coverings. The black cap stage follows as the waxy coverings turn gray-black.
Host Range: San José scale feeds on a variety of fruit hosts including apple, pear, plum, cherry, peach, apricot and berries (e.g., raspberry, blackberry), as well as on nut-bearing trees (e.g., walnut) and many ornamental trees and shrubs (e.g., elm, maple, mountain-ash, serviceberry, juniper, white cedar, yew).
Symptoms and Effects: San José scale sucks sap from branches, leaves and fruit causing overall decline in plant vigor, growth, and yield. If left uncontrolled, San José scale can ultimately kill plants. On fruits, San José scale feeding causes slight depressions with red to purple haloes. If San José scale populations are low, fruit damage is usually concentrated on the bottom of the fruit. When infestations occur early in the season, fruit may become small, deformed, and poorly colored. Damage by San José scale (even cosmetic spotting) decreases fruit quality and in commercial settings makes the fruit more difficult to sell.
Life Cycle: San José scale can complete its life cycle in approximately 37 days. There are typically two generations of the insect each year, and generations overlap so that all stages of the insect occur at the same time during the summer. San José scale overwinters in the black cap stage. Development of the insect resumes in spring when temperatures exceed 51°F. Around petal fall, mature females and short-lived males emerge. Males can fly from tree to tree, but females move very little. After mating, females produce approximately 400 live crawlers over a period of six-weeks. The first generation of crawlers appears between early and mid-June, with white and black cap stages developing over approximately the next month. A second generation of adults appears between July and early September. If warmer temperatures continue into the fall, a third generation of San José scale can occur between late October and early November.
Monitoring: The first indication of a San José scale problem may be when infested fruit is found at harvest or (in commercial settings) at packing. However, sometimes the insect can be found earlier on branches. If a San José scale infestation is detected, careful examination of trees/orchards during dormancy can help determine the level of infestation and the extent of spread. Watch for trees that retain leaves during winter (a good indication of a San José scale infestation) and check both branches and trunks for the insect. Mark (e.g., with flagging tape) infested areas on trees to identify where sprays should be applied the following growing season.
In the spring and summer, use pheromone traps to detect the presence of males. Begin using traps at the pink stage of apple flower bud development, in areas where infestations have been detected. Place traps on the northern or eastern side of trees at a height of six to seven feet. Check traps at least weekly. Traps are effective for four to six weeks.
Monitor for crawlers by wrapping two-sided sticky electrical tape (coated with a thin layer of petroleum jelly) around infested tree limbs at both ends of the infested area. Start checking tape for crawlers approximately four to six weeks after bloom.
Control: The best strategy for managing San José scale is to prevent serious infestations. The best cultural control is to prune out infested branches. This reduces scale numbers and opens up the tree canopy so that if spray treatments are used, there is better penetration. Several parasites and predators attack San José scale; however, use of these alone does not provide enough control to prevent damage.
The most effective spray control for San José scale is the use of 2% horticultural oil with or without an insecticide just before or right after bud break, but before flowers open. During this period San José scale resumes its development after being dormant during the winter and the sprays will smother the insects. After applying horticultural oil, continue to monitor for adults and crawlers (as described above) and if you still find active San José scale, consider using chemical insecticides for additional control. Insecticides containing insect growth regulators (e.g., pyriproxyfen or buprofezin), neonicotinoids, organophosphates, or spirotetramat can be effective. Start applications when you find the first adults in pheromone traps or the first crawlers on sticky tapes (usually around early to mid-June). Apply another spray approximately 10 days later if you continue to find active crawlers. When using two applications, be sure to use two products with active ingredients in different Insecticide Resistance Action Committee (IRAC) chemical classes (i.e., with different modes of action) to delay development of insecticide resistance. See http://www.irac-online.org/modes-of-action/ for guidance. Note that late-fall and postharvest applications are NOT effective for San José scale control. Also, remember that whenever you use insecticides, you should consider the effects of products on non-target and beneficial insects. Check the current year “Midwest Fruit Pest Management Guide” (available at https://learningstore.uwex.edu/) for additional insecticide recommendations.
For more information on San José scale: Contact your county Extension agent.
What is basil downy mildew? Basil downy mildew is a devastating disease that affects the leaves, branches, and stems of many types of basil (i.e., plants in the genus Ocimum), plants commonly used for cooking. Green-leafed varieties of sweet basil are particularly susceptible to the disease, while purple-leafed varieties of basil, Thai basil, lemon basil, and spice basil are less susceptible. Certain ornamental basils (e.g., hoary basil) appear to be highly resistant to the disease. Basil downy mildew was first reported in the United States in 2007 and has since spread widely to wherever basil is grown, including Wisconsin.
What does basil downy mildew look like? Symptoms of basil downy mildew typically develop first on lower leaves, but eventually an entire plant will show symptoms. Initial symptoms include leaf yellowing (which gardeners often think is due to a nitrogen deficiency) followed by leaf browning. Affected leaves also curl and wilt, and on the undersides of the leaves, a gray-purple fuzzy material will develop.
Where does basil downy mildew come from? Basil downy mildew is caused by the fungus-like water mold organism, Peronospora belbahrii. This pathogen can be easily introduced into a garden each year via contaminated seed, on infected transplants, or via wind-borne spores (technically called sporangia). Once introduced into a garden the pathogen can spread by wind, by rain splash, or via items (e.g., hands, clothing, garden tools) that come into contact with infected plant and then are used to work with healthy plants. The pathogen thrives in humid, warm environments and can spread rapidly, decimating an entire basil crop.
How do I save plants with basil downy mildew? There is no known cure for basil downy mildew. If you see basil downy mildew, harvest any asymptomatic leaves on infected plants, as well as other healthy basil plants in your garden. Use these materials immediately (e.g., to make pesto). Remove and bag any symptomatic plant remains and dispose of this material in your garbage.
How do I avoid problems with basil downy mildew in the future? Avoid planting sweet basil if possible. Instead, plant other types of basil that are more resistant to basil downy mildew. If you decide to grow sweet basil, try growing the variety ‘Eleonora’ which has been bred for at least some resistance to the disease. If you grow basil from seed, check to see if the seed you are buying has been steam-treated to kill the downy mildew pathogen. Be aware however, that this information may be difficult to find, because steam treatment of basil seed is relatively new and the use of this technique is not widely advertised (at least to home gardeners).
Whatever type of basil you choose, try to grow your plants in a manner that will keep them as dry as possible, thus creating an environment that is less favorable for the downy mildew pathogen to develop and infect. Plant basil in a sunny location, space plants as far apart as possible and orient rows in the direction of prevailing winds to promote good airflow and rapid drying of plants when they get wet. Avoid overhead watering (e.g., with a sprinkler) that will wet leaves and spread the pathogen; instead, use a drip or soaker hose to water.
Use of fungicide treatments to control basil downy mildew is NOT recommended. Products that currently are available to homeowners, even when applied in the best manner possible, will likely not control the disease adequately, if at all. Using these products would be a waste of time, effort and money.
For more information on basil downy mildew: Contact your county Extension agent.
Slugs are legless, soft-bodied creatures that resemble snails without a shell. Slugs feed on a wide range of plants including ornamentals, vegetables and fruits. A number of species of slugs are found in Wisconsin, but gray and spotted garden slugs are the most common, and the most likely to cause damage.
Slugs overwinter either as adults or as eggs, and develop slowly, often living for more than one year. They are often found in higher numbers in areas where sod or other plant residues have been tilled under during the previous growing season. Slugs become active during the first warm days of spring, and thrive under cool, damp conditions. Slug populations will be high during and following damp, rainy weather, and will almost disappear during dry periods. Slugs cannot survive direct sunlight, and without protective hiding places during the day, they rapidly lose body moisture and die. Rock walls, boards, pots and plant debris, as well as shaded flower beds and heavily mulched gardens, serve as ideal daytime resting sites.
Slugs damage plants by chewing large, irregular holes in leaves, stems, flowers or fruits. Most feeding occurs at night or during dark, cloudy days. A shiny trail of mucus (slime) may be associated with the damage. Vegetables and fruit in direct contact with the soil are attacked more frequently than those off the ground. In Wisconsin, tomatoes are often damaged just as they ripen.
Control: There are a number of cultural control options available for controlling slugs. These include:
- removing plant debris, boards or other places where slugs might survive during the day;
- strategically placing boards, carpet patches or inverted flower pots, and collecting and destroying slugs that congregate under these items;
- watering in the morning so that gardens dry out before evening hours when slugs are most likely to be active;
- raking mulch in the winter to expose slugs to adverse environmental conditions that are likely to lead to increased mortality;
- placing copper strips around flowerpots or flowerbeds that will deter slugs from crawling onto plants;
- using regular applications of an abrasive, sharp-edged material such as diatomaceous earth, gravel or sand to deter slug movement;
- placing fresh, undiluted beer (heavy, yeasty varieties work best) in a container set flush with the ground to bait and drown slugs;
- searching your garden at night with the aid of a flashlight and destroying any slugs that you find.
Slugs are often naturally controlled by the feeding of toads, frogs, birds, ground beetles and firefly larvae. In Europe, parasitic nematodes (Phasmarhabditis hermaphrodita) have been used for slug control. Unfortunately, these nematodes products are not commercially available in the US.
Slugs can also be controlled using chemical products. However, keep in mind that slugs are not insects, and will not respond in most cases to insecticides. There are however, a number of commercial products that contain iron phosphate or metaldehyde that can be used as baits for slug control. These products are sold as pellets, or can be packaged in plastic feeding stations. Baits are most often used in large scale agricultural settings when slug control is needed. If you decide to use a commercial bait for control, be sure to read and follow all label instructions of the product that you select to insure that you use the bait in the safest and most effective manner possible.
For more information on slugs: Contact your county Extension agent.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.What are lichens? Lichens are organisms that arise from mutually beneficial interactions between certain filamentous fungi, algae and yeasts. The filamentous fungi provide the physical structures of the lichens, as well as protection for the algae and yeasts. The algae produce food for the fungi and yeasts via photosynthesis. The yeasts are thought to produce compounds to fend off disease-causing organisms and insect pests.
What do lichens look like? Lichens come in four basic growth forms. Crustose lichens are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens are leaf-like and composed of flat sheets of tissue that are not tightly bound together. Squamulose lichens are composed of scale-like parts. Fruticose lichens are composed of free-standing branching tubes.
Where do lichens come from? Lichens are everywhere. There are an estimated 13,500 to 17,000 species of lichens, and lichens can be found growing in tropical, temperate and polar regions throughout the world. Lichens will grow on almost any surface that is stable and reasonably well-lit. In temperate regions, lichens can often be found growing on the bark of trees or old fence posts. Others lichens grow in less hospitable places, such as bare rock surfaces or old headstones in graveyards, where they aid in the breakdown of rocks and the formation of soil.
How do I save a tree with lichens? DO NOT PANIC! Lichens do not harm trees; they are not pathogens or parasites, and do not cause disease. Lichens are self-reliant, with the algal component of the lichen producing food for the organism via photosynthesis. Lichens absorb water and minerals from rainwater and the atmosphere, and because of this, they are extremely sensitive to air pollution. As a result, the presence or absence of certain lichen species can be used as an indicator of levels of atmospheric pollutants. Information on the abundance and species of lichens growing in an area can give a good indication of the local air quality.
For more information on lichens: Contact your county Extension agent.
Freezing rains and ice accumulation in Wisconsin during the winter often lead to questions about potential ice damage to alfalfa stands.
How does ice cause damage to alfalfa? The freezing temperatures of ice often do little damage directly to alfalfa. Temperatures significantly below freezing (less than 15°F when plants have hardened off well) are required to cause damage in alfalfa crowns, one to four inches into the soil.
Ice primarily damages alfalfa because alfalfa roots require oxygen during the winter. This oxygen diffuses into the soil from air above ground. A solid layer of ice restricts air diffusion and suffocates the alfalfa. Lack of oxygen is a common reason for loss of alfalfa in low spots in fields. Alfalfa covered by ice for three to four weeks will likely suffer injury or death.
How can I assess potential ice damage to my alfalfa? If plants have been fertilized according to soil test recommendations, allowed to accumulate good root carbohydrate levels during the fall period, and have hardened properly in the fall, cold injury due to ice is less likely. When determining whether or not ice sheets will cause suffocation damage to alfalfa stands, one must consider whether the ice is in a solid sheet. If the ice is non-uniform, cracked, or has holes, it will not completely restrict air movement into the soil and as a consequence, less damage will result. In addition, alfalfa stems sticking up through the ice will help create holes that will aid in the diffusion of oxygen.
What can I do about ice on my alfalfa? The short answer is nothing. Some growers have used a large tractor and disc in an attempt to break the ice. This technique generally will not break the ice unless there is a layer of snow in between the ice and ground. Other growers have applied fertilizer to ice with the idea that salts in the fertilizer will melt through the ice and create holes, allowing for air movement. Often, however, fertilizer particles will not melt totally through the ice and thus, fertilizer applications are not effective. In addition, subsequent rainfalls or even melting of the ice can lead to substantial runoff problems, particularly if the fertilizer contains nitrogen or phosphorus. Because of runoff issues, current nutrient management standards prohibit most nitrogen and phosphorus fertilizer applications on frozen or snow-covered soils, except when an application is made to winter grains. Even broadcast applications of potassium fertilizers on an ice cover are problematic. If runoff occurs, distribution of the fertilizer can be quite variable and off-site losses could represent an economic loss to growers.
While growers may be concerned about ice on alfalfa, their best course of action is to wait and see how long the ice lasts and afterward assess what damage, if any, occurs. In the recent past, ice sheeting has occurred across central Wisconsin for at least eight weeks with very little resulting winterkill or injury.
For more information on ice damage to alfalfa: Contact your county Extension agent.